Landracing Forum
Bonneville Salt Flats Discussion => Bonneville General Chat => Topic started by: Richard Thomason on December 13, 2014, 01:39:39 PM
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I read with great interest the Bonneville Racing News interview with George Poteet concerning his unfortunate crash this past year. A great tip of the hat to George and the whole team for everything they have accomplished. I wish them well in their coming endeavors, whatever they may be. I too understands George's concern regarding father-time and its relentless movement. Ed and I are not getting any younger either, and building our new car is taking longer than expected. We retired the original Danny Boy, because it was 25 years old and we were starting to push up over 350 mph and were concerned about the ravages of time and salt on the chassis. We had seen several older cars crash in the previous few years and just didn't want to repeat history. Thus the decision to embark on a new project.
The article about George's crash got me to thinking about our earliest days at the salt and getting started.
In 1979, when Ed Tradup and I were considering a build for the salt flats, we did a lot of research and investigation. I contacted my dad's cousin (Clinton Wilkinson), who was a very high up engineer with Boeing. When I explained what we were attempting to do, there was a long silence, then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer.
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Richard, we all need more input from folks like yourself, Tom Burkland, George Fields, et al who have learned from their own experiences and listening to the advice and "real life" experiences both on and off the Salt and Dirt of El Mirage.
I also want to thank George Poteet for sharing what He learned and would do differently if ever in a similar crash to His recent one.
With a site like this one "newbees" and "oldbees" have a chance to both learn and to build safer cars that go faster.
I would urge you to share more with the rest of us and consider posting on our BUILD forum.
Hopefully speaking for everyone on this site............
"One Run" Bob Drury
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....In 1979, when Ed Tradup and I were considering a build for the salt flats, we did a lot of research and investigation. I contacted my dad's cousin (Clinton Wilkinson), who was a very high up engineer with Boeing. When I explained what we were attempting to do, there was a long silence, then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer.
You certainly got some good advice way back then :cheers:,
Sumner
P.S. In 4 years you will be as old as I am now. Wish I could hang around for you to catch up with me but time marches on and is dragging me along with it, or something like that :-)
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Richard thank you for posting that . Anybody with any doubt about the CP of their car make a 1 or 2 ft wooden model of your car and with it hanging from a thread and aim your blow gun at the nose . Staple a thread to the nose with washers to make it hang level . If it doesn't point into the wind move the thread forward till it does . If the model is cut roughly accurate on the bandsaw and smoothed up a little that's good enough . Don't be surprised when the CP you find is way forward of the traditional center of side area . The CP model of my streamline was at about 40% from the nose .
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If a simple analysis of a 2-dimensional (elevation, profile, silhouette) area of the car is good enough, an accurate scaled drawing will produce good results.
E-mail me for details.
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Good comments Richard. I choose to ignore the one about us all getting older, but you and George are correct. I also read the article in TBN. Interestingly I trashed my previous ‘liner for pretty much the same reasons. The car was a rear wheel drive, weight biased to the rear, turbo motor. I was on a warm up run (around 270)when the turbos came on about the time I ran into to some damp salt. The rear tires spun, car went side ways and that was the end of that. I knew the car had the CP/CG relationship backwards but it had never been a problem, until ……When we started designing the current car, we discussed the CP/CG issue and the folks at Riley Tech and Bob Riley insisted we build a model and do a wind tunnel test for stability, including yaw. We did that and proofed the design. The other issue was to go with all wheel drive. When running for the higher speeds, I am convinced AWD is a necessity for not only acceleration but for stability reasons as well. In describing the problem, I suggest to folks they take a pencil and put it between their two index fingers (inline) and push, no problem. now move one finger out of line and stability is no longer in the cards. One finger can be viewed as the driving force and the other the aero force. Now think about having a driving force at the same end as the aero force pushing in the opposite direction. Pretty simple. I will also offer that George and Ron’s car was not the only one with that problem running at Bonneville, there are others and it is only a matter of time as speeds increase and salt conditions continue to be marginal that the issue will persist. My incident was in 1990, really surprises me we are 24 years past and the issue is still around and being discussed like it is some new revelation. That said, it what makes Bonneville so cool, we all get to do our way.
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Roger, 24 years. I remember it like it was yesterday. Happy you are still with us and involved in LSR. Since Ben Jordon got me into LSR, I understand it can be a life long adventure.
John
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Some great advise from many people.......
Hopefully it will change some peoples mind set......
Some people feel the longer the dart the less likely it will happen......
In the end that is what makes Landracing so great, everyone has there own ideas, some work, some not so well.
Tom G.
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Anyone know the CP vs CG of a top fuel dragster,funny car,formula 1 or Bville lakester like the DMR?
What changes does the CP-CG have when the spoilers and wings add downforce?
How come Street roadster spin more often when their CP is better than other roadsters?
In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.
JL222
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How come Street roadster spin more often when their CP is better than other roadsters?
What moves the street roadster CP rearward relative to a roadster?
In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.
Was this added weight in front of the front axle?
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Street Roadsters have less engine set-back than (Altered) Gas and Fuel Roadsters.
Street Roadsters also have to put the driver in the original "cockpit" where Gas and Fuel Roadster can move them rearward as long as they stay in front of the rear end. And the cage goes with them.
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How come Street roadster spin more often when their CP is better than other roadsters?
What moves the street roadster CP rearward relative to a roadster?
In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.
Was this added weight in front of the front axle?
Like Stan says the engines are not allowed the setbacks other roadster classes are.
Not adding weight forward and upsetting weight on rear wheels of 222 Camaro.
I would like more answers on the static vs dynamic down force on CP that no one is talking about.
Indy and formula 1 didn't go to rear engines because they were worried about CP. What is their CP-CG?
I can see Airplanes designed with correct CP because of landing and flying straight, but rear wheel driven cars especially short wheelbase cars have different problems.
JL222
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I would like more answers on the static vs dynamic down force on CP that no one is talking about.
You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.
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I would like more answers on the static vs dynamic down force on CP that no one is talking about.
You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.
That's what I'm trying to get going on the thinking. Just because airplanes are designed that way, don't be moving weight forward to improve CP and make thinks worse by increasing chances of wheel spin.
Streamlines and a few other classes can design in CP and CG but others are limited.
I still would like to know the CP vs CG on a top fuel car at 330 mph when the wing is putting hundreds of lbs. of down
force on rear tires.
JL222
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If the traction is good enough the CP can be at the push bar . Salt and dirt traction is poor so the CP location matters to us .
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. . don't be moving weight forward to improve CP and make thinks worse by increasing chances of wheel spin.
So don't move weight forward, add weight forward. You may have to live with some aero instability, though, if this added weight slows acceleration too much to get to speed in the distance available.
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If the traction is good enough the CP can be at the push bar . Salt and dirt traction is poor so the CP location matters to us .
John did you mean CG at the push bar. Having CP back there would be good,
Sum
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Of course CG vs CP is only one factor in going consistently fast and straight at B-Ville. Traction, aero and Polar Moments of Inertia are all also critical components. Part of the design criteria should hopefully be to make the vehicle as inherently stable as possible. All things have to work together and a lot of principals can be overcome by other factors. Witness the Thrust SSC, which was essentially a fork lift. They used that particularily unstable design because of other necessary design criteria, but had the technology to make it work. Anyone else want to give that a try? Not me.
What we tried to do, was to incorporate any and every stability positive factor that we could. Different classes and build types have their own unique circumstances that can and will require compromise, adjustments and just plain experience and know how.
That's what certainly makes it fun.
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I would like more answers on the static vs dynamic down force on CP that no one is talking about.
You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.
The question is, does the CP get worse from the down force of wings-spoilers-ECT?
One question 2 forces. Seems to me as down force is added at rear at speed, the CP is going to get worse.
jl222
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"did you mean CG at the push bar. Having CP back there would be good"
Thanks Sum , meant the nose .
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. Seems to me as down force is added at rear at speed, the CP is going to get worse.
For The CP, "getting worse" means moving forward. I can't see how rear down force would do that.
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The percentage difference between them will decrease as speed increases due to the added weight on the wing at the rear of the vehicle. The CG will move rearward.
Sid.
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The CG will move rearward.
CGs don't move, unless you move mass in the vehicle. Aero force is not gravity.
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The wing down force is applying weight to the vehicle. It gets heavier at that location, that changes the CG unless the change is on the axle centerline.
Sid.
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From Wikipedia:
"Center of gravity is the point in a body around which the resultant torque due to gravity forces vanishes. Near the surface of the earth, where the gravity acts downward as a parallel force field, the center of gravity and the center of mass of an arbitrary body are the same."
What you are referring to might be called the "center of down force". It does affect what forces the wheels can produce, but that's a different question. When the CG is ahead of the CP, the vehicle yawing produces a self-correcting aerodynamic torque.
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The wing down force is applying weight to the vehicle. It gets heavier at that location, that changes the CG unless the change is on the axle centerline.
Sid.
This is nonsense. Tortoise’s reply #22 is correct.
Rear wing drag and downforce aft of the rear axle may, and likely will, reduce front axle contact with the salt and produce reduced steering capability and effectiveness. Also a possible change in pitch attitude.
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Then is centre of gravity the right force to be using for calculating stability ?
maybe it should be cg +other mechanical or aerodynamic downforces at any given speed
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Maj;
CG and CP are the basis for aerodynamic stability but vehicle "stability" also depends on lots of other factors-- bump steer F & R can cause instability: front toe- out can, too. Roll centers, roll stiffness, tire sidewall lateral stiffness, height of the CG, weight transfer, etc., etc., are all factors affecting vehicle stability.
One is tempted to think that in straight- line LSR, these things don't matter since the car is not going around a corner as in circle track or road racing. As long as the car is going in a straight line these things matter little but once the vehicle deviates from a perfectly straight line, these factors begin to come into play. If one or more of these factors cause the deviation to increase, it gets out of hand quickly-- i.e., instability.
Regards, Neil Tucson, AZ
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My partially streamlined bike, when analysed as a solid shape, has a CP farther forward then the CG. It is very stable and does not behave like it should given this. My figuring is my toes, the rear wheel, my and helmet top all catch the wind and this moves the CP back behind the CG.
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My partially streamlined bike, when analysed as a solid shape, has a CP farther forward then the CG. It is very stable and does not behave like it should given this. My figuring is my toes, the rear wheel, my and helmet top all catch the wind and this moves the CP back behind the CG.
I'm getting into tricky, controversial territory here, but bikes are different than cars; they lean over. When a bike gets turned left a little bit, the wind pushes on the right side, leaning it left. If the CP is way back, the wind has a long lever arm to push the bike to turn back right. Now you've got a bike leaned left and turning right: maybe not a good thing.
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....Tortoise’s reply #22 is correct....
I agree. A wing can provide downforce that would help to keep the tires from spinning, but once they do and it they do then if is going to be the relationship of the CG to CP that is going to keep the car from going around as the downforce from the wing has most-likely left the room at that point,
Sumner
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I'm guessing the wing fell off... ? If the down force left... :cheers:
Bob
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I'm guessing the wing fell off... ? If the down force left... :cheers:
Bob
If the car starts to spin what air is the wing seeing in relation to the direction the car is still headed? Is it still going directly across the wing? As long as the wing is seeing the air like it was designed for it will be applying aero downforce that is going to help to keep the tires from spinning and as long as they don't spin too long and upset the car all is well but once the car does start around the wing is going to be less and less effective in providing that downforce. A wing is effective in providing downforce for traction which sure helps but does not change where the CG on the car is.
In the case of the Stude when the throttle stuck and the tires were spinning for 5 seconds the CP didn't stop the car from going around until the back and the vertical stabilizes were at maybe a 60 degree angle (guessing) to the direction of travel. There is going to be a big difference between say having the CG ahead of the CP by 6 inches vs. 6 feet. Both will help but one is not going to help as quickly and maybe not enough once inertia is involved.
As George said in the interview in BRN (more or less) the car was stable for many runs before conditions finally were too much and it went around. He notes that the cars CG was behind the CP and also notes that it won't be if there is a new car,
Sum
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Studebakers with wings... What has this world come to.. Just.. Kidding.. :-D
This subject is interesting to me.. That's why I have one of the biggest tails out there.. And a wing..
A great debate..
Hard to build a proper car as a RWD car..
Danny Thompson's liner is a very good 4wd design.
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The percentage difference between them will decrease as speed increases due to the added weight on the wing at the rear of the vehicle. The CG will move rearward.
Sid.
Yeah Sid I don't know how the CG wouldn't change as speed increases.
If the CG was measured at rest then 100 lbs. added to a rear wing the CG would move back. But at speed adding
100 lbs of down force the CG doesn't change? Just can't buy that.
JL222
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I am kind of new to the LSR thing, but have noticed that most of the near stock bodied Studebaker coupes have a vent cut from above the rear axle up to just behind the rear window. I was told by several teams that these were added because the negative pressure in the window area and air under the car caused lift at speeds over 175 and promoted spins. If that is true, is there any application to the CG vs CP involved?
