front stiffer/rear stiffer?

[schyawn]

I've been autocrossing my '89 fastback lately, and i'm pretty happy with the daily drivability of my setup and it's autocross performance. AGX/Prokit/Whiteline rear sway. Being that the S13 has a lot of understeer in stock form, It made sense that my prokit springs were stiffer in the rear. The rear is also stiffer in some of the other "softer" aftermarket spring setups.

I was curious as to why (it seems anyway), that as soon as you go with an even stiffer spring or coilover setup, that the front spring rates are often stiffer than the rear? Seems it would put more push back into the handling characteristics of the car.

Suspension geeks, have at it.

[TurboKA37]

thats because there is more weight on the front wheels. so it kind of equals out because of more weight being on a stiffer spring and less weight on a less stiff spring. i am not 100% sure on this but this is what made since to me.

[schyawn]

That may make sense, but why (my actual question) does my prokit (and other softer spring sets) have the rear stiffer?

[Exar-Kun]

because the front compresses more, and since its a progressive spring it almost evens out

with eibachs, both springs have different "compression" maps, so to speak...as they compress they get stiffer(achieving different rates between the two) at varying rates.

also, to reduce understeer, you increase the roll resistance(stiffness) the rear of a