I just dug out an '88 front spring and took some measurements. I thought I'd use those to calculate spring rate and see how that number jived with the other numbers I posted.
Here's the formula, from page 142 of How to Make Your Car Handle, by Fred Puhn:
code:
(W^4)*G
K = ----------
8*N*(D^3)
K is the spring rate in lbs/in
W is the wire diameter in inches
G is a constant, it's 12,000,000 for steel (from the same page of the same book)
N is the number of active coils
D is the diameter of the spring measured at the center of the wire
The measured OD of the spring is 4.384" and the wire diameter is 0.488" so D = 3.896.
Here's where things get interesting. When the spring is free, there are 8.25 active coils. Punching this number into the above formula gets a spring rate of 174 lb/in, very close to what I measured.
When looking at my spring I can see wear marks on the spring where the top and bottom partial coils hit the next coil. If you assume that these coils are touching when the spring is installed that gives us 6.25 active coils. Punching this into the formula gets 230 lb/in which is very close to what T-top Fiero quoted from the MVMA book in the other thread I posted above.
Interesting.
Well, I just happen to have a Formula up in the air right now so I crawled under it and looked. Sure enough, the two end coils of the spring are unused when installed in the car.
I am now changing my official answer to this:
The ARD front spring has a rate of approximately 230 lb/in when installed on the car.
The rear will be more complicated since the spring is somewhat cone shaped. I'll figure out a formula for that and post those results here eventually.
I hope this helps somebody.
Doug
[edit - I bolded my official answer]
[This message has been edited by Doug Chase (edited 12-16-2003).]