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I think we're dealing in semantics here. Physically the CG is made up of all the components in the vehicle and the way they're distributed throughout the chassis. The only thing that changes there throughout a run is the fuel used. Downforce or lift is produced by the motion of the vehicle and whatever outside forces may be applied such as cross wind. These are dynamic forces and vary with the speed and wind, both force and direction plus the effect of any changes in elevation which would cause the chassis to react. I think probably what we're talking about when the vehicle is in motion is balance which would be a combination of the static CG plus the dynamics of downforce and lift.
Is it possible that a lack of competition last season is allowing us time to over think what's going on??? :-D :-D :evil:
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Yeah Sid I don't know how the CG wouldn't change as speed increases.
If the CG was measured at rest then 100 lbs. added to a rear wing the CG would move back. But at speed adding
100 lbs of down force the CG doesn't change? Just can't buy that.
There must be a way to get this idea through.
Think of 2 externally identical vehicles, vehicle A with most weight in front, vehicle B with most weight in rear. Both have adjustable wings, front and rear. Adjust the wings so that at 200 mph the wheel loadings are the same for both vehicles.
By your thinking, both vehicles now have the same CG at 200 mph, right?
Drive over a patch of black ice at 200. (Funny weather at Bonneville, huh?)
Will vehicle A and vehicle B behave the same?
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I've spent a lot of time over the last 10 years explaining vehicle dynamics to student mechanical engineers that have had little real work experience with vehicles of any kind… So, I'll wade in on this:
The center of Mass (Gravity) is an imaginary point where aLL the Mass (weight) of a vehicle can be considered to be concentrated. It only changes when components (fuel for example) are consumed or moved about the vehicle (relocating a battery).
When static and level, the CG location determines how much of the vehicles mass is supported by each wheel (tire loading). This in turn, determines the traction available for each wheel.
Aero package "Downforce" increases the vehicle's "TIRE LOADING" (traction)where the aero package is located, but it does not increase the Mass of the vehicle (except for what the air directing devices actually weigh of course).
Aerodynamic device downforce is very sensitive to the "relative wind" flowing over or through them.. Tire Loading can suddenly go away when a aero device no longer has the relative wind properly flowing over it from the normal direction…..
I hope this helps some to see the difference.
Robert "Smitty" Smith
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It is not a matter of semantics, it is a matter of being unclear and confused about what exactly the CP and CG actually are. Sid and JL222 need to review and come to grips with their definitions. Attempting to apply them while misusing them, not surprisingly, leads to an erroneous understanding of the situation. Ascribing aero forces to the CG instead of the CP is the root of their problem.
At Bonneville there are essentially two sets of forces acting on the vehicle to maintain or disturb directional control. They are the tractive forces generated by the tires interacting with the surface, and the aero forces acting on the body. At slow speeds the tractive forces dominate and directional control is easily maintained via the steering system. As speeds rise and aero loads increase, the tractive requirements also increase. There comes a point that the tires lose traction with the surface, spinning (with sufficient horsepower), and with that, a loss of directional control. At that point tractive forces are negligible and aero forces are substantial. The aero drag forces are acting as if through the CP to slow down the vehicle while the inertial forces due to the mass of the vehicle, acting at the CG, are trying to continue down the course. If the CG is in front of the CP, the mass is essentially pulling the vehicle down the course in a stable manner. With the CG behind the CP the situation is unstable until the vehicle rotates to the point that the CG is in front of the CP. (Putting aside the fact that the CP location while travelling backwards is probably not in the same location as going forward) The spin unfortunately introduces rotational inertia forces which may overpower the possibly tenuous stability of the CG/CP relationship, resulting in more spinning. A deployed parachute can really move the effective CP to the rear and produce good stability.
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It is not a matter of semantics, it is a matter of being unclear and confused about what exactly the CP and CG actually are. Sid and JL222 need to review and come to grips with their definitions. Attempting to apply them while misusing them, not surprisingly, leads to an erroneous understanding of the situation. Ascribing aero forces to the CG instead of the CP is the root of their problem.
At Bonneville there are essentially two sets of forces acting on the vehicle to maintain or disturb directional control. They are the tractive forces generated by the tires interacting with the surface, and the aero forces acting on the body. At slow speeds the tractive forces dominate and directional control is easily maintained via the steering system. As speeds rise and aero loads increase, the tractive requirements also increase. There comes a point that the tires lose traction with the surface, spinning (with sufficient horsepower), and with that, a loss of directional control. At that point tractive forces are negligible and aero forces are substantial. The aero drag forces are acting as if through the CP to slow down the vehicle while the inertial forces due to the mass of the vehicle, acting at the CG, are trying to continue down the course. If the CG is in front of the CP, the mass is essentially pulling the vehicle down the course in a stable manner. With the CG behind the CP the situation is unstable until the vehicle rotates to the point that the CG is in front of the CP. (Putting aside the fact that the CP location while travelling backwards is probably not in the same location as going forward) The spin unfortunately introduces rotational inertia forces which may overpower the possibly tenuous stability of the CG/CP relationship, resulting in more spinning. A deployed parachute can really move the effective CP to the rear and produce good stability.
YEAH, no shit. Its you that doesn't understand the question or doesn't want to answer it.
You remind me of the engineers in the 50ths that said a dragster could never go more than 150mph in the 1/4 mile.
You need to understand that wings and spoilers increase down force from aero loads changing CP VS CG.
JL222
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I've spent a lot of time over the last 10 years explaining vehicle dynamics to student mechanical engineers that have had little real work experience with vehicles of any kind… So, I'll wade in on this:
The center of Mass (Gravity) is an imaginary point where aLL the Mass (weight) of a vehicle can be considered to be concentrated. It only changes when components (fuel for example) are consumed or moved about the vehicle (relocating a battery).
When static and level, the CG location determines how much of the vehicles mass is supported by each wheel (tire loading). This in turn, determines the traction available for each wheel.
Aero package "Downforce" increases the vehicle's "TIRE LOADING" (traction)where the aero package is located, but it does not increase the Mass of the vehicle (except for what the air directing devices actually weigh of course).
Aerodynamic device downforce is very sensitive to the "relative wind" flowing over or through them.. Tire Loading can suddenly go away when a aero device no longer has the relative wind properly flowing over it from the normal direction…..
I hope this helps some to see the difference.
Robert "Smitty" Smith
So the added weight to the car which is then transferred to the tires doesn't increase the mass [or weight]?
JL222
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.... So the added weight to the car which is then transferred to the tires doesn't increase the mass [or weight]?
No, because it is a force not a weight, you did not add weight to the car you are subjecting it to a downward force. Does your weight go up when walking or biking into a strong wind?
Sumner
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.... So the added weight to the car which is then transferred to the tires doesn't increase the mass [or weight]?
No, because it is a force not a weight, you did not add weight to the car you are subjecting it to a downward force. Does your weight go up when walking or biking into a strong wind?
Sumner
So down force is not measured in lbs?
JL222
Well devils don't have wings, but if they had inverted wings standing on a scale, into a head wind, wouldn't their weight register more?
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Sometimes we all (myself included) confuse pure theoretical science and precise definitions with actual observations. It's somewhat like comparing Newtonian physics with Einsteinian physics. Newton was correct as far as he went but there are other factors at play as speed increases. A similar issue as we add aerodynamic forces and roll pitch and yaw. The observable results are that the "effective locations" resultants are not static. That's what control surfaces on aircraft do, they change the forces acting on the plane.
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.... So the added weight to the car which is then transferred to the tires doesn't increase the mass [or weight]?
No, because it is a force not a weight, you did not add weight to the car you are subjecting it to a downward force. Does your weight go up when walking or biking into a strong wind?
Sumner
So down force is not measured in lbs?
JL222
Well devils don't have wings, but if they had inverted wings standing on a scale, into a head wind, wouldn't their weight register more?
You would be measuring the force of the wind. Not weight. Torque from your engine is a force, does it add weight to the car?
Once the car becomes unstable you can quickly loose the down-force from the wing or body itself, as mentioned above, so now what you have left to keep the car from swapping ends or spinning is the car's actual weight and where it is located in relation to the center of pressure,
Sumner
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As a purely pedantic aside, not directly related to the discussion, the center of mass and the center of gravity are not exactly the same point. Mass further from the center of earth is subjected to a lesser gravitational force. Thus, for a tall building, (or to a much lesser extent, a car), the center of gravity will be a bit lower than the center of mass. Sorry, I couldn't help myself.
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Its you that doesn't understand the question or doesn't want to answer it.
You remind me of the engineers in the 50ths that said a dragster could never go more than 150mph in the 1/4 mile.
You need to understand that wings and spoilers increase down force from aero loads changing CP VS CG.
JL222
"‘When I use a word,' Humpty Dumpty said, in rather a scornful tone, ‘it means just what I choose it to mean—neither more nor less.'
‘The question is,' said Alice, ‘whether you can make words mean so many different things."
‘The question is,' said Humpty Dumpty, ‘which is to be master—that's all.'"
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.... So the added weight to the car which is then transferred to the tires doesn't increase the mass [or weight]?
No, because it is a force not a weight, you did not add weight to the car you are subjecting it to a downward force. Does your weight go up when walking or biking into a strong wind?
Sumner
Well devils don't have wings, but if they had inverted wings standing on a scale, into a head wind, wouldn't their weight register more?
You would be measuring the force of the wind. Not weight. Torque from your engine is a force, does it add weight to the car?
Once the car becomes unstable you can quickly loose the down-force from the wing or body itself, as mentioned above, so now what you have left to keep the car from swapping ends or spinning is the car's actual weight and where it is located in relation to the center of pressure,
Sumner
Sumner..are you saying wings don't add lbs?
I know the dynamics of loosing down force. we're tilting the spoiler up adding a flap and more and bigger side plates.
This to increase aero lbs. of weight on rear tires for more traction with out adding lbs of static weight.
Bigger side plates should help for better CP, but mainly more wind resistance to stop a spin.
I think this added aero weight will change the CP VS CG ratio at speed, others don't.
jl222 :cheers:
P.S. If spoilers keep a car from getting lite and unloading the suspension, why wouldn't wings make them heavier and load the suspension?
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Sumner..are you saying wings don't add lbs?
If I stand on my bathroom scale under the doorway and push up on the door frame, do I gain weight? The scale reading goes up, so I must be heavier, right?
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Sumner..are you saying wings don't add lbs?
If I stand on my bathroom scale under the doorway and push up on the door frame, do I gain weight? The scale reading goes up, so I must be heavier, right?
The scale thinks so.
JL222
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The scale thinks so.
Not an answer.
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I want to COMMEND everyone who has participated in this discussion..!!!!!!!
Seriously, most of us care deeply about this type of discourse, and many of us have levels
of understanding and or ignorance related to this type of subject matter similar to many who have posted....
this is priceless, and I and many are better for seeing it
thank you
Joe :)
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AMEN Joe
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Here is an old thread that will help on this subject.
CoG vs down force
http://www.landracing.com/forum/index.php/topic,9301.0.html
Tom G.
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The scale thinks so.
Not an answer.
The scale shows more weight, it doesn't know our care where it came from. Same as aero lbs of force or added weight on tire patch.
OK I'm standing on the scale holding a beer it goes up 1 lb. Now I drink it-scale goes up 1 lb. Scale doesn't know what happened.
JL222
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The scale thinks so.
Not an answer.
The scale shows more weight, it doesn't know our care where it came from. Same as aero lbs of force or added weight on tire patch.
OK I'm standing on the scale holding a beer it goes up 1 lb. Now I drink it-scale goes up 1 lb. Scale doesn't know what happened.
JL222
John a couple pages back you asked....
.....What changes does the CP-CG have when the spoilers and wings add downforce? .....
You've gotten the answer over and over from a number of different people but just don't want to think they are right for some reason that I can't figure out so I guess it comes down to you will have your view of the effects of spoiler/wings on CP-CG and we will have ours.
Good luck with the car I'm sure it will go fast,
Sumner
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Ok... My turn..
"OK I'm standing on the scale holding a beer it goes up 1 lb. Now I drink it-scale goes up 1 lb. Scale doesn't know what happened."
200 lb guy + 1 lb beer (in hand) = 201 Now, move beer from hand to stomach, still holding bottle, very big bottle, lol 200 lb guy + 1 lb beer (in stomach) = 201 :cheers:
What we are really doing with a wing / spoiler is increasing the friction / load on the tire, so as to keep it from allowing tire spin and or loss of directional control. This friction is separate from the CP of the car. The wing / spoiler can add a component of drag to the whole package, that can affect the center of pressure though.(think top fuel wing on front of a roadster, bad things will happen, although front end has got hell of a load ) The matter of size / design of side plates on the wing / spoiler, is another factor in the CP story. Bigger Side plates will almost always help... if they're in the back of the car.
Now ...
Why do wings and spoilers work ? Newton or Bernoulli or Coandă ? :-o :-o :-o
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I so love this "techy" stuff and having to twist what little brain function I have around it! When referring to CG though, my question would be are we looking at it while "static" or while at speed? I'm building my streamliner to go fast and yes, chase records at "speed". If I have it on scales with me in it and CG is is in one point while sitting " static", but at 300mph (using my magic "speed" scales) it's now in a different location because of airflow, isn't that what we're actually looking for? In this sport/hobby it's actually ALL about airflow. Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective. Am I wrong?
