Author Topic: CP vs CG  (Read 102262 times)

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Offline NathanStewart

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Re: CP vs CG
« Reply #75 on: December 17, 2014, 04:40:50 PM »
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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Offline ronnieroadster

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Re: CP vs CG
« Reply #76 on: December 17, 2014, 04:49:52 PM »
  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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Offline tortoise

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Re: CP vs CG
« Reply #77 on: December 17, 2014, 05:00:04 PM »
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.

Offline 631

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CP vs CG
« Reply #78 on: December 17, 2014, 05:23:32 PM »
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

Offline manta22

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Re: CP vs CG
« Reply #79 on: December 17, 2014, 05:59:46 PM »
Good post, Rex. Merry Christmas to you, too.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline Interested Observer

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Re: CP vs CG
« Reply #80 on: December 17, 2014, 07:31:32 PM »
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.

Offline jl222

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Re: CP vs CG
« Reply #81 on: December 17, 2014, 09:30:46 PM »

  It the center of gravity doesn't change from acceleration, why do drag cars have wheely bars? :-D

                          JL222

Offline wobblywalrus

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Re: CP vs CG
« Reply #82 on: December 17, 2014, 10:10:29 PM »
The first volume of John Bradley's "The Racing Motorcycle" discusses these issues in detail with pictures and diagrams.

Offline Interested Observer

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Re: CP vs CG
« Reply #83 on: December 17, 2014, 11:41:46 PM »
Quote
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.

Offline Ron Gibson

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Re: CP vs CG
« Reply #84 on: December 18, 2014, 11:12:26 AM »
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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Offline Sumner

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Re: CP vs CG
« Reply #85 on: December 18, 2014, 11:46:39 AM »
.....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
« Last Edit: December 18, 2014, 11:49:02 AM by Sumner »

Offline Interested Observer

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Re: CP vs CG
« Reply #86 on: December 18, 2014, 12:00:04 PM »
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.

Offline Ron Gibson

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Re: CP vs CG
« Reply #87 on: December 18, 2014, 12:55:36 PM »
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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Offline Sumner

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Re: CP vs CG
« Reply #88 on: December 18, 2014, 01:09:24 PM »
...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
« Last Edit: December 18, 2014, 01:14:58 PM by Sumner »

Offline Interested Observer

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Re: CP vs CG
« Reply #89 on: December 18, 2014, 01:37:10 PM »
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.