Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: Salty Nuts on October 04, 2008, 01:46:37 AM
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What is preferred as far as weight distribution is concerned (roadster)?
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What is preferred as far as weight distribution is concerned (roadster)?
Talk to Wilson and Waters or Chuck Salmen both are record holders over 250mph and run suspension and i don't think a lot of weight. Formula 1 cars run around 65% on rear tires not counting downforce from wing.
Some roadster run more weight than the above cars weigh.
Depends also how much hp and how fast you go.
Also if you add weight its better to keep it from being to far back and keep low.
Good luck JL222
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This is a dilemma, since good high speed handling (viz. sports cars, F1 etc.) is very different from LSR.
Too much weight in the rear is (always, assuming RWD) good for traction, but not necessarily for handling unless the suspension rates, tire width differentials, etc. are on track or oversteer results. You don't really want to run tires that wide in the back - too much rolling resistance, too much width, more unsprung weight.
The 2 (main) methods of increasing tractive effort are weight and aerodynamics.
Weight is predictable, linear (more is better in direct ratio), reversible, cheap (comparatively), and easily adjustable. Downside: it reduces acceleration, and changes center of gravity location.
If rules permit, downforce is preferred over weight for traction. It doesn't reduce acceleration (i.e., mass is constant), and the amount of added traction can be very large.
The problem is that it's not legal in all classes, expensive to make or buy, not very adjustable as to location (position & height), and very difficult to anticipate the exact effect it will have. It also changes the car's angle of attack (relative nose height) as speed increases, and may unload the front wheels and permit air under the car. Higher downforce generally requires higher drag (larger surface, greater angle of attack), which eats power at high speed (but need not be a problem if the car is traction limited). It also stops working if the car yaws more than a bit, and lifts the tail if the car spins, so handling in an incident becomes scary.
Both load the suspension and structure in proportion to their location, exactly above the rear wheels = 100%, but cantilevered behind the rear wheels = more than 100% (and removes some weight from the front wheels). It can even be added directly to the axle (illegal for aero devices in some racing), within reasonable limits (or bending results, plus unsprung weight penalty).
Whatever method is used, the aero center of pressure (in elevation or side view) should be behind the center of gravity for stability. When you add weight or downforce to the rear wheels, the center of gravity may move back too close to (or even behind) the aero center, which is an unstable condition.
There's more, but look at the rules 1st, then figure how "do-able" hiding weight is vs. adding a wing.
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Too much weight in the rear is (always, assuming RWD) good for traction, but not necessarily for handling unless the suspension rates, tire width differentials, etc. are on track or oversteer results. You don't really want to run tires that wide in the back - too much rolling resistance, too much width, more unsprung weight.
Can you run a FWD roadster? Seems to have worked to advantage on a lakester before. I wonder how wide tires in front, skinny in back would work out in the wind tunnel, easy enough to try without fully committing.
Just a bike guy that will have to run OneWD :mrgreen:
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No more 4Wd roadsters. :cry:
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They killed the 4WD roadster because the powers to be did not want to see a 4WD roadster go over 300 before a normal fuel/gas roadster did. :evil:
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FWD might mean Front Wheel Drive. There is at least one of those running and doing well. But it's ugly I think
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Salty: I believe that there is at least one discussion of this topic in the archives. It was quite extensive. Some of those people are still on the forum.
Kip
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Salty
I agree with panic about land speed racing being different than sports car or other types of racing but most of high speed handling has come from road racing when it was discovered that high speed oversteer was caused by aero lift resulting in tires with little contack with track. Jim Hall ''Chaparral'' were one of the first to use spoilers and then wings to spoil lift and get downforce. 4 link and latter bars came from drag racing and high boost and hp came from formula 1. Also Hall used an adjustable spoiler before the wing came along which landracers can still do ''wings also''.
We need to look at pro racing they have a lot that we can use.
In landspeed racing to little weight on rear as in road racing-oval track -grand national-drag racing, results in wheel spin and oversteer.
Get ''Tune to Win'' by Carroll Smith which explains handling problems and what to do.