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It would seem to be..... until your airflow (downforce) didn't stop the tires from spinning... then you are subject to the CG/CP theory for airplanes, we have been enjoying. :-D
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This is what I was referring to .. "Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective."
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I so love this "techy" stuff and having to twist what little brain function I have around it! When referring to CG though, my question would be are we looking at it while "static" or while at speed? I'm building my streamliner to go fast and yes, chase records at "speed". If I have it on scales with me in it and CG is is in one point while sitting " static", but at 300mph (using my magic "speed" scales) it's now in a different location because of airflow, isn't that what we're actually looking for? In this sport/hobby it's actually ALL about airflow. Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective. Am I wrong?
As mentioned above the CG is the same place except for a small difference with the fuel burning off and changing the car's weight and where it is. Center of Gravity is center of gravity, it is static. Other forces can change loadings but they are forces not changes in the car's weight and where the center of that weight is located. The other forces such as a wing or spoiler are going to play into things and need to be considered and studied and respected but do not change the CG of the car,
Sumner
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This is what I was referring to .. "Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective."
Not really since once you loose that traction that the wing helped with the CG as found in the shop along with the CP will now come into play and will influence which end of your car will be facing down track. Your 2 tails are going to go a long ways towards ensuring the car goes straight but it is very simple to get an approximate location for the CG and CP so why not do it. 60 minutes or less and you would at least know the approximate locations of both,
Sumner
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Sum, would you not concede to the idea of a "dynamic" center of gravity? You and Tortoise and a few others are batting around semantics. Yes, CG is static but what Sid and JL are talking about is DYNAMIC. Look at the larger picture and don't put CG in a box. If CG is found by establishing a fore-to-after "balance" point when the vehicle is not moving, then it's entirely conceivable to me that the EFFECTIVE CG would move rearward as aero devices add down force to the back of a vehicle. Just like in a wind tunnel - car is setup on scales. With no air moving we see static CG. Then with the air moving either in scaled or real speed, we can now measure the dynamic forces applied to the car which will indicate a shift in weight and effective "make the car heavier" on one end or the other. Or conversely it could just as well make the car effectively lighter but the scales under the car will indicate a change - this is what they're talking about. It is a real change and it is measurable. Yes, we all know and understand that the mass of the car obviously didn't actually change therefore the static CG is fixed. Without regard to the chance of having zero tractive force, I think it makes perfect sense to say that aero forces will affect the effective weight and balance of the car.
This discussion isn't far from an argument about calling motors engines and vice versa.
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So how how is "air" force different than "static weight (fuel loss)" force? It's weight regardless (gravity) of how it's there isn't it? Our deal is always at speed and with airflow causing lift or down force. If my car is jacked up in the shop and sitting balanced (like a teeter totter) but at speed (our main goal) and that balance point is now a foot farther forward or back, isn't that the center of gravity we're actually looking for?
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Next time you guys head off down the road with your race car in tow, make sure to leave the tie downs lose so it can roll further back on the trailer & you have a nice day! :-D
Sid.
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Tires connected to salt... go very very fast... with HEMI :cheers:
tires not connected to salt... crash very very fast... :cry:
unless CG is in front of CP
aero planes always have CG in front of CP.. well since the Wright Flyer.. :-D
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For your entertainment.. one run two views NO WING .. Most Steering provided by throttle.. aka sprint car ..
note to self... focus harder to keep it on the track... :cheers:
https://www.youtube.com/watch?v=PBqKu1zbR-Q front view
https://www.youtube.com/watch?v=DCBLSkynrv8 rear view
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I'm with Brad on the little brain part. :-D :-D To begin with, we're not talking spinning, yawing, losing traction, side wind, tractive force, torque, sidewalls or any thing other than cg /cp in a straight line. All that other stuff just muddies the water.
If I have enough wing at the rear and far enough back so that it lifts the front wheels off the ground at speed, (as some dragsters have done). It sounds like I'm supposed to believe that the cg of the moving car is still the same at speed with all the weight of the car plus all the downforce on the rear tires. You could theoretically could build a front motored rear wheel drive car that had 80/20 static front bias and with enough wing far enough back and enough speed, lift the front wheel off the ground.
. If you have a bar with a weight on one end and an equal weight in the middle and both weights sitting on two scales. If you apply down pressure to the free end of bar til the end weight is off the scales, the second scale will show the weight of both weights plus the weight of the bar, plus the weight of whatever downforce it took to lift the first weight.
With the logic that if a wing doesn't add or remove weight, an airplane while flying would have no weight whatsoever and therefore would not have or need a center of pressure. (good luck on that one)
YMMV
Ron
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Much of the confusion in this discussion would be removed if we stopped calling the center of mass the center of gravity.
If the vehicle for any reason (loss of traction, side wind),the vehicle finds itself not moving directly into the wind, there will be a side force. The inertial resistance to this side force acts at the center of mass.
If the car is yawed to the left, the wind force will be to the left, centered at the CP. If the resistance to the side wind force is behind the CP, The wind will push to yaw the vehicle further left.
If the inertial resistance to the side force is ahead of the CP, the wind will push to yaw the vehicle right, straightening the vehicle relative to the wind.
Yes, of course it's important to have the appropriate down forces on the wheels, and this can be done with wings; nobody's disputing that; but it does not move the center of mass.
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I love this! Airplanes and our deal are TOTALLY different from my perspective though. Is their CG/CP figured in the shop floor or calculated at cruising speed? Most planes I've seen have the nose pointed in the middle as to shove air over AND underneath. In our world this is a bad thing. The basic physics theories here ARE the same old thoughts that said it was IMPOSSIBLE to accelerate past 150mph in the 1/4 mile aren't they? If the rules hadn't changed, fuel cars would be in the low to mid 340mph range now. Theories and ACTUALLY shoving bricks through the air are two totally different things. If Speed Demons CG/CP were wrong like George said AND he still went 460+mph with a small motor, doesn't that kind of kill the theories here? What we do with horsepower and aero out there is still very much always changing uncharted territory!
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The basic physics theories here ARE the same old thoughts that said it was IMPOSSIBLE to accelerate past 150mph in the 1/4 mile aren't they?
That theory was presented in an article in HOT ROD by an engineer named Roger Huntington. He was a real smart guy, but he made two mistakes; he didn't consider using downforce and he didn't realize that a very low inflation pressure slick tire, heated to where it got sticky, could have a friction coefficient greater than one. NOBODY ELSE HAD THOUGHT OF THIS YET EITHER. Give the guy a break. Predicting the future is hard.
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Tortoise said:
Much of the confusion in this discussion would be removed if we stopped calling the center of mass the center of gravity.
If the vehicle for any reason (loss of traction, side wind),the vehicle finds itself not moving directly into the wind, there will be a side force. The inertial resistance to this side force acts at the center of mass.
If the car is yawed to the left, the wind force will be to the left, centered at the CP. If the resistance to the side wind force is behind the CP, The wind will push to yaw the vehicle further left.
If the inertial resistance to the side force is ahead of the CP, the wind will push to yaw the vehicle right, straightening the vehicle relative to the wind.
Yes, of course it's important to have the appropriate down forces on the wheels, and this can be done with wings; nobody's disputing that; but it does not move the center of mass.
He is completely correct! Aero down force has NOTHING to do with the center of gravity (mass). You are confusing lbs of down force with lbs of mass. Actually the mass of an object is a property of that object and does not change even if you would take it to a place that did not have gravity. The mass that we are concerned with is called inertial mass, which is the property of a body that resist acceleration, linear or rotational. The basic physics formula F=MA (Force=Mass x Acceleration), which came from Newton, can also be written as A = F/M which shows that acceleration increases proportionally to force and inversely proportionally to mass, i.e. more force faster acceleration, less mass faster acceleration. To use Kustombrads example of a top fuel car if the down force of the wing(s) added mass then they would actually would accelerate slower!!
Rex
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I'm not trying to argue here, but now I'm kind of confused. Center of Mass vs. Center of Gravity. Say for arguments sake we'll use a neutral (even on all sides but airflow is even) shape like a boat tail bullet. We do the see saw/teeter totter thing again and have it balanced evenly from the bottom. Is this mass or gravity? Now we slide our pivot point up (any distance up to the top edge) and it'll still balance evenly. Where in there is it the CG if it's two different things? Now I fire said bullet and stop it at 500mph, will it balance on the same spot? If it does, is that considered mass or gravity? Is the bullet designed to balance sitting on the floor or at 500mph? On a fuel car last time I checked it was around 7000lbs down force in the lights. When you talked about adding mass I'm assuming you referring to just adding that weight to the back. Yes a static 7000lbs will move slower since it's not progressively being added like it would be during a pass. If your talking about mass (as in size) moving slower, wind resistance would again come into play if we're talking about 7000 lb mass of wood or the same weight in tungsten (much smaller mass) being moved. I guess I should've gone to to college instead of becoming a custom painter...
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Where in there is it the CG if it's two different things?
The CG and the CM are at the same point (almost, see my post #46, but that's an insignificantly small difference for our purposes). I proposed referring to it as the center of mass to focus attention on the inertial property of mass relevant to the discussion of aerodynamic stability, rather than just how hard the wheels are pushing down on the ground. Yes a static 7000lbs will move slower since it's not progressively being added like it would be during a pass.
That downforce itself does not slow the vehicle; that wing has a lift/drag ratio, and it's the resultant aero drag. The 7000 lbs. of downforce makes much less than 7000 lb. induced aero drag at top speed, and no inertial resistance to acceleration.
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More semantics. Take aero out of the discussion. Even something as simple as an accelerating car will have inertial loading that's rear focused. Ever heard the old salt racer adage that you should always be accelerating and as soon as your speed plateaus your cars handle will change dramatically? This is true for lots of roadsters and other cars - everything goes straight until speed tops out and then you usually get a spin. With less acceleration there's less rearward inertial loading and the "dynamic effective" center of whatever you want to call it shifts forward thus unloading the rear tires which causes slippage and a spin. Every drifter kid in the world knows this and is why they slam on the brakes before initiating a drift - because it shifts the cars weight forward thereby unloading the rear tires allowing them to slip.
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Seems to me its smarter to build a vehicle with the correct CP and CB so if or when you get into a situation similar to what George experienced the chance of saving the vehicle will be greater than if the pressure and balance points are off. I know that's what I want for my car. Sure it may not be easy getting the balance forward of the pressure point but I'm sure we can all agree on one thing if this was easy everyone would be doing it. :-)
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Sure it may not be easy getting the balance forward of the pressure point but I'm sure we can all agree on one thing if this was easy everyone would be doing it. :-)
In the case of roadsters at Elmo, without adding so much front weight that you kill the acceleration, it may be impossible to build an aerodynamically stable car. If you lose traction, you spin.
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i agree with joea, this is a great discussion. A couple of thoughts from my experience; military jet fighter weight and balance guys do talk about a 'dynamic' CG. Computer controls and massive thrust adjust for any imbalance and potential departure from controlled flight, there is a flight control adjustment for bombs on vs bombs off. An LSR vehicle with front and rear wings will add down force in lbs to the vehicle and make it go faster / safer. Should the vehicle yaw enough to render these devices inoperable or limited in down force then the LSR vehicle reverts to its static CM vs CP with tire friction and new high yaw associated air loads factored in to the process. Wheel down force from wings and vehicle center of mass are similar definitions, both exert down force, both are discussed in lbs.However, when the wings stop working for any reason the wheel down force goes away. When long narrow vehicles turn into a high yaw angle for any reason their carefully crafted aero shape turns generally into a lifting body. When an aircraft yaws past its mfg limits one or two wings generally stop flying and you fall out of the sky for a bit. I have always looked at LSR cars as air craft in ground effect (hanging around the Burkland clan) and have had pretty good luck so far keeping the nose pointed north. As Mr. George stated his car had many good 400 plus runs with an imbalanced CP CM, it just takes that one incident to bring the laws of physics to the forefront. Private pilot training handbook has an excellent discussion on this subject. thanks and Merry Christmas
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Good post, Rex. Merry Christmas to you, too.
Regards, Neil Tucson, AZ
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This discussion would be a lot more straight forward if the willful heretics would simply accept the long established prevailing convention that the center of mass and center of gravity are, for all practical purposes coincident, and are inherently a product of the mass distribution of the object, only. It doesn’t move with speed or any other condition unless you start burning off fuel or throwing the crankshaft out the bottom, collecting bugs on the windshield, or some other redistribution of the MASS. Other external forces (aero, traction) have no influence on the position of the CG.
The external forces may, and do, cause redistribution of the ground contact forces and may likewise affect the magnitude of those forces and consequently the behavior of the vehicle. BUT THEY DON’T AFFECT THE CG LOCATION!
An aside re Speed Demon: If, indeed, the CG/CP relationship was disadvantageous, it can only be said that up until the crash they were just lucky. Note that the crash occurred at relatively high speed on a decidedly marginal surface--poor traction. Once it yawed it just went around and around. Same thing for roadsters.
