I found a used set of Bilstein coilovers with QA1 springs for sale locally. The set up is coming off a LS1 Miata. The seller told me the spring rates are 7kg front and 6kg rear and the Bilsteins were revalved to match the springs and the car. That seems to translate to (roughly) 392 and 336 lbs. The set up is barely used supposedly. My goal for the set up would be to use as a track suspension for my turbo Miata.
My question to the group is if those spring rates would work well on a Miata that I hope sees the track regularly. Here are some pics.
In reply to dyintorace:
You really have to include the sway bar rates in the formula. A really good Miata specific reference for figuring all this stuff out is here:
http://www.fatcatmotorsports.com/FRC_1_8NA/FCM_MSDS_1_8NA.htm
If this is a track only on sticky tires, I'd say both ends are too soft. These cars are pretty "rolly", big springs and bars in front are the usual solution. Another reference - Spec Miata rates are 700F/325R with 27mm front & 12mm rear bars if I recall correctly.
The fat car calculator leaves out a few things such as the length of the arms on the sway bars - that can have a massive effect on sway bar rate. But it's fun to play with, and much easier than actually testing 
That's a fair bit of rear spring bias. The Spec rates aren't ideal, but the current fashion is for higher rates than that. On a pure track car, you could certainly handle something higher. I'd be tempted to keep the rears ands get the fronts revalved to handle a 9kg spring.
Thanks Keith. I thought the front spring rate was probably a little low. Any idea how much it would cost me to get a different set of springs and to have the Bilsteins revalved?
Springs vary on whether you buy new or raid the local circle track guys. Figure up to $70 per spring.
Revalving - you'll have to ask someone who does it Keep in mind that the ability to revalve Bilsteins and the ability to valve them properly is not the same. I'd go by personal recommendation.
