BC RACING/KIDO/FORTUNE/ETC. How do these companies come up with their spring rates?

[MJ DOOM]

Have a 2000 750il.

Weight distribution is 50/50. Stock spring rates are 151 lbs/in in the front. 166 lbs/in the rear.

But every coilover application I see for the e38s have higher spring rates in the front.

BC Racing - 9k front 6k rear
Fortune - 10k front 8k rear
KW Comfort 343 lbs/in front 314 lbs/im

Even lowering springs have higher rates in the front.

Why is this? Does it help oversteer/understeer? Is it to make the ride more comfortable?

Any ideas guys?

Thanks,
Mike

[Oh Damn, it's Sam]

There's a couple things going on here.

1. Since the front wheels have to steer, you often want a stiffer spring rate to keep things from running into each other if you hit a bump while turning.

2. You may have 50/50 weight balance when the car is sitting still, but when braking, the nose of the car dives as the weight transfers sharply forward. Stiffer front springs help combat this tendency.

3. Common wisdom suggests that stiffer springs in the front vs. the rear will help balance handling slightly toward more neutrality (or oversteer if you go too far in that direction). I do not know enough about suspension dynamics to explain why, so take this with a grain of salt.

4. I know very little about your 7-series's suspension configuration, but sometimes the motion ratio of the suspension factors into this as well. Depends on the car and how the suspension is designed, of course.

5. Strangely enough, the stiffness of the rear suspension is what you feel the most as compliance or harshness on a vehicle. You can get away with very stiff front suspension if the rear feels somewhat compliant.