Also, a la F-16’s, if George had a quick enough computer running the steering/rudder of the Demon, he may have been able to get away with piloting an inherently unstable vehicle. But in that case you’re not piloting, you’re just part of the CG.
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It the center of gravity doesn't change from acceleration, why do drag cars have wheely bars? :-D
JL222
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The first volume of John Bradley's "The Racing Motorcycle" discusses these issues in detail with pictures and diagrams.
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It the center of gravity doesn't change from acceleration, why do drag cars have wheely bars?
JL222
Because the external accelerating force applied in the forward direction to the tires at ground level induces an opposite acting (rearward) inertial force (F = ma) at the CG which is above ground level, thereby creating an overturning moment tending to raise the front end--if the overturning moment is greater than the moment acting in the opposite direction due to the weight acting downward at the CG forward of the rear axle.
The CG may move, rotating about the rear axle with the chassis, but it does not move relative to the chassis.
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Me and I think Brad, :-D are still having trouble with this. This post is about center of gravity, not center of mass. I under stand the center of mass would not change and the center of gravity is where the gravity is reacting equally on both ends of the car (we're talking cars here, right?)
I'll use a dragster for ease of visualization. If you have a 1200 lb dragster sitting on a scale in a wind tunnel, just guessing you would probably have 150 lbs on the front end and 1050 lbs on the rear tires. Again just guessing but the center of gravity (where both ends balance) would be about the center of the engine due to the long wheelbase.
At speed the wing applies aprox 7000 lbs downforce which is weight on the tires, no matter how it is obtained. In a 320 mph wind the total weight on the scales would show 8200 lbs
Static, if you added 7000 lbs to the rear end so it is 8050 lbs and 150 lbs on the front, the center of gravity (balance point) would not be the same place as before the weight was added therefor the center of gravity moved.
Ron
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.....Static, if you added 7000 lbs to the rear end so it is 8050 lbs and 150 lbs on the front, the center of gravity (balance point) would not be the same place as before the weight was added therefor the center of gravity moved. Ron
Ron this comes back to the same thing that we have been talking about. The term center of gravity only applies to the effects "gravity" has on the car. It's weight is determined by gravity not any other forces. Sure these other forces can be used to help make the car more stable, have more traction, etc.. We are not denying that what we are saying is that those are forces and they don't effect the cars center of gravity. They might effect how the car behaves while the forces are in effect but what if they are no longer in effect.
If those forces go away what you are now relying on to keep the car going down the course in a more or less nose in front of tail position is the relationship of center of gravity (CG) to center of pressure (CP). This is the point that George P. was trying to make in BRN and Tom Burkland made in a different issue and they and others on here and other places have made in the past. These are people who have been there and done that for years and years. They know what it takes to run well over 400 mph. We can appreciate their advice and use it or not. Personally I'm trying to use it.
Most people are envisioning a wing or spoiler taking care of improper use of what we have learned about where CG should be in relationship to CP. There are a lot of things that can put the car in the position where you would of wished you had taken advantage of keeping the CG ahead of the CP. I'll name a few that could take the downforce the wing is giving out of the equation pretty quick.
1. You run over something or for no apparent reason a tire blows.
2. You blow an engine and the drivetrain locks up the rear end.
3. You encounter a freak crosswind.
4. You hit a slick spot in the course (Speed Demon).
5. You have more HP than in the past and misjudge it (Speed Demon again combined with the slick part of the course).
6. Your transmission or rearend locks up again locking up the rear tires.
7. You might loose a body panel.
8. You might not have the wing or spoiler set to the correct angle needed to insure you aren't going to spin those tire. (this brings up the point we are struggling with. What is the angle that will give us the downforce we need on the Stude. About the only way we are going to know is trial and error and some of the error might involved tire spin and we don't want the car spinning so will keep the CG ahead of the CP. I doubt very few people that are using their spoiler or a wing for downforce know what that down force actually is at different speeds. Anyone actually know that?)
There are probably a number more. The point is we have been given good info from good well respected sources. It is now up to us if we use it or not?
Sumner
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Ron and Brad--you’ve got it!
Your basic car has the longitudinal location of the CG at 150/1200 = 12.5% of the wheelbase forward of the rear axle.
When you add wind, the rear tire load goes up but the aero forces are external forces acting on the car and have no influence on the mass distribution--CG is still at 12.5%.
When you add 7000 pounds of weight (mass), the mass distribution is changed and the CG is then 150/8200 = 1.83% of wheelbase in front of the rear axle--a real wheelie machine.
Again, for all practical purposes, there is no point in differentiating between the CM and CG locations since the CG is merely the CM being acted on by gravity.
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Sum, I understand what you are saying and was just discussing whether cg changes or not. Just trying to get my brain around the statement that cg never changes. It seems to me that as a car goes down the track and speed increases, the amount of downforce on the rear and therefore cg is changing (per IO's post). It is all theoretical, with no variables, yaw, wind, tire spin, etc. taken into account, "just cold hard facts, Mam" Not arguing cp/cg relationship, agree with that completely.
After all I have a roadster which would go better backwards than forward so no dog in this fight.
Ron
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...It seems to me that as a car goes down the track and speed increases, the amount of downforce on the rear and therefore cg is changing (per IO's post)....
Sure the forces on the rear change due to the aero factor but the CG doesn't change and I don't think IO was saying that it does if the car's weight doesn't change (weight, not forces on the car).....
.... but the aero forces are external forces acting on the car and have no influence on the mass distribution--CG is still at 12.5%....
The gravity for our purposes determines the cars weight and that stays the same and that is what CG is all about and it stays the same regardless of it the car is stationary or moving. If we are considering forces other than gravity then we should use a term other than center of "gravity" to describe them.
It is someways similar to shooting a bullet parallel to the ground and drop one at the same time. Which hits the ground first or do they hit at the same time?
Sumner
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Wait a minute Ron, you don’t got it.
IO’s post said the CG moves when you change the mass distribution by adding mass (weight, not force) to the car, not when you add external aero downforce when going down the track.
There seems to be some confusion in that people apparently think the CG magnitude and location is a result of the ground contact loads. That is not the case. It is instead a result only of the mass distribution. However, when sitting statically, that is, without external forces, the CG can be and most often is determined by the contact loads since the only force acting is that of gravity acting on the mass.
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I guess I don't understand how an external force of 7000 lbs (downforce) that will flatten tires if not for centrifugal force is not to be considered as weight. In my other post if you could have a 320 mph wind in the tunnel, the car would weigh 8200 lbs on the scales. How is that not weight?
Ron
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It the center of gravity doesn't change from acceleration, why do drag cars have wheely bars?
JL222
Because the external accelerating force applied in the forward direction to the tires at ground level induces an opposite acting (rearward) inertial force (F = ma) at the CG which is above ground level, thereby creating an overturning moment tending to raise the front end--if the overturning moment is greater than the moment acting in the opposite direction due to the weight acting downward at the CG forward of the rear axle.
The CG may move, rotating about the rear axle with the chassis, but it does not move relative to the chassis.
Yea, that's the point CG does move in dynamic situations.
Gas coups, not allowed engine setbacks have a better CP vs CG but are more prone to spinning than a comp coup or
altered class. See videos on U-tube.
So whats better on cars not able to make body changes, a good CP or more rear CG?
I always liked Joe Laws mid mounted wing on his lakester that still holds the C/BFL AT 349+mph and pretty sure on gas :-o
JL222
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I guess I don't understand how an external force of 7000 lbs (downforce) that will flatten tires if not for centrifugal force is not to be considered as weight...
Real simple it is a force and not a weight. Weight can push down on the tires and also a wing can supply downforce on the tires. They both push down on the tires but only the weight of the car and where it is located is going to determine where the CG of the car is while at rest or underway and it is not going to change.
Once the car is out of control we are no longer interested in the weight of the car to maintain its stability but where the CG of that weight is in respect to the CP.
The relationship of CG to CP is of importance to us not to provide traction for the car but to maintain car stability so the back doesn't become the front.
Sum
P.S. For another time and thread but I don't think just because roadsters have spun in the past is a reason to believe you can't do better. There are options for them related to this discussion, especially if they are blown gas or blown fuel.
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Sum..the options on blown fuel roadsters are not a better CP vs CG.
jl222
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Yea, that's the point CG does move in dynamic situations.
The whole car is moving, so of course the CG is moving, but it remains in the same position relative to the car.
Gas coups, not allowed engine setbacks have a better CP vs CG but are more prone to spinning than a comp coup or
altered class. See videos on U-tube.
So whats better on cars not able to make body changes, a good CP or more rear CG?
You can, you know, put more weight on the rear axle without moving the CG. Just add weight centered at the existing CG. The only problem with that is if your existing weight distribution is, say, 70/30, you need to add 1000 lb. to increase the weight on the rear axle by 300 lb. Maybe not a problem on the long course on the salt for most cars. If you can't make it to speed in the distance available with that much weight, than you have no choice but to add ballast at the rear, let the CG move back, and live with the aero instability. Just don't get off the gas until the chute hits. You've probably figured this out from practical experience.
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Sum..the options on blown fuel roadsters are not a better CP vs CG.
jl222
If you don't want to spin they are, but just my opinion,
Sum
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One way to look at it is to draw an outline of the side view of the vehicle on a piece of graph paper and to make five copies. The first one will be the forces on the vehicle at 0 mph. It is the weight on the wheels. The force arrows from the gravitational attraction are straight down through the axle centers. Weigh the thing to figure that out.
Measure the frontal area and pick the centroid of that area as best as you can. The first analysis will be at 50 mph. Figure out the force from the air pushing on the front at that speed. This force, acting on the centroid, tends to roll the vehicle up and over itself. That does not happen. Force on the rear tire contact patch increases and it is counteracted by the salt being there. Downward force on the front contact patch decreases. There is also the tractive force vector at the bottom of the rear tire pushing the vehicle forwards. It equals the rolling friction and aero forces pushing against it. All of the forces should equalize each other. In other words, the upward forces should be the same as the downward, the forces on the front equalizing those on the back, etc.
Do the analysis at 100 mph, 150 mph, etc. There will be a speed at which there is no downward force on the front tire. Try the exercise with front wheel drive. See how fast the vehicle can go without the front tire lifting. Do everything with a smaller frontal or lower area.
This is a simplistic way of modeling things. It is a big help to figure out the critical relationships. A free body diagram it is.
This has been a big help. It told me to lower the center of pressure on the front, to lengthen the swingarm a bit, to add weight up front, not too far forward, but just enough. What I end up with is a bike that is front heavy while sitting still that has a decent weight distribution at speed. The target speed for me was 150 mph and I wanted no more than a 45% front and 55% rear distribution at that speed. I do not get that, the front end is lighter, but is handles much better than it would if I had not done the changes based on math.
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The moments that try to spin the vehicle around its center of mass should be cancel each other out, too, as well as the forces. This I forgot to say.
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I guess I don't understand how an external force of 7000 lbs (downforce) that will flatten tires if not for centrifugal force is not to be considered as weight...
Real simple it is a force and not a weight. Weight can push down on the tires and also a wing can supply downforce on the tires. They both push down on the tires but only the weight of the car and where it is located is going to determine where the CG of the car is while at rest or underway and it is not going to change.
Once the car is out of control we are no longer interested in the weight of the car to maintain its stability but where the CG of that weight is in respect to the CP.
The relationship of CG to CP is of importance to us not to provide traction for the car but to maintain car stability so the back doesn't become the front.
Sum
P.S. For another time and thread but I don't think just because roadsters have spun in the past is a reason to believe you can't do better. There are options for them related to this discussion, especially if they are blown gas or blown fuel.
IF down force is not a weight [of course it is and measured in lbs] what unit of measurement do You give it.
Of course CG changes dynamically. Would you agree that a CG taken at rest at a certain rear level and another
2 inches lower at rest would be different? But you and others would say a lower rear height from speed or acceleration doesn't change the CG.
JL222
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. . .you and others would say a lower rear height from speed or acceleration doesn't change the CG.
Not me.
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I have been eating popcorn while following this discussion :lol:
I think most involved aren't going to change their opinions.... however food for thought. If you think aero adds weight not force, does your parachute then have to be sized for the extra 5000 lbs you need to stop? No it doesn't because there isn't actually 5000lbs more weight. Your parachute needs to bisect the center of mass too function properly and not upset the car. This is calculated with the car static not moving and no aero "weight" factored in (at least that's how I have done it ). Mount and size your chute with a calculation using a lot of aero "weight" factored in and your chute could really upset the car when it deploys. Have you ever heard of DJ asking how much aero "weight" your car has at speed so they could properly size your chute.....
Just wanting to be involved in the conversation :evil:
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I have been eating popcorn while following this discussion :lol:
I think most involved aren't going to change their opinions.... however food for thought. If you think aero adds weight not force, does your parachute then have to be sized for the extra 5000 lbs you need to stop? No it doesn't because there isn't actually 5000lbs more weight. Your parachute needs to bisect the center of mass too function properly and not upset the car. This is calculated with the car static not moving and no aero "weight" factored in (at least that's how I have done it ). Mount and size your chute with a calculation using a lot of aero "weight" factored in and your chute could really upset the car when it deploys. Have you ever heard of DJ asking how much aero "weight" your car has at speed so they could properly size your chute.....
