Like racecomp says, front and rear suspensions are entirely different, putting the same spring rates will create two vastly different feelings at each end of the car, if I may quote them, the rough math below illustrates it pretty well:
Quote:
Originally Posted by Racecomp Engineering
Failing to account for the motion ratio
Sometimes you see spring rates for a set of coilovers and it's something like 7kg/mm front and 5kg/mm rear. Makes sense, since the BRZ/FRS is a little front heavy right so we can make the rear a little softer? Well actually there's another factor at play.
The BRZ/FRS has a multi-link rear suspension. The rear spring is mounted inwards on the control arm just a little bit. Unlike the front strut which is mounted at the hub, a 5 kg/mm spring rate in the rear does not mean a 5 kg/mm rate "at the wheel". Essentially the rear control arm is a moment arm....placing the spring inwards towards the pivot point means it needs to be stiffer to have the same effect as it would if it were mounted at the end of the arm or at the hub. Sometimes you'll see people mention the "motion ratio." For the front BRZ/FRS with the strut based suspennsion, it's close to 1. For the rear, it's around 0.75.
To get the "wheel" rate of the spring, or the spring rate at the wheel, you multiply the spring rate by the square of the motion ratio.
So that 7k/5k is really more like 7 kg/mm front and 2.8125 kg/mm rear.
Is that bad? Well, if it's not what you were planning on then, yeah, that's not good. It has it's advantages and disadvantages. Some of these can be tackled through swaybars and alignment settings but my philosophy is to get the spring rates right first as a foundation. Softer rear rates CAN work.
- Andrew
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Edit: Got motion ratios wrong so this section was totally wrong, but here's the link to the thread with lots of details on the suspension:
http://www.ft86club.com/forums/showthread.php?t=26661
imo swapping front and rear springs sounds like a good start and the adjustable valving should make accommodation easy, I've even heard non-adjustable dampers can easily take +/- 50 lb/in (0.9k) spring changes without adverse effects.