...hope it's ok for me to dig up an old thread...
I have a theory that the minimum possible spring rates represent the static weight distribution of the car. The NSX-R is 6.5/5.0 so I reckon the physics engine models the car as 57% / 43% F/R weight balance (slightly FR).
Even accounting for this, the stock spring rates are still unusually biased towards a softer rear compared with any other car I can remember. So is this to do with the motion ratio? No idea, sorry! If you were keen, you could try finding another car with similar mass, weight distribution etc and compare how they react on landing jumps when they are using the same spring rates. Or (just brainstorming...maybe this isn't possible), if you could somehow
- measure stopping rate using the data logger
- tweak the brake controller until you've got the same deceleration rate on 2 cars
- measure the dive during braking (perhaps using photoshop on a screenshot taken from an external camera angle)
then this could tell you if there's a multiplier involved in the spring rates
Personally, I've never found anything to suggest motion ratios exist in GT4 (although now that I think about it, maybe they could be influencing things and you'd have no way of knowing anyway). And like you said, tuning should be based on driving feel and not numbers.
Back to my tangent about the NSX, anyway I was thinking maybe Polyphony softened the rear to tame the car a bit so it wasn't too scary with the stock settings. But then playing with the spring rates, the effect on balance was very minimal. (I didn't play around with the brakes but it felt like the handling was being dominated by overly strong front brakes, which induced an understeer stance that remained right throughout the corner) But then maybe it was done so that when kiddies throw a Stage 4 turbo at the car with stock FC suspension settings, then having a soft rear meant the car wouldn't be totally undriveable. This is a lot of second guessing, though...
Sorry this doesn't answer your question, hope it is at least thought provoking!