Just wanting to be involved in the conversation :evil:
:cheers: :cheers: :cheers: :cheers:
I think I've exited the building on this so glad someone else entered the room 8-) 8-),
Sumner
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I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.
So here is a video that will explain it in such a way that everyone should understand.
Tom G.
http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip
PS. Sumner, don't leave yet.
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I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.
So here is a video that will explain it in such a way that everyone should understand.
Tom G.
http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip
PS. Sumner, don't leave yet.
Thanks Tom, explains how aero makes weight.
JL222
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I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.
So here is a video that will explain it in such a way that everyone should understand.
Tom G.
http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip
PS. Sumner, don't leave yet.
Good video and it explains downforce over and over and some of us are not disputing downforce (force, force not weight, weight) I didn't see anything in the video about moving the cars center of gravity with downforce.
They mentioned that to the tires the downforce looks like added weight to the car but they then explained that the good thing was that it didn't actually add weight to the car. No weight change, no change in the center of gravity and that is what this whole discussion has been about. About the relationship of CG to CP . to the stability of the car once it looses traction.
I've watch about every NASCAR race since 1990 and it seems like every time one of those cars looses traction it spins. A second before it had all that downforce on the car but once it started to spin that downforce did nothing to halt the spin and that is what this is all about.
Can I leave now :-) :-) :? :?,
Sumner
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“weight n. ...attraction of a material body by gravitational pull toward the center of the earth”
It is clear that JL222 prefers to operate in his own universe, apart and different from everybody else’s.
You can lead a horse to water . . .
At this point, I’m with Sumner.
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“weight n. ...attraction of a material body by gravitational pull toward the center of the earth”
It is clear that JL222 prefers to operate in his own universe, apart and different from everybody else’s.
You can lead a horse to water . . .
At this point, I’m with Sumner.
AND the flattening of top fuel tires at speed is from what?
And the video is wrong too?
Why don't you answer the questions that Summer won't?
JL222
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If you think aero adds weight not force, does your parachute then have to be sized for the extra 5000 lbs you need to stop? No it doesn't because there isn't actually 5000lbs more weight.
Whether jl222 wants to call downforce "weight" isn't too important. Speaking casually, a lot of racers, even engineers, might do the same. But they'd understand your point, and know you're correct.
Jl222, do you?
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It is unfortunate that we use the same terms for external forces and weight. That is why some people here think that an external force such as aerodynamic force and weight are the same as they are measured in pounds.
As several have pointed out CG is the center of mass acted on by gravity. External forces do not change the center of mass or the CG. lbs force are not the same as lbs weight. The fact that both are measured in lbs does not make them the same.
In the metric system people tend to talk about weight in kilograms (kg) and forces in Newtons (N).
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AND the flattening of top fuel tires at speed is from what?
And the video is wrong too?
Why don't you answer the questions that Summer won't?
JL222
In the video I thought it explained mechanical vs. aerodynamic force pretty well, and I know it did not talk about CP, but it was the best video I could find that I thought might help this discussion.
I really like FORCE as the word used to describe what is going on aerodynamically, as IMO it differentiates from the word weight, and is easier to get a Grip (no pun intended) on the forces involved.
I posted the video because I am a visual guy and I thought it explained what Sumner and others had pointed out and explained.
JL222 also thanked me for posting the video.
IMO we all agree on the video. Now it would seem to me that there is an interpretation or word that might be throwing a wrench in the works.
I think using the word weight instead of Force is the problem.
In physics, a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull.
I also think GRIP is a good word to use.
So the wing on a T/F car at a given speed uses Force to increases the Tire Grip on the surface of the drag-strip, with out adding mechanical weight to the car which would slow down its acceleration and stopping ability.
Tom G.
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I think we have lost sight about the intent of this thread which was...
....then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer....
The title is "CP vs CG" . The thread is/was suppose to get that message out. Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,
Sumner
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It is a complicated issue. Hard to isolate to just two topics. The CG stays the same at all speeds, excepting the loss of fuel or coolant. The CP varies in its location, and the magnitude and the direction of the forces acting on it change greatly depending on the circumstances. Ultimately, the forces acting on the tire to salt contact patches are what matters and this is a combination of many.
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The title is "CP vs CG" . The thread is/was suppose to get that message out. Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,
Sumner
I read the entire thread and learned a bunch of stuff, but i'm with Sumner.
I thought that the static mass distribution of the car results in the location of its center of mass.
On earth we most easily determine the the vertical component of the center of mass by measuring the force of gravity on its 4 wheels (weights)
Then we calculate the center of mass and call it CG
As the car goes down the track, all the forces that act on that mass dynamically push it around.
Wings, traction, parachutes, wind, steering input, etc generate forces which will cause that mass to move. F=Ma
It will move differently according to its specific distribution of mass, but generally having Cp aft of Cg results in greater stability.
This seems too simple, i must be missing something.
Karl
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... Ultimately, the forces acting on the tire to salt contact patches are what matters and this is a combination of many.
I agree totally that is what matters in order to maintain traction and that should be the subject for a different discussion.
The point of this thread was to enlighten us on what we can do to maintain some semblance of car control after traction is loss by paying attention to our car's CG in relation to its CP,
Sumner
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Re: "it is a force and not a weight"
Part of the confusion is that identifying down-force (or any non-gravity based effect) as weight is that it conflates weight (gravity pull, proportionate to the relative masses of the object in question and the astronomical body on which the event occurs) with mass (directly proportionate to the number of atoms present in an object).
This is why chute selection is not down-force compensated - it only controls mass, not weight.
On Earth (and at sea level, to be specific) weight (in lbs.) = mass (in slugs) × 32.172 (acceleration rate by Earth gravity). On the Moon weight (in lbs.) = mass (in slugs) × 5.322 (acceleration rate by Moon gravity).
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. . .mass [is] (directly proportionate to the number of atoms present in an object).
Oh, really?
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I guess I don't understand how an external force of 7000 lbs (downforce) that will flatten tires if not for centrifugal force is not to be considered as weight. In my other post if you could have a 320 mph wind in the tunnel, the car would weigh 8200 lbs on the scales. How is that not weight?
Ron
Ron...You not drinkin either? :-D
And all that 7000 LBS of downforce [ or what ever that's flattening the tires] doesn't change the CG dynamically.
It'S like a car doing a wheel stand and these guys say the center of gravity hasn't changed.
JL222
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I finally figured this out . . .
Convinced People vs. Convinced Guessers
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It'S like a car doing a wheel stand and these guys say the center of gravity hasn't changed.
I think everyone here would agree that wheelstanding raises the center of gravity.
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I think we have lost sight about the intent of this thread which was...
....then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer....
The title is "CP vs CG" . The thread is/was suppose to get that message out. Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,
Sumner
Sumner... CG ahead of CP is a great design for Streamliners and other classes of cars that allow changes to body styles.
But I would warn about moving CG forward to improve CP on some cars, loosing traction and causing a spin.
I was trying to point out that even if CG was ahead of CP, aero down forces as from a wing would cause the CG to move dynamically. You don't think so. Explain how Top Fuel tires are flattened without added weight on chassis and how the CG hasn't changed dynamically.
Some one reading the original warning and thinking its gospel, then moving weight forward for a better CP could make things worse.
Some of the things we have done on the 222 Camaro are completely wrong according to a lot of advice from seasoned
racers.
JL222
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I finally figured this out . . .
Convinced People vs. Convinced Guessers
I am still trying. Tony
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I'm pretty sure it's come to this... If you've figured it out, you'll go fast. If you haven't, you'll always be chasing your tail. If you don't have a car it doesn't matter anyways...
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....You don't think so. Explain how Top Fuel tires are flattened without added weight on chassis and how the CG hasn't changed dynamically....
This is my last response to a question such as this. Aero downforce flattens the tires and the CG of the car itself is in the same location relative to the car. If the car does a wheelstand then the CG goes to a new location in the universe along with the car.
You just keep coming back to aero forces on the car that for sure are there and I'm not disputing that but they don't change the CG. A NASCAR car should have what you are talking about with all the downforce built into the car and if you are correct it should keep the car from spinning once it loses traction but it doesn't.
I'm not saying you need to give up trying to gain traction, that would be foolish. We now have the CG ahead of the CP on Hooley's Stude and I"m convinced that was what kept it from going around in 2013. Now we can more or less leave the car alone and stop adding more weight since we can gain traction via the rear wing, using its size, profile and angle of attack. But while experimenting with that if we loose traction with the CG ahead of the CP that relationship will go a long ways towards keeping the car from spinning. It isn't just the spin that is dangerous, some cars once sideways tend to start flying. Long cars like streamlines also don't tend to spin very well as witnessed by the number that receive a lot of damage once sideways.
My remark about the blown gas/ blown fuel referenced that now with the development of turbos and in your case centrifugal blowers it isn't that hard to build any motor larger than an F motor that will put out over 1000 hp and a lot more. So with that kind of HP available a lot of the cars that can't have a wing and the vertical stabilizers that can really, really help with the CP part they can add both weight ahead of the rear axle for traction and weight further forward to move the CG forward.
I've already repeated myself to the point I'm even tired of hearing myself on this so people will just have to decide what they want to take from this thread. I'm not trying to convince you to change your car. If it is working for you stick with it.
Sumner
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Because the "cg" is up in the center of the mass, maybe for practical purposes we should call what we're discussing the balance point. As I understand it, the static "cg" would be somewhere along a vertical line at the balance point. As long as the tires are on the ground this is what we're talking about. The balance point would change as force is applied to either end. The cg can be low and the cp much higher, think Nascrap cars blowing over backwards after sliding sideways while going forward.
Ron
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So with that kind of HP available a lot of the cars that can't have a wing and the vertical stabilizers that can really, really help with the CP part they can add both weight ahead of the rear axle for traction and weight further forward to move the CG forward.
You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.
But what some of the others are saying, I think rightly, is you want the least possible or even no ballast if you are drag racing, even a 1.3 mile drag race on dirt.
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It'S like a car doing a wheel stand and these guys say the center of gravity hasn't changed.
Don't ask me why, but I'll try once again to explain how weight distribution can change without the CG moving. Think of your car moving at a constant speed up a gradually steepening hill. If the hill gets steep enough, and you maintain traction, the car is going to tip over backwards. The CG is at the same point in the car, but the direction of the gravitational force relative to the road surface changes. Accelerating is like going up a hill: the vector sum of inertial and gravitational forces is no longer straight down but slanting toward the rear.
By the way, according to Einstein, gravity and inertia are really the same thing, but let's not go there.
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.....You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.
I agree with that and probably 80% of the weight we have added to the car is in weight boxes ahead of the rear tires and back of the driver with the CG about where the steering wheel is. Running out of room there we also have weight under the transmission area and a small amount just ahead of the engine. Cars are still compromises and we have had to make some.
I once built a 'pump trailer' that was used to do water testing and pump-down tests on wells for an anticipated strip mine. You could pump or bail wells up to 400 feet deep with it. To keep the cost down I used the drive-train from a 4 wheel drive Bronco to supply power for the winches and generator and such. The drive train was pretty much intact except for swapping the front axle and rear axle locations and was mounted on a trailer. So a lot of weight at both ends of the trailer. The trailer was a real pain to tow since if it started to wag its tail it only got worst. The next one I made had a different design and all the weight central to the trailer. Lesson learned. That is why I really try and tell people to not put the weight they need for traction back behind the rear axle. We have nothing back there except for 1 normal size battery and another for data-logging only.
Edit: Added what is below this at a later time.
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(http://1fatgmc.com/car/14-Hooley/974-CG-CP-10.jpg)
I added the photo above to show the 'approximate' locations of the CG and CP on the Stude. Actually the CP would be a little further back as Tom B. pointed out to me that I should use the side view area of the vertical stablizers twice in predicting the 'approximate' location of the CP since both of them see the air.
I'll grant that it is going to be very hard to get a 'precise' location for the CP, but John Burk has pointed to one method to get close and I've used a different method that comes from people with far more knowledge than I have to figure ours. I outline that method here for anyone interested....
http://1fatgmc.com/car/14-Hooley/14%20-%20hooley-construction-2014-2.html
I'm not saying you are going to be within inches of the CP with John's method or what I presented but it sure beats doing nothing about something that you can do to build a safer car,
Sumner
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I think looking at the car as if it were traveling down track at a 15 deg angle helps a lot for center of pressure. Lets face it if the car is straight all is good, but at 15 deg sh&*ts about to get real. At that angle the cars center of pressure can change dramatically and aero downforce is typically nearly gone. The shape the air sees is totally different, this shape/cp and the center of mass is what will make the difference on a car going around or not. Some car with a taller front cockpit/body will see a significant change in the cp at an angle and suddenly the tail is not enough to overcome the pressure trying to push the front around. In the video it appears as though speed demon's cockpit is ripped off from the air prior to the roll. Imagine the cp while the car is sliding sideways. With a large/tall cross section forward of the cg it seems the car would be harder to recover.