Good luck JL222
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John, if you were going to build a new car with a smaller, less hp engine, would you build it as light as possible and add weight later if needed, or build it heavy to start with?????
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John, if you were going to build a new car with a smaller, less hp engine, would you build it as light as possible and add weight later if needed, or build it heavy to start with?????
Hi Gary
I would go light as possible with engine set back as far as rules allow and as much weight on rear tires as possible with out getting to much behind rear axle centerline but some. Try to get down force from spoiler or wing and low pressure under car and add weight later if needed. Well you no me always preaching light springs and 4 link suspension.
Roadsters are at a disadvantage with out spoilers or wings.
What are you putting that 6 cyl in?
JL222
PS. I'm not saying light as possible, i like funny car cages in coupes and midplates for engines and strong frames.
Weight is not as much of a penalty in LSR as other types of racing but some [unless you need it for traction as a last resort]
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FWD might mean Front Wheel Drive. There is at least one of those running and doing well. But it's ugly I think
Yes, I meant front wheel drive.
Ugly? I think that sort of thing is an abonimnation of all that is holy! :-D
But if it's in the rules, and makes life easier/better, why not?
Lots of engine choices in 2.0 liters and under in FWD, and no so many in RWD. Many of the FWD platforms are well proven in drag racing and have many off the shelf parts available to boot.
BWDIK
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Think of an arrow...the weight is on the nose, with tail feathers to guide it straight. I believe that for a door car, 51 or 52 percent on the nose helps. Keep all liquids and ballast between the wheelbase ( avoiding the barbell effect), mounted low. Long cars have drag to aid in keeping them straight.
Just an old chassis guy with all the scars to prove it,
joe
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I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.
JL222
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Think of an arrow...the weight is on the nose, with tail feathers to guide it straight. I believe that for a door car, 51 or 52 percent on the nose helps. Keep all liquids and ballast between the wheelbase ( avoiding the barbell effect), mounted low. Long cars have drag to aid in keeping them straight.
Just an old chassis guy with all the scars to prove it,
joe
An arrow doesn't have wheels trying to drive it. Also if you have too much weight on the front you limit the amount of traction on the rear wheel. :-o
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Thanks for the suggestions all!
Panic- how do I calculate AC?
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I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.
JL222
The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.
I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!
Ryan
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Unless you have a wind tunnel wide enough to place the car sideways, it's not easy.
However, you can approximate it using a wide F stop elevation photo (side view, as little perspective as possible) of the car.
Use any photo program, and color in the entire silhouette into a solid (wing sideplates, roof fins, hood scoop) primary color with no outline.
Now convert it to a .bmp, and use a little program I found (e-mail me for a copy).
Using the program, open the .bmp, and select the color, and it maps the color field and locates the AC and CG (but this is useless since weight not related to area).
The location will be at a proportion to the length in pixels, so just crank in your total length in inches blah to get the real AC on the physical car.
Not exact because cannot compensate for radii, effect of open wheelwell, etc. but a good start, and you can save the file, and photo the car again with new details (bigger scoop?), and see how far the AC moved.
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Panic;
I'd like to have a copy of that .bmp calculator.
Thanks, Neil Tucson, AZ neil@dbelltech.com
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I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.
JL222
The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.
I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!
Ryan
The Lindsley-Leggitt Firebird is the fastest one way car ever. I think he went 300 in the first mile, and 323 in the middle mile and shut down. :-o
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I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.
JL222
The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.
I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!
Ryan
The Lindsley-Leggitt Firebird is the fastest one way car ever. I think he went 300 in the first mile, and 323 in the middle mile and shut down. :-o
Ooops that was 333mph. :-o
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Thanks for the suggestions all!
Panic- how do I calculate AC?
You can get pretty close by taking a picture like panic suggested. Print it as large as possible. Cut the car out. Lay the cutout on some thicker cardboard and trace the car on the cardboard. Cut that out. Now take some thing with a sharp edge on it like a knife. Put the sharp edge up and put the cardboard cutout on it like a seesaw. Move the cutout back and forth until it balances. Where it balances is close to the Center of Pressure since there will be the same area (weight of cardboard) on both sides of the balance point.
I have some thoughts on CG/CP here...........
http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-4.html
.......... but they are the same as what panic is saying.
c ya,
Sum
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Yup - and if you feel like living dangerously, you could "adjust" the cardboard by trimming off an extra (example only: .010" for every 1" of radius) of the outer edge surface, windshield, roof etc. to tilt the shape away from accepting the silhouette literally as if it were a billboard.
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I have to respectfully disagree with the centroid method of estimating the center of pressure.
The side on center of pressure (even if the centroid method worked which is highly debatable), only matters if the car is already spinning.
You want to know the center of pressure when the car is in its normal attitude and pointing within a few degrees of the local air flow. The bad news is that the center of pressure moves forward as the car goes faster so the center of pressure at 150 mph will be different than the center of pressure at 200 etc. That is why some cars have a "magic speed" which you can put money on them spinning if there is the slightest upset to the car. That is the speed where the center of pressure gets very close to the center of mass. Once the center of pressure is no longer behind the cars center of mass, there is no aerodynamic stability (tail feathers on an arrow) provided by the air flow, and the car is just as happy going backwards as it is frontwards.
Once the center of pressure gets ahead of the center of mass (if you can keep it straight that long) you have absolute aerodynamic instability where the aerodynamic forces are trying very hard to turn the car around so the center of pressure a behind the center of mass.
The only way I know of to get a good guess on the location of the aerodynamic center of pressure at low angles of attack (car pointing more or less in the direction it is supposed to be) is to use an accurate model and suspend it so it is free to swing about the support point and blow air over it or flow water over it. When you get the point of suspension at the center of pressure the model will become very unstable and try to spin in the flow.
Either that or buy wind tunnel time or find an aeronautical engineering student who can use some of the newer numerical aerodynamic modeling software to get you a center of pressure.
The centroid method only gives you good results when the car is way out of shape ie nearly sideways, and if and only if the side on drag of the front half of the car body is similar to the rear half. If you have a shape that is smooth and sleek from the side in the front so it has low drag like many tubular streamliners but has a flat fin like body in the rear your area centroid and side on center of pressure will be totally wrong, even for side on air flow.
See the last post in this thread --
http://www.landracing.com/forum/index.php/topic,2991.0.html
and this thread --
http://www.landracing.com/forum/index.php/topic,1594.0.html
Larry
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And I have to respectfully disagree with your comment.
Nothing said even suggests that any prediction of a tendency is precise, or includes all relevant factors, or should be maintained against empirical evidence.
Your comment, by making the case that since a prediction based on the CP in a static profile is not accurate at higher speeds it has no value and must be disregarded completely, suggests that no attempt at all at prediction (i.e., a random guess) is a better basis on which to begin construction and testing.
You're also offering a truism (CP ahead of CG is unstable) as if it favors only your method, and if not corrected precludes construction.
Inherently, all RWD cars are unstable under power - their stability relies on geometry external to the problem. A 400 mph car with CP < CG can still be driven - it just takes more skill, more precise controls, and fewer design errors. Many, many cost-no-object military aircraft are unstable immediately outside their design environment, that's why we spend millions to train pilots.
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Your comment, by making the case that since a prediction based on the CP in a static profile is not accurate at higher speeds it has no value and must be disregarded completely, suggests that no attempt at all at prediction (i.e., a random guess) is a better basis on which to begin construction and testing.
No not at all,I am also saying it could be in accurate at low speeds too. It is highly dependent on the body shape. On one car it might be close and on another car be very far off the mark.
I am saying bad data is worse than no data, especially if you do not realize that the data might be bad. You need to know what the limitations are of the testing process you are using. Yes the projected side on silhouette centroid will give you a scientific wild A** guess of where the center of pressure is when the car is only yawing 5 degrees to the air flow but it could just as easily be off by 10% -50% of the true location of the center of pressure at a 5 degree yaw. In short, unless your car fits a very small family of shapes that have very similar aerodynamic drag both front and rear and very little "fin area" in the rear, it might give you totally useless information, and even very bad information.
It is sort of like trying to do corner weighting on an unleveled floor with bathroom scales. It can be done but the value of the effort is doubtful.
Larry