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I just ran across this post, and figured I'd toss in my 2cents.
I've been fooling with circle track cars for quite a few years, and have done a couple that started out as chalk lines on the floor, and I've done a good bit of studying on springs, both book learnin' and trial and error.
One thing I've learned about coil overs, as opposed to stock springs in stock location, is that where and how you mount the shocks/springs has a ton of effect on what rate you actually see at the wheel. The rate of spring that will work is directly contingent on how it's mounted- if it's half way in on the control arm, then you're seeing half the rate, but twice the travel, at the wheel. The math is pretty simple- It's the old force times distance from your fulcrum deal-- how much the coilover is angled where it's mounted also affects the rate, ideally you want it directly perpendicular (I said perpendicular, huh,huh,huh) to the load, which never happens, because the control arm it's mounted to is traveling on a radial path, so the best we can do is figure in a little extra spring rate to compensate. That's one of the reasons torsion bars are really cool- the loading of the spring is consistent all the way through the travel of the arm it's mounted on- they still use them a ton on sprint cars, except for a breif period when coil overs were being experimented with, something about response rate, but now they're all back to torsion bars. Anyhow, when you're talking about springs, keep in mind that differences in how your springs are mounted will cause a lot of variation from vehicle to vehicle. With a camero lower control arm, and a stock spring in stock location, I've run as much as 1400 lbs./in on the right front of a 3000 lb. car. A dirt late model with coil overs- about the same weight, totally different mounting situation- might run a 400lb. right front spring. Weight distribution also affects spring rate a bunch, although I'd guess that since we all have LUV pickups, that should remain pretty consistent between trucks of the same engine configuration.
Anyhow, didn't mean to write a book- It's just that there's a ton of stuff involved in figuring out what springs are the right ones. As a general rule, I like to start on the soft side, and bump up the rate in small increments until I get what I need- a car that doesn't head for the wall when I try to turn left.
AFCORACING.com has some really good stuff about springs on their site, if anybody wants to get into it a little bit more... or quite possibly I've gotten into it too much already...
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