Struggling with race car set-up? Get expert advice here!

[Ranger Mike]

This thread is to assist the racer or inquiring mind regarding the whys and hows of chassis set-up. Where possible, the applicable principle of Physics should be noted. Any inquiry should include a short, concise problem statement along with as much background information as practical. I have enlisted Kevin Gertgen of Performance Trends Software to assist in explaining the math used for various set up calculations ( caster , camber gain, Toe Out etc..). This thread is " free advice" and you know what they say about things that are free so any input should be taken at face value and I advise you to act accordingly.

Regarding a race car , there are too many variables in play for the average mind to juggle. The old axiom " a short pencil is a good substitute for a long memory" is now in effect. This is why we use Set Up Sheets. If you have not done so, I recommend you start using these and keep a running history of adjustments on your car and there resultant effect.
Nuff said..questions?
RM

 

[Ranger Mike]

nb89 asked the question in a different post about weight transfer and roll centers. After discussing, the bottom line question is ...How to determine the proper spring size (rate) to handle the sprung weight transferred when cornering. Its Saturday morning and our first race of the 2011 season is next week so I must clock out now and dig thru my set up notes..finish up on the Formula Car..will be back on line later.
nb89 - can you look up race car suspension class and read post #17 to 20 and post the wheel load rate?
also do you know Gs when cornering?
I think you prob have all the other info so will not take long to plug in formula...

 

[nb89]

Using the wheel load rate equation with variables A, B, C and D in the picture in slide #17 i get Kwl = 0.566Ks

As for the Gs when cornering i am still not sure about, but for now i could just use an estimate and change it for different speeds.

At the bottom of slide #19, is the 800lbs the weight transferred? I don't get what you have done after that where you did 0.54 x 0.54 x 800. Is this using the wheel load rate equation to find spring rate?

 

[Ranger Mike]

Wheel rate = (Length of A-arm divided into inside frame mount point to center of spring mounting point) squared , times spring rate is correct...

 

[Ranger Mike]

Wheel rate = (Length of A-arm divided into inside frame mount point to center of spring mounting point) squared , times spring rate is correct...
the 800 is a spring for this particular application that compresses 1 inch if 800 pounds is placed on it. Due to the linkage mechanism and location of the spring mounting point, 800 pounds is considerably reduced. so we must calculate the true degree of transferred weight that this set up will handle. we do this by figuring the motion rate.

Wheel Rate (WR) is the effective rate of the spring at the lower ball joint (BJ) located on the lower A-arm or control Arm which compresses the spring (modern independent suspension). WR is Spring Rate (SR) actual effective value after the mechanical advantage or leverage. This factor is Motion Ratio (MR) is the linkage squared.

 

[nb89]

I see, so what this basically means is that your spring rate can have a much lower value than the weight transfer load and still be able to support it due to the mechanical setup of the suspension?

 

[Ranger Mike]

see, so what this basically means is that your spring rate can have a much lower value than the weight transfer load and still be able to support it due to the mechanical setup of the suspension?

no exactly..the spring rate in this case, is 800 pounds,,,due to mechanical leverage , the rated value is dramatically less than 800 pounds. If the spring is properly selected, it will be able to accommodate 1/3 of the weight that is transferred during cornering. ( two coil springs and one sway bar, all having equal spring rates)

understand?

 

[nb89]

I'm confused :S
When you did '.54 x .54 x 800 = 233 lbs. spring required to handle weight transferred' at the bottom of slide #19 you are using a spring rate to find a spring rate? Does the 233lbs mean only 233lbs of the 800lbs spring is being used?

 

[Ranger Mike]

I should have been more precise ..
from my post on #19 race car suspensionOk, here is how we determine the proper springs for each corner of the race car. you asked about loads on each wheel..well here is an example of our old door slammer running on a medium banked asphalt track.
Stock suspension with solid rear axle.
We calculated that it is under 1.3 Gs in the turn. " F= ( m*v^2 ) / R " is correct formula
one more piece to ponder..

from our cone killing days in SCCA Autocross..skid pad testing ,,go to parking lot, airport,,what ever, set up circle 200 to 300 feet in diameter, drive around the cirle as fast as you can without spinning out..
G = 1.225 x R / T squared
R= Radius of the turn in feet
T = Time in seconds to complete a 360 degree turn

typical Corvette corners at .84gs
road race sedan like Tran Am 1.15 Gsif you know the tire performance curve from the manufacturer charts weight (vertical load in static pound) vs Traction (lateral load in lbs) you can calculate the Cornering efficiency.

This particular car weighs 2800 lbs. of 35% of weight will transfer under 1.3 G
and 75% will be on front end due to engine weight and corner loading

2800 lbs. X .35% = 980 lbs. transferring or loading tires

75% of 980 lbs. = 720 front end weight
divided by three to determine wheel rate ( two front springs and sway bar )
so we need wheel rate of 240

we have used an 800# SPRING RATE to find the wheel rate to handle the 720 lbs of front end weight..in this case it is 233 lbs.

 

[nb89]

Ah ok, so ideally you would want a spring rate slightly higher than 800 so that you get closer to 240lbs.

 

[Ranger Mike]

in a perfect world, yes..just too many variables involved to try to get the rate perfect..get it real close and take it to the track...we have not even discussed Rate of transfer , dampers or shock absorbers, stagger growth yet..