Coming from motorcycles we used to set rider and free sag to determine if uu had the correct spring rate as well as how much to preload your springs:

How do the pros determine the correct spring rate on a car? Do they install a set of springs and then throw it up on a lift, letting the suspension fully extend and take measurements, and then again when the car is on the ground?

 

Corner-weight the car.

Disassemble the suspension.

Weigh all the pieces.

Subtract to get sprung weight.

Calculate resonant frequency based on spring rate and sprung weight. Shoot for 2.2 Hz front and 2.5 Hz rear.

Test, test, test, swap springs, test, test, test, swap springs, test, test, test, swap springs, test, test, realize all the work you did calculating put you not really all that close to where you actually needed to be.

 

http://www.worksevo.com/Spring_Rates_1.pdf

 

Pretty much.

Then remember it will all change with tire choice/size, aero, driving style, what type of competition, etc.


- Andrew

 

Forgot to mention finding out the motion ratios (and of less importance, angle correction factor) that will give you your wheel rates.

 

There is some good info on springing a car in here:
http://www.koni-na.com/pdfcatalogs/K...ortCatalog.pdf

Natural frequency is important, but the natural frequencies you use on a street car may not be the same as what works well on a race car. For example, the natural frequencies on my Lancer are nothing like what was recommended above, but it works very well. In any case, you should not have similar natural frequencies front and rear, or you will have problems with resonance.

FWD, AWD, and RWD all have different needs for springing, and it also depends on what kind of tires, how the car will be driven, and what kind of tracking/racing will be done.

Unfortunately, there is no one formula that will get you there.