[wannafbody]

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?

[GlennCMC70]

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.