How do you know what the perfect spring rate is for a car? How do you know if the car is under or over springed?

trial and error. different tracks and surfaces dictate more less rate.
through trail and error findings by other F-body racers here we seem to have it pinned down. but then again, is there downforce involved or not. so there is no absolute w/ the number of variables.

street car? 550-600F/175-200R
track car? what class? CMC? 650-900F/200-300R AI? who knows.
auto-cross car? what class?
combo of any above?
how much "perfect" can your ass tack on the street? my tolarance my be higher than yours.
give us more info.

Like Glenn said, it really depends on application. The range is 500-1200 F and 125-500 R. The rear rate is also dependent on rear roll center height (PHR brackets or Watts link will effect this).

Can you corner weight a vehicle and match the spring rate to the corner-in a 1:1 ratio? Would this provide a starting point? Or is this all just random guessing as to what is ideal?

Lets say the front corner weight of a car is 500 pounds. A 500# spring would compress 1 inch under the weight of the vehicle. As that weight is shifted additional compression would occur. Based on suspension travel couldn't you calculate an ideal spring rate per application?

idea grip?
if so, then we would assume a road race car. we should use a CMC car as it limits you to OEM geometry and parts (for the most part) which drastically limits the variables in set-up.
the weight of the car will determine the rate needed to control the car for a given corner or bump. 3200lbs would be a good number.
since most all tracks are different, this number (ideal number) will be different solely based on the surface type. more grip will allow a stiffer spring. imagine driving on a wet track. you want 1200lb springs or 500lb springs? i'll take the 500's in the wet. this was just an example of a needed changed based solely on surface grip.
so like Jason said above, dry track, CMC car, 255/50/16 RA1...... your looking at a range of 650-1200 for the front.
driver ability is another factor, but we will avoid that. but if your ability does not allow you to be fast w/ 1200's in the car, then 700's may be YOUR perfect rate.

my ideal rate is based on my driving style, my camber, my caster, my air pressure settings, my compromise to braking grip, so on and so on. i know for a fact that i run more camber than most CMC 4th gens and its mostly due to running less spring rate. i could up the rate, drop the camber and gain straight line braking grip, but at a cost of having a car that is only fast "on line". when is the last time you passed a car "on line"? also, what about rain? what if it starts raining during a race? the rate I run is a slight compromise based on the possibility of rain. don’t put much weight on this factor, but it is a factor in picking a rate.

if you find a way to figure this out, i would love to know. for now, you have to put XXX rate on the front, make adjustments to balance the car, then make another front change up or down, make adjustments to re-balance the car and compare lap times. then make a change the opposite direction from your original front rate, re-balance the car and re-test and compare lap times.
adjustments to re-balance the car should include: camber, caster, air pressure, toe.
an example of this is my last event. I added 2.5 to 3 degrees of caster w/ no other changes. my car was super loose. why? more front grip added to a car already slightly loose. I had to drop the rates in the rear 25lbs. I went from 1-2 seconds off the leaders, to dropping the track record by .6 seconds on Sunday.

if it was me, I would use a proven set-up from someone you trust and respect and adjust from there to suit your ability and style.

Be prepared to change the rates. Here's just a very small example.

Take 3 cars we have on course. A softly-sprung LT1 Camaro, a stifly sprung BMW and my car (in between). In some twisty-but-smooth turns, the BMW is fastest, me second, with the LT1 behind.

However, when we are about to come onto the back straight, there are some bumps right at the beginning coming out of the turn. The BMW jumps over them, delaying his ability to get on the gas. The LT1 is able to lay into it and run away from people. I'm in between.

So, it depends where you want to be fast, also. Ideally, it's easier to pass people on straights. But, if you go too slow in the turns, you're gonna get passed (and probably spin out because the car isn't handling well). Plus, we're road racing - we like turning!

As Matt stated, how bumpy your tracks are and how grippy the surface is has a lot to do with how stiff you go. As does driver ability and taste as Glenn mentioned.

Jason S.

Spring rate really is a moving target. The best you can hope for is to get it close and then use tire pressures and wheel spacers to adjust the effective rate in a range.

I've gone from 900 to 800 to 700 to 650 and back to 750 over the last few seasons.

That's why it's a great idea to keep meticulous notes and record as much data as you can.

Also, tuning for autox and tuning for road racing is apples and oranges. Completely different approaches.

Or, just leave the 700s in there, and wonder why you can't keep up with people on one day but run away on the next. That's my philosophy when I go to the track - change nothing, drink beer, get out of the heat.

hey! I like your style!

Here's an example. My TA has 1LE front springs and stock rear springs with Strano revalves on the front and 3rd gen Koni's. On street tires it was turning times 2.5 seconds slower than a C4 Vette with electronically controlled Bilsteins on R compounds. His best run was 40.something and my car turned a 42.5. My car is less than an ideal setup since it has hardly any negative camber and a stock ride height(we were running 44 PSI cold in the front).

I'm reading of guys who AX Miatas who are claiming that stock springed cars are turning better times than cars with $1500 coilovers.