Race Suspension Set Up

[JamesTheDriver]

I just bought a 1993 318is that I plan on turning onto a track car. I've already stripped out the interior and now I need to start working on my suspension. Since I'm a beginner I'm pretty set in doing a ground control coilover kit, but I'm unsure of what spring rate to tell them. Whats too hard? Whats too soft? I've seen other E36's with around 700 lbs, is that the limit? Cause I was thinking 700lbs/in for the front and 800lbs/in for the year. Let me know what you guys think. Thanks!

[JamesTheDriver]

Sorry I just realized I forgot a lot of big details. The car is going to be set up for Auto-x and road course racing down the line.

[fcvapor05]

That is REALLLLLYY stiff. Softer is better, contrary to popular belief. The softer the car is, the more grip it will have, and the easier it will be to drive. If you call ground control and tell them what you want, they can help you pick spring rates. Just remember, softer is better. You want the car to be as soft as possible and still control the body roll. In a car as light and low as a race-prepped E30, this means you can go pretty soft.

[Hodge]

Well, my Tein Basic Coilovers have been really good for daily driving and auto-cross so far.

The rates are 448lbs/in all the way around. I want to control the body roll a little more with some thicker sway bars, but the car already stays very flat when cornering.

[fcvapor05]

450 ish is more like it. At 700/800 the car is going to lose grip over bumps in a big way, and will be severely underdamped.

[MIKEGTR]

I just bought a 1993 318is that I plan on turning onto a track car. I've already stripped out the interior and now I need to start working on my suspension. Since I'm a beginner I'm pretty set in doing a ground control coilover kit, but I'm unsure of what spring rate to tell them. Whats too hard? Whats too soft? I've seen other E36's with around 700 lbs, is that the limit? Cause I was thinking 700lbs/in for the front and 800lbs/in for the year. Let me know what you guys think. Thanks!

800 rear?????? wow... take it down a notch...

rule of thumb... use as little spring as possible... adding spring and taking out camber is not going to be equal to taking out spring and adding camber... this is real life

of course this is only a simplified way of saying that spring rate/camber should be set up depending on the amount of steering input necessary for a given track... further more this refers to the amount of weight transfer that u will get on both x and y planes of the car (left to right and front to back [or viceversa])

let's say for our discussion's sake, that the only thing u can change on ur car is spring rate... and that we are analyzing a 90 degree corner (be it left or right), let's assume car is perfectly 50/50 and no problems with the car in any way.
after all calculations and trials were performed, the optimum spring rate for ur car set up is determined to be 450/300 (it's just a number don't read too much into it).... this means that if u add spring three things will happen:

1) if u add front spring only (let's say to 700/300): you will not allow weight transfer from the back to the front under braking, u will also not allow the front axle to rotate left or right thus not using the complete amount of thread on the tire (based on a given camber angle) and thus u'd get a bad handling car which will tend to have understeer

2) if u add rear spring only (let's say 400/800): u are promoting weight transfer to the front during braking, thus making the back end very "light", the back end of the car will not rotate left to right as it should and once the spring has gone to the maximum deflection it will take, the rest of the moment about the rear axle will cause the tail to slide...

so unless u decrease speed considerably, u're bound to live with terminal oversteer

3) if u add to both springs (let's say the same amount of 200 [650/500]) u might say well this won't affect cause i've added the same amount to both axles... yet if u look at the spring ratio (ratio of front spring to rear spring) they have not been maintained constant. original ratio was 1.5:1, now u have a ratio of 1.3:1, therefore ur car is no longer neutral given the other constant settings of ur car...

Furthermore consider that if u were able to calculate increments in spring rate that will yield a constant front to rear ratio, this will only be coupled with other changes to the suspension such as a new camber value or a new dampening value... and you might not be able to reach the new settings because they are physically impossible (i.e. need 15 degrees negative camber to accomplish a desired setup)


RECOMMENDATION: start off with a good proven fixed rate and height suspension, and only allow urself one variable to play with... camber. it is easier (read: cheaper) to change camber than to buy a new spring with a different rate each week.

hope this helps... i had more on my mind but i've got limited time right now

sorry for the long post,

Mike

[MIKEGTR]

btw this was a simplified analysis which could be expanded to prove that some points are not necessarily true but it is a basic understanding...

however i didn't feel the need to dive to deep into this...

Mike

[CTC BM]

That is REALLLLLYY stiff. Softer is better, contrary to popular belief. The softer the car is, the more grip it will have, and the easier it will be to drive. If you call ground control and tell them what you want, they can help you pick spring rates. Just remember, softer is better. You want the car to be as soft as possible and still control the body roll. In a car as light and low as a race-prepped E30, this means you can go pretty soft.

I agree one purpose for sway bars is to make the springs act stiff then they are in the corners, that way you can run softer springs and still get the desired stiffness. Dont think that came out the way i wanted it to but ya know what i mean?

Just call up turner or some other place that builds race cars, pretend your gunna buy shit from them and ask a tech.