The cg and or mass of the car is what is continuing to push the car down track so the pressure will rotate the body around the cg point depending on which end of the car is easier to push sideways (considering air and scrub/tires). So a car with less than ideal cp/cg relationship can get away with it up untill it sees a certain degree of slip/yaw.
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So that we are clear on CP. It is the point that if you turned the car 90 degrees to the air the car wouldn't rotate one way or the other if a pivot point was put vertically through the car at the CP (this is if we discounted the CG for a moment). The pressure behind and in front of the CP would be equal.
Now bringing CG into the equation the car should weather-vane around with the nose still down track if the CG is ahead of the CP just like a weather-vane keeps pointing into the wind. If the CG is behind the CP it is going to weather-vane around trying to put the tail in the lead. Likely it won't stay there because now the car is spinning and inertia is involved but if we can keep it from going around in the first place that will be a good idea.
It is too bad that all that are reading this don't get BRN as George's article is a good one and explains the canopy coming off and other items of interest. That article and others like it are a good reason for anyone involved in this to subscribe. What you will learn from the interviews and articles that Bob Hellmuth and Bill Hoddinott write and others could save you thousands in building a better car and not having to repeat and relearn what many have learned before us. You can also order a lot of those articles from back issues. I go back and re-read them from time to time and always pick up something new,
Sum
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Correcting my earlier typo (duh):
"mass (directly proportionate to the number of atoms present in an object)."
Yes, it's the atomic weight, not the atomic number.
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Re: "if you turned the car 90 degrees to the air the car wouldn't rotate one way or the other if a pivot point was put vertically through the car at the CP"
True with the car in the intended vector and alignment.
However, with the car in yaw (unless it's radially symmetrical!), the CP almost always moves - the questions are:
1. how much? and
2. forward or backward? and
2. acting at increased or decreased height (will it induce a roll moment)?
I'm sure we all agree that yaw resulting in an aero change that moves the CP backward (at least, in the range of predictable steering corrections, not 90°!) is a useful safety feature.
Has anyone heard of spill plates (e.g., end & end "capture" fins on a rear wing) linked to the steering (toward motion parallel with steering corrections) so that they tend to sustain down-force by controlling spillover?
Another "wing" thought: if yaw begins to become a slide, isn't increasing wing down-force also helpful (since drag is no longer a concern)? How about a dive brake, which cranks the leading edge's nose down (ref.: Stirling Moss's 1955 300SLR, linked to brakes)?
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OK, I think this is about exhausted............
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I like that icon, Neil, I have been reading along on this discussion but haven't tried very hard to absorb the information. Not that it isn't interesting or perhaps not important, but there's so much of it. :evil:
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Relax Jon. Not all of it is information. :wink: Wayno
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Sorry, I didn't really mean to start a treatise on aero, design etc., I was just reflecting on my own experience and thinking about George's comments. As I posted before, there are so many differant classes and design constraints that there is no one size fits all. Yes it is more simple in a streamliner to achieve ideal configurations, but there can be other design constraints that violate the ideal rule, such as the fork lift Thrust SSC. They made it work, as many of you have in other situations. I applaud you all for your ingenuity and cleverness to make things work the way you have. I'm certainly not as smart as many of you in "tricking the laws of physic" and therefore that is why we built the way we did. It eventually (after two relatively high speed endos) worked for us and hopefully our experience will help us in our new build.
Richard
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Thanks to all for tolerating a nonracer's presence. I love this stuff.
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.....You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.
I agree with that and probably 80% of the weight we have added to the car is in weight boxes ahead of the rear tires and back of the driver with the CG about where the steering wheel is. Running out of room there we also have weight under the transmission area and a small amount just ahead of the engine. Cars are still compromises and we have had to make some.
I once built a 'pump trailer' that was used to do water testing and pump-down tests on wells for an anticipated strip mine. You could pump or bail wells up to 400 feet deep with it. To keep the cost down I used the drive-train from a 4 wheel drive Bronco to supply power for the winches and generator and such. The drive train was pretty much intact except for swapping the front axle and rear axle locations and was mounted on a trailer. So a lot of weight at both ends of the trailer. The trailer was a real pain to tow since if it started to wag its tail it only got worst. The next one I made had a different design and all the weight central to the trailer. Lesson learned. That is why I really try and tell people to not put the weight they need for traction back behind the rear axle. We have nothing back there except for 1 normal size battery and another for data-logging only.
Edit: Added what is below this at a later time.
=======================================
(http://1fatgmc.com/car/14-Hooley/974-CG-CP-10.jpg)
I added the photo above to show the 'approximate' locations of the CG and CP on the Stude. Actually the CP would be a little further back as Tom B. pointed out to me that I should use the side view area of the vertical stablizers twice in predicting the 'approximate' location of the CP since both of them see the air.
I'll grant that it is going to be very hard to get a 'precise' location for the CP, but John Burk has pointed to one method to get close and I've used a different method that comes from people with far more knowledge than I have to figure ours. I outline that method here for anyone interested....
http://1fatgmc.com/car/14-Hooley/14%20-%20hooley-construction-2014-2.html
I'm not saying you are going to be within inches of the CP with John's method or what I presented but it sure beats doing nothing about something that you can do to build a safer car,
Sumner
Sum...how is traction with the CG so forward? Seems like it would suck. Whats Hooleys best 2/14 and 1st mile time?
JL222
Ps...its well known that trailers should have at least 10% of their weight on the tongue.
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.... Sum...how is traction with the CG so forward? Seems like it would suck. Whats Hooleys best 2/14 and 1st mile time?..
We don't have a good run with the new motor as I was doing license runs and we felt the motor might of gotten hurt so quit after the last run where I got my A license. On the one B lic. run where I got into probably 75% throttle and then 100% for a second until the throttle stuck the car was running 185 at the 1 1/2 mile marker before it started to spin and I'm sure it would of in the old configuration (pre-vertical stablizers/wing) but didn't.
With the old B motor that maybe made a little over 800 HP Hooley would run about a 217 at the 2 1/4 and set the record at 249+ with a 253 exit speed. With a 2.47 rear and a 1.91 first he could run about 50% throttle max in 1st gear (this is data logged data not driver impressions). Would get to 100% throttle in the 1 to 1 3rd and in the overdrive 4th (.93).
It isn't just about the CG as you can have all different rear axle weights with the same CG. In our case right now the car with the big block and the 2 foot extended wheel base is at 6420 lbs. minus a driver and has 3320 on the rear....
(http://1fatgmc.com/car/14-Hooley/974-CG-CP-11.jpg)
... other figures are in the pic above.
Now with the wing we will be able to add downforce without having to add weight hopefully (won't help a lot at lower speeds but at some point you will meet the aero wall where the car is going to spin the tires on the top end too and then we will have the benefit of the wing). Even with the wing working we will still have the CG ahead of the CP as shown above since the wing hasn't moved the CG.
I think the car has a lot of potential. On my A lic. run I didn't push it at all getting up to speed and ran the last couple miles constant throttle between 212 and 218 with the throttle in the mid 40% range. We had changed gearing in the transmission and thought we had different drive gears than we really had and went less overdrive instead of more. According to the tack I thought I was running 240 but that was based on having the ratio in the transmission we thought we had. Since the timing slip and my calcs didn't agree back home I counted teeth in the transmission and then saw we had different drive gears than what we thought. Plugging those in the calcs and timing slip now matched. We were going back to World Finals with the right gears in the transmission but it rained out and then the same happened this year so we have yet to find out what the new combo is going to do but I think it will run well over 217 at the 2 1/4 :-),
Not sure what the comment about tongue weight was about but if it was about that pump trailer I built it had well over 10% tongue weight (about 400 lbs. if I remember right). The problem it had was the weight at the two ends of the trailer which still put the CG close to the axles or ahead but the weight, especially at the far back was not in a good place once inertia became a factor.
Sumner
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In hydraulic engineering we have a term called center of buoyancy, center of buoyant pressure, or sometimes informally called the center of pressure. Lets say you need to hold a car or dead horse shaped styrofoam shape under water with a stick. The procedure done in the previous post would be used looking at the front, top, and sides of the shape. Looking at the front would tell how far down the CB is in the shape, looking at the top would tell how far back the CB is from the front of the shape. Looking at the side will tell how far back the CB is from the front of the shape. Note there are two distances back from the front. In practice we average them to get the distance from the front. The gets us close.
Next, we look at the CG. It should be at the CP if the thing is to stay horizontal under water. Picture in your mind a freight submarine carrying a load of lead. Carrying the lead too far towards the nose will cause the submarine to always want to dive. Constant correction will be needed on the diving planes and it will need to be moving to stay level. The plane angle will vary in proportion to the sub's speed. Similar but opposite happens if the load is too far towards the rear. Subs have ballast tanks they can partially fill with water to adjust their buoyant trim.
How the heck this method applies to a moving land vehicle is beyond me. My feeling is it is the wrong method and its value is it is making people do the right things.
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.... Sum...how is traction with the CG so forward? Seems like it would suck. Whats Hooleys best 2/14 and 1st mile time?..
We don't have a good run with the new motor as I was doing license runs and we felt the motor might of gotten hurt so quit after the last run where I got my A license. On the one B lic. run where I got into probably 75% throttle and then 100% for a second until the throttle stuck the car was running 185 at the 1 1/2 mile marker before it started to spin and I'm sure it would of in the old configuration (pre-vertical stablizers/wing) but didn't.
With the old B motor that maybe made a little over 800 HP Hooley would run about a 217 at the 2 1/4 and set the record at 249+ with a 253 exit speed. With a 2.47 rear and a 1.91 first he could run about 50% throttle max in 1st gear (this is data logged data not driver impressions). Would get to 100% throttle in the 1 to 1 3rd and in the overdrive 4th (.93).
It isn't just about the CG as you can have all different rear axle weights with the same CG. In our case right now the car with the big block and the 2 foot extended wheel base is at 6420 lbs. minus a driver and has 3320 on the rear....
(http://1fatgmc.com/car/14-Hooley/974-CG-CP-11.jpg)
... other figures are in the pic above.
Now with the wing we will be able to add downforce without having to add weight hopefully (won't help a lot at lower speeds but at some point you will meet the aero wall where the car is going to spin the tires on the top end too and then we will have the benefit of the wing). Even with the wing working we will still have the CG ahead of the CP as shown above since the wing hasn't moved the CG.
I think the car has a lot of potential. On my A lic. run I didn't push it at all getting up to speed and ran the last couple miles constant throttle between 212 and 218 with the throttle in the mid 40% range. We had changed gearing in the transmission and thought we had different drive gears than we really had and went less overdrive instead of more. According to the tack I thought I was running 240 but that was based on having the ratio in the transmission we thought we had. Since the timing slip and my calcs didn't agree back home I counted teeth in the transmission and then saw we had different drive gears than what we thought. Plugging those in the calcs and timing slip now matched. We were going back to World Finals with the right gears in the transmission but it rained out and then the same happened this year so we have yet to find out what the new combo is going to do but I think it will run well over 217 at the 2 1/4 :-),
Not sure what the comment about tongue weight was about but if it was about that pump trailer I built it had well over 10% tongue weight (about 400 lbs. if I remember right). The problem it had was the weight at the two ends of the trailer which still put the CG close to the axles or ahead but the weight, especially at the far back was not in a good place once inertia became a factor.
Sumner
Good job Sum on building a safe car.
JL222
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The center of buoyancy does not consider pressure differential over the object's surface. It is like the center of gravity in this respect. It depends on the volume of water displaced by the various parts of the shape and it stays pretty constant if the shape in not changed.
This is the question to ask yourself during the design. Is the CP I am using the center of buoyant pressure or the center of the dynamic pressure. They are different. One is static for practical purposes and one is not.
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Wobbly:
Actually, the center of buoyancy does depend on the pressure acting at the various points of the object’s surface. It is the surface integral of the pressure distribution.
However, for the special case of complete submergence, the sum of all the surface forces works out to be equivalent to the weight of the displaced fluid, as you have stated.
For Pete’s sake, don’t tell JL222 that his car is losing “weight” because it is submerged in air! (Which would be an advantage of running at Bonneville vs. El Mirage.) That’s liable to start another endless and pointless discussion.
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.... Good job Sum on building a safe car.
JL222
Thanks but Hooley did 90% of the build and John and I and a few others filled in the other 10% :-). Hoping for 1300-1400 HP, but that is only about 1/2 of what you guys make :cry:,
Sum
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Wobbly:
Actually, the center of buoyancy does depend on the pressure acting at the various points of the object’s surface. It is the surface integral of the pressure distribution.
However, for the special case of complete submergence, the sum of all the surface forces works out to be equivalent to the weight of the displaced fluid, as you have stated.
For Pete’s sake, don’t tell JL222 that his car is losing “weight” because it is submerged in air! (Which would be an advantage of running at Bonneville vs. El Mirage.) That’s liable to start another endless and pointless discussion.
IO..Some cars actually do loose weight at speed and the suspension can go into droop. Racers couldn't figure out the
ill handling at speed for years then Jim Hall and the Chapperall put on a spoiler, then a wing.
Didn't you know that ?
JL222
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That is something I do not know. The work I did, and do, on this subject is on objects submerged in liquid fluid. The center of dynamic pressure moves forward as velocity increases. Centroids by definition are at the center of what they define. More pressure on one side of something due to movement in a fluid shifts the dynamic centroid of pressure in that direction. The problem with using buoyancy CP is that it might work for awhile. Eventually, as the vehicle goes faster and faster, the dynamic CP moves too far ahead for stability.
This was demonstrated in a flume when I was in skool. An object was hung by a cable and lowered into the flume where it was submerged. Water velocity is slow at first and the object hangs from the cord without wiggling. The water velocity speeds up. The object starts to wiggle at higher flow velocities. The dynamic CP is too far forward. We monkeyed around with different shapes, fins, where we attached the cord, etc. These had effects on the flow velocities when wiggling started.
I never figured out a method of determining dynamic CP that I am proud enough about to share. A problem of that nature comes up once every eight to ten years so I never put much effort into refining the method. My profession is civil and environmental engineering so I rough in a math based design and "field adjust" to get it to work. The mechanical and structural folks on our staff get paid to think so they get the problem if it needs to be done correctly.
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Didn't you know that ?
JL222
Spoilers and wings on cars happened long before Paul Lamar, Chevy R&D, and Jim Hall made notable use of them.
Wobbly:
From your water flume description, it sounds like the “wiggling” was more a product of vortex shedding than a stability issue. Vortices forming alternately on either side of the object travel down the side and then are shed off the tail, inducing an alternating pressure distribution and “wiggling”. Have you ever heard or seen a powerline “singing” in the wind?
If it was an instability, the object would have turned and remained in another orientation (and possibly wiggled there).
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Vortices forming alternately on either side of the object travel down the side and then are shed off the tail, inducing an alternating pressure distribution and “wiggling”.
I seem to remember reading on this forum about one of the prominent motorcycle streamliners of our day having a cover enclosing the chute exit. They ran without it because early testing showed just such a problem. Is there a straightforward fix for this?
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The problem with discussing CP vs CG is knowing where the CP is . The simplest method is a wooden model on a string and an air hose like I've done which showed the CP to be 40% back from the nose.
The closest things I saw on the internet that look like a streamliner and mention CP are aircraft fuselages and bullets .
"The subsonic center of pressure of the fuselage will be located at or forward of the quarter-length point , and since the airplane c.g. is usually considerably aft of this point , the fuselage contribution will be destabilizing" .
http://thearmsguide.com/wp-content/uploads/2013/11/Featured-static-stab-620x350.jpg
My test and these disprove the CP is at the center of body area . Why is it so far forward . From what I've seen it's because pressure on the nose is a bigger part of drag than skin friction . That disagrees with "skin drag is 70% of total drag" . Still hoping to answer that contradiction . Door cars have the same forward CP but still looking for information on that .
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And one proposal from Harry Miller while building the " Golden Submarine" for Oldfield was to install a moveable wing atop the car for downforce. In 1917 :cheers:
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That bullet stuff is interesting , i could not find further reference from those pages , but was CP dependant on velocity ?
Other info i found after this yrs stability issues on our bike was rocket design where they aim for one body diameter between CG and CP as ideal for stability
http://www.rockets4schools.org/images/Basic.Rocket.Stability.pdf, not sure i can achieve that sort of spread
also i wonder on 2 vehicles with same Cg and CP but different wheel placement (probably more on liners and motorbikes) , one with wheels near either end of the vehicle and other with one or both wheels closer to centre , just how stability would compare , i suspect any instability would be exaggerated by closer wheels
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The CP of most wing profiles change little or not at all at Reynolds numbers that would be 133 , 167 and 400 mph for a 24 ft streamliner .
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Today I talked with some engineers who know about this subject. The method shown by Sumner is perfectly legit. The car, when the back end swings out to be perpendicular with the line of travel, has a pressure centroid close to the areal centroid. The method of weighting the car as shown is what is needed to help it regain stability. Fins on the back help and as long as one is not in the turbulence shadow of the other, each can be considered a full addition to the exposed side area.
The center of pressure does not move in some shapes like symmetric airfoils if they are pointed in the direction of travel. An assumption of the CP being 25 percent behind the front edge is often used. The CP moves to the near the middle of the airfoil if it is tilted so it is perpendicular to the flow. I was cautioned about applying this principle to other shapes.
There are stabilizing and destabilizing forces acting on vehicles. They vary according to speed. The impression I have of the CP moving forward might be partially due to the destabilizing forces gradually overcoming the stabilizing ones as I go faster.
The above was explained to me. My thoughts are that a bike guy is dead meat if the back end slides out on the salt and the motorcycle is perpendicular to the direction of travel. That is why I developed the idea described a few pages back. In it I look at the forces acting on the vehicle rather than the CP. Two reasons for this. No one I know including myself can accurately determine the CP of a bike facing forward, and we cannot depend on the areal CP to CG relationship to save us in a slide.
I was wrong during the recent posts and I apologize. My dinner tonight will be crow. Sorta tough but tasty with plenty of BBQ sauce.
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Aerodynamic stability prevents wild rides . It shouldn't show up while you're having one .
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....The method shown by Sumner is perfectly legit. ....
Let me make something clear and that is I didn't devise using the area as viewed from the side to derive an "approximate" (note that word) CP location I think it was Tom B. who first brought it to my attention. Using the area method or about any other means that most of us can use and that is in our reach is going to end in an "approximation" In the case of the 'area method' that I documented on my site a flat vertical surface, like a tail, is going to have a greater effect/value than say the same area as seen from the side that is rounded, like say a fender or roof top.
I feel the main purpose of the thread is to try and get you to at least come up with some idea where the CP is even if it isn't exact. Then you can explore what your options are in regards to trying to move it rearward or the CG forward. If you are building a streamliner or lakester you have a lot better chance of designing a vehicle that has more stability from the get-go but with a car you need to work on at least trying to stop the disaster before it gets totally out of hand.
You don't have to be Einstein to get some idea where the CP/CG is on your car. Get the car weighed and a side view taken from a distance and at least take a look (see my other post for the link on how to do the rest) or try John's approach, at least do something :-) :-),
Sum
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This was posted here in 2006 .
Yes that is one of the reasons why cross winds (even minor ones) can make such a huge change.
For example the high pressure area that forms in front of the windshield quickly disappears as the wind begins to move from a quartering angle. This means two things, one a slight cross wind can drastically change your front down force (or rear for that matter) and it can very rapidly shift the center of pressure fore and aft on the vehicle. A body that is very stable in dead quiet wind conditions can suddenly turn into a car that wants to turn away or into a cross wind and unloads the front tires when it is hit by a cross wind gust.
In Norbye's book Streamlining and Car Aerodynamics (long out of print copyright 1977) he covers this issue a bit in Chapter 5. He states that the center of pressure on a typical sedan is near the cowl area and on sports GT's it is near the front wheels. (streamlining tends to move the center of pressure forward as drag at the rear of the body is eliminated, unless measures are taken such as fins to increase the rear sail area).
On the 1963 Corvette its front down force due to the high pressure area at he base of the windshield essentially disappears at a wind vector of 30 degrees from the axis of the body.
According to Norbye, the early Ford GT 40 had 696 lbs lift at 200 mph, 528 lbs front and 168 at the rear. When the wind vector swung to 15 degrees off axis the cars lift went up to1170 lbs, 786 at the front, and 384 in the rear. This car had a static weight on the front wheels of only 766.5 lbs, so in cross winds the front wheels would come off the ground.
The problem was solved with the addition of a front air dam and a change in the shape of the cars nose. After those modifications front lift at 0 degrees angle of attack was 236 lbs, while rear lift increased to 272 lbs. At a 15 degree angle of attack front lift still was only 309 lbs, and rear lift was at 343 lbs.
Larry
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John;
Here is an excerpt from a 1969 wind tunnel report on a "GT40-like" race car. It supports what you have stated.
Regards, Neil Tucson, AZ
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Nose shape and its effect on vehicle behavior in cross-winds was the reason for using this rounded shape. The shape it presents to a wind from straight ahead or at mild yaw seems to be the most uniform, in comparison to other shapes.
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Clearly the solution is to go really really fast to reduce the effective yaw angle from side winds.
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OK. I will..... :cheers:
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Usually when a vehicle goes sideways we think it is the rear end breaking loose from too much applied power. At high speeds tractive forces exerted by the back tires on the surface are weakening as one shifts through the gears. Is there another cause of high speed spins? Aero drag is increasing with speed. The CP is pretty far forward when going straight. Is the wind catching the front of the vehicle and spinning it around?
A few years ago there were some horrific bike crashes on runways. I do not know the exact causes of them. They got me to thinking. Suppose the bike hit a bump, the front end lifted, the wind got under the bike, and it lifted the front end up and spun the bike onto its side?
This is something I think about. Is there any evidence of it happening?
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Funny thing about tires , when available traction is used up for acceleration there is none left to resist yaw . That's the reason for spins . Speed Demon has been so stable because their traction control kept a little traction in reserve for side grip . The crash happened because the traction was worse than they expected and had the traction control set for .
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Yeah Sid I don't know how the CG wouldn't change as speed increases.
If the CG was measured at rest then 100 lbs. added to a rear wing the CG would move back. But at speed adding
100 lbs of down force the CG doesn't change? Just can't buy that.
There must be a way to get this idea through.
Think of 2 externally identical vehicles, vehicle A with most weight in front, vehicle B with most weight in rear. Both have adjustable wings, front and rear. Adjust the wings so that at 200 mph the wheel loadings are the same for both vehicles.
By your thinking, both vehicles now have the same CG at 200 mph, right?
Drive over a patch of black ice at 200. (Funny weather at Bonneville, huh?)
Will vehicle A and vehicle B behave the same?
Where did you, or anyone else, get the idea that I think extra downforce [weight] on rear tires makes it more stable in a spin?
I think more aero dowforce helps traction and prevents tires from spinning and causing a spin, but my original point was, if aero downforce puts more weight on rear tires and if it did not help traction wouldn't the CP VS CG get worse because of added weight on rear tires and make the car less stable?
JL222
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Where did you, or anyone else, get the idea that I think extra downforce [weight] on rear tires makes it more stable in a spin?
I don't claim to know what you think, other than that aero downforce moves the CG.
I think more aero dowforce helps traction and prevents tires from spinning and causing a spin . . .
Everyone would agree, I think. but my original point was, if aero downforce puts more weight on rear tires and if it did not help traction wouldn't the CP VS CG get worse because of added weight on rear tires and make the car less stable?
No, aero downforce doesn't move the CG. There's a question in the post you're quoting, and I wish you'd answer it. If those 2 different vehicles have the same downforces (from mass plus aero) on each wheel at speed, do they thus have the same CG location at that speed, and if they lose traction, will they handle the same?
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Where did you, or anyone else, get the idea that I think extra downforce [weight] on rear tires makes it more stable in a spin?
I don't claim to know what you think, other than that aero downforce moves the CG.
I think more aero dowforce helps traction and prevents tires from spinning and causing a spin . . .
Everyone would agree, I think. but my original point was, if aero downforce puts more weight on rear tires and if it did not help traction wouldn't the CP VS CG get worse because of added weight on rear tires and make the car less stable?
No, aero downforce doesn't move the CG. There's a question in the post you're quoting, and I wish you'd answer it. If those 2 different vehicles have the same downforces (from mass plus aero) on each wheel at speed, do they thus have the same CG location at that speed, and if they lose traction, will they handle the same?
The vehicle with the most weight in the front will be less likely to spin [duh]
Now please explain how down force is not a weight that puts more load on the tire and does not change the
CG. And while your at it explain how weight is not a force as Sumner says.
The 222 Camaro 3000 lbs on rear tires 1680 lbs on front. tilting up spoiler and adding a 2'' piece of metal
at 45 deg angle for more down force. lets just guess 500 lbs. Now rear weight is 3500 lbs at speed and you guys
say that's not moving the CG back at that dynamic point and I don't have to worry about it?
JL222
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The vehicle with the most weight in the front will be less likely to spin [duh]
According to you, aero downforce and mass are both "weight". Both vehicles have (at speed) the same "weight" (as you define it) on each wheel. Now please explain how down force is not a weight that puts more load on the tire and does not change the CG.
I cannot speak for Sumner or anyone else, but I have no problem calling downforce "weight". After all, we speak of weight jacking by adjusting the suspension without moving any mass around. Forget about "CG" when talking about aero stability; think of center of mass. When you've lost traction, inertia is resisting the slowing of the vehicle, and this forward inertial force is centered at the center of mass. The rearward aero force (drag) is centered at the CP. If the force pulling forward is in front, the vehicle "wants" to face forward.And while your at it explain how weight is not a force as Sumner says.
I think he would say that weight is a force, but not all force is weight, even if it's pointing down. I agree, but don't think it's important.The 222 Camaro 3000 lbs on rear tires 1680 lbs on front. tilting up spoiler and adding a 2'' piece of metal
at 45 deg angle for more down force. lets just guess 500 lbs. Now rear weight is 3500 lbs at speed and you guys say that's not moving the CG back at that dynamic point and I don't have to worry about it?
The center of mass has not moved. Aero stability is unaffected. Fear not; go fast.
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...
A few years ago there were some horrific bike crashes on runways. I do not know the exact causes of them. They got me to thinking. Suppose the bike hit a bump, the front end lifted, the wind got under the bike, and it lifted the front end up and spun the bike onto its side?
This is something I think about. Is there any evidence of it happening?
WW,
I was at the events where most of those horrific asphalt crashes ocurred, even had "the best seat in the house" at one at Texas in Mar 2012. Thus i've given this issue a bunch of thought.
Unfortunately I have no profound conclusions.
I do know that most of the bad crashes happened in shut down with Bill Warner being a notable exception. Personally, i believe Bill's crash at something pushing 300mph was somehow aero-related, but no one knows.
karl
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The vehicle with the most weight in the front will be less likely to spin [duh]
According to you, aero downforce and mass are both "weight". Both vehicles have (at speed) the same "weight" (as you define it) on each wheel. Now please explain how down force is not a weight that puts more load on the tire and does not change the CG.
I cannot speak for Sumner or anyone else, but I have no problem calling downforce "weight". After all, we speak of weight jacking by adjusting the suspension without moving any mass around. Forget about "CG" when talking about aero stability; think of center of mass. When you've lost traction, inertia is resisting the slowing of the vehicle, and this forward inertial force is centered at the center of mass. The rearward aero force (drag) is centered at the CP. If the force pulling forward is in front, the vehicle "wants" to face forward.And while your at it explain how weight is not a force as Sumner says.
I think he would say that weight is a force, but not all force is weight, even if it's pointing down. I agree, but don't think it's important.The 222 Camaro 3000 lbs on rear tires 1680 lbs on front. tilting up spoiler and adding a 2'' piece of metal
at 45 deg angle for more down force. lets just guess 500 lbs. Now rear weight is 3500 lbs at speed and you guys say that's not moving the CG back at that dynamic point and I don't have to worry about it?
The center of mass has not moved. Aero stability is unaffected. Fear not; go fast.
ON THE black Ice, both vehicles would loose traction and probably spin. the #1 wouldn't spin as many times. [maybe]
OK...Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels?
Don't say because it doesn't change. And I know after aero down force is lost MASS CG goes back to static state.
You keep talking about what happens when you loose traction I'm talking about before you loose traction and aero
Down force.
JL222
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OK...Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels? Don't say because it doesn't change.
Because it doesn't change. What you call moving weight (by aero force) doesn't move the CG. Aerodynamic directional stability is about side forces, not down forces. The mass of the vehicle resists a side force. This resistance to the side force is not transmitted through the wheels, it's inertial. It's acts at the center of mass. If the center of mass is behind the center of gravity, the vehicle yawing to the left creates a side force on the vehicle pushing it to the left, and the inertial resistance creates a moment yawing it further left. If the downforces at the wheels, and their coefficient of friction don't allow them to produce enough force to resist this yaw, around you go. No matter how much downforce you have, it's better to have aero producing a self-correcting force when you yaw.
And I know after aero down force is lost MASS CG goes back to static state.
"MASS CG" is not a term I'm familiar with. CG and CM are the same point, they never moved in the vehicle. I'm talking about before you loose traction and aero Down force.
Me too.
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Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels?
Because aero downforce does not, in any way, redistribute the center of MASS. And until you understand that, you will continue to have an incorrect and confused concept of vehicle dynamics. In reality, the conventional practice is a much simpler and more useful in that the mass and external forces are considered separately and combined as appropriate to predict a particular situation.
most of the bad crashes happened in shut down
A motorcycle does not have the advantage of having the center of mass between the front wheels while under braking as a car would, which for a car provides a somewhat stable situation under deceleration.
Assuming that M/C shutdown on a relatively short, paved course would tend to invoke fairly heavy braking, weight transfer to the front relieves the rear wheel of vertical load and its stabilizing effect, leaving one with the large rearward acting braking force of the front wheel in opposition to the inertial force of the bike mass which is well behind the front wheel--an unstable set of opposing forces. This could easily lead to putting the bike down. How many crashes occurred in the absence of front braking?
Aero drag would tend to stabilize the situation. Bikes with dive brakes or parachutes?
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The events, including Bill's, caused me to take a hard and long look at vehicle stability. A challenging aspect of airport racing is pushing the bounds of speed in both directions. Accel and decel.
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IO & WW,
I'd love to have a beer or 2 with you guys, kick around MC asphalt LSR handling issues.
As you point out, 2 wheels can make things different than 4 wheels
Now back to topic trying to get consensus that Cg is a static quality of the car with implications on dynamic handling :cheers:
With my recent switch from 2 wheels to 4 wheels and my lakester's perceived handling issues at 2014 WOS, i am keenly interested in this topic.
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JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.
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Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution?
First, I would think, does the car take maximum advantage of the rules to move the CP as far aft as possible? Strakes and spill plates don't need to add much drag.
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Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution?
First, I would think, does the car take maximum advantage of the rules to move the CP as far aft as possible? Strakes and spill plates don't need to add much drag.
We have added spill plates close to as large as allowed. My rule change request for additional plates have been approved and we are adding 2 or 3 more. When we added the original spill plates we made the mistake of not raising the spoiler rods to where we usually have them so were losing down force. Strakes are not allowed in our class.
We also have the 2% body stretch in cowl area.
javajoe.. Yeah, it would be nice and SAFE if we changed the CG ahead of CP But it wouldn't perform as well.
I also believe that 3000 lbs of weight on rear helps to keep the 222 car from lifting in a spin.
A lot of other cars have much better CP vs CG than we do but still spin. Gas coupes and street roadsters
are some.
JL222
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This interests me because as you know I am building a similar car. It will be real heavy for starters but we will have lots of options for placing extra weight.
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JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.
Yeah, I saw that hood up in the air and thought of all the time we had in it and knew it was going to be thrashed :-o Tacking 280 mph at that time and spun going through the traps at 260+ with the chute out.
Where were you when I spun? It was sprinkling on the windshield at the start, just a few drops, after I spun in the 1st
timed mile, my chute was completely soaked when I picked it up. So course was wetter down there. And our wide tires
don't like water. Did you notice any rain if you were near the 3 mile?
Other spins have been because of 2 much throttle even though I never give it to much until hi gear.
Other cars have gone faster, but 294 mph in the 1st mile with 2 more to go keeps us coming back.
JL222
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Not to hijack my own thread, but just a quick thought. You cannot apply more forward force than the down force (weight plus aero and any other force that acts as weight [corealis? Ha] times the coefficient of friction. On the salt, the CF can and does vary greatly. In the 60's, no one expected a tire to exceed a 1.0 CF. Dragsters exceed the then supposed theoretical limits. Guess that's why a cog wheel train works.
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Meant to say "weight" on the driving wheels.
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JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.
Yeah, I saw that hood up in the air and thought of all the time we had in it and knew it was going to be thrashed :-o Tacking 280 mph at that time and spun going through the traps at 260+ with the chute out.
Where were you when I spun? It was sprinkling on the windshield at the start, just a few drops, after I spun in the 1st
timed mile, my chute was completely soaked when I picked it up. So course was wetter down there. And our wide tires
don't like water. Did you notice any rain if you were near the 3 mile?
Other spins have been because of 2 much throttle even though I never give it to much until hi gear.
Other cars have gone faster, but 294 mph in the 1st mile with 2 more to go keeps us coming back.
JL222
Keep it up! We were in line and ran behind you in the black sonoma. Would have bumped our own record up a couple mph but we also spun. There was alot of wind and we didn't want to run but they yelled at us in the staging lane to go
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Not to hijack my own thread, but just a quick thought. You cannot apply more forward force than the down force (weight plus aero and any other force that acts as weight [corealis? Ha] times the coefficient of friction. On the salt, the CF can and does vary greatly. In the 60's, no one expected a tire to exceed a 1.0 CF. Dragsters exceed the then supposed theoretical limits. Guess that's why a cog wheel train works.
Guess that's why 4 wheel drive is better, more contact patch.
Wider tires? Naw everyone knows they don't work, you know, traps air at speed, blaw,blaw,blaw, like Indy cars,formula 1 Sports cars :roll:
JL222
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Good info in this article Rick Yacoucci shared on FB the other day
http://scienceenvy.com/race-car-engineering-400-mph-how-to-stay-straight-and-on-track/
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Killacycle beat you to it, but bit buried perhaps:
http://www.landracing.com/forum/index.php/topic,14924.msg272851.html#msg272851
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Good info in this article Rick Yacoucci shared on FB the other day
http://scienceenvy.com/race-car-engineering-400-mph-how-to-stay-straight-and-on-track/
I don't know where the CP is on the street roadster and coupe is but the center of gravity should be pretty far forward
on those front mounted engine cars.
Kind of contradicts what they think their talking about.
Of the 2 types least likely to spin ''according to the article'' funny that they shows these :roll:
2OO mph like a pig on ice. What a bunch of crap, if the 222 Camaro ever felt like that its shut off time.
JL222
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I just thought it was a good tutorial for us idiotos the do a quick CP equation :wink:
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I just thought it was a good tutorial for us idiotos the do a quick CP equation :wink:
It is a good as figuring the CP-CG I just don't agree with adding weigh to front with a car having traction problems making traction worse.
Example-Street Roadsters they have a lot of front end weight so their CG should be ahead
of CP but still spin same as gas coupes why if CG and CP are correct?
I know that above 160 mph '' Chaparral Jim Hall'' discovered enough lift on coupes to unload
the suspension calling for a spoiler for downforce. Roadsters and gas coupes are not allowed
spoilers, or body mods, so weight keeps the lift down with less wheel spin.
How about you roadster and gas coupe guys posting some results.
I'M just saying that wheel spin causes the instability and solving that should be looked at
just as much.
I guess a pig on ice could ''and have'' been built, just have a wheel rate of infinity, skinny tires
with 90 lbs in them and not much weight on rear tires. Hey spin city. :-D
JL222
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Our Street Roadster is 40% front and 60% back.
Stable as hell until it spins the tire(s). At El Mirage with a cross wind in the "brown stuff" . . .
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Here are the top secret weights . . .
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<quote> Example-Street Roadsters they have a lot of front end weight so their CG should be ahead
of CP but still spin same as gas coupes why if CG and CP are correct?</quote>
Part of the answer to that, is that the approximation in that article is only an approximation. It just gives you a ball park idea of where the cp is under ideal conditions.
In the real world the center of pressure moves forward as you go faster.
The center of pressure at 100 mph will be behind the center of pressure at 200 mph.
The center of pressure also moves around depending on the angle of the air flow over the car.
Likewise down force can change drastically due to small changes in the wind direction.
A good portion of your down force and drag force (which contributes to center of pressure) is due to air pressure build up on the windshield and cowl area at speed. This is negligible at speeds below about 60-70 mph but becomes more and more significant as you get to 200+ speeds.
For example on the 1960's corvette body, if it was placed in a side wind so the effective airflow was coming from about 15 degrees instead of straight ahead (this as I recall from a very old book) the diagonal wind would cause that high pressure bubble of air at the base of the windshield to basically disappear as it dumped off the down wind side of the windshield.
Result, mild cross wind caused huge increase in front end lift, higher front end stance due to unloaded springs increased exposure of the suspension and undercarriage and increased frontal area and aero drag shot up (moving the center of pressure forward of where it was at lower speeds).
Same effect is the result of flow separation toward the rear of the body. As that transition point moves forward at higher speeds so does the center of pressure as only the front portion of the car still has more or less attached flow and once the air flow separates the aerodynamics of the rear of the body become meaningless.
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Stan, we both use the same performance tuning method, a #2 pencil and some not previously written on paper.
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8888 views and 188 replies. Not anymore. :evil:
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What's New is Old... From Bonneville Racing News early 90s Harry Hoffman Jr.... 25 years ago....
maybe we should look toward Galileo .... He might have known ... :-D
Center of Gravity of A Horse drawn Carriage is...?
Wait.... Roadster or Coupe? one or two horsepower?
:cheers:
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Doing some calculations on my race car I find the CG is 2 1/8 inches forward of the CP this is without moving any weight around by relocating stuff in the car. Is there a prefered distance forward of the CP for the CG?
Ronnieroadster
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It depends on what problem you are trying to cure.
For aerodynamic stability only, move the weight as far forward as possible
The bad news is that as you do that, you will reduce rear wheel traction and the ability to put power down by the rear wheels.
Not as big a problem if you are front wheel drive, then forward weight distribution would be a benefit to both.
As they say moderation in all things. Move it as far forward as you can without creating traction problems.
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The problem I experienced as the speeds increased the front of the car would move around more and more. Checking the CP and CG last year I discovered my CG was behind the CP. At 150 MPH no issues at 160 a little movement at 170 lots more movement and over 180 a whole lot of movement. As the speeds increased I thought it was front end issues tried many different things nothing would help. Then I learned a bit about this CP and CG after checking the cause was finally figured out.
Now with a longer wheel base and a few other changes the CG is finally ahead of the CP so now the next test is to see how this all effects the traction. Interesting stuff for sure.
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That could also be due to front end lift.
Regards, Neil Tucson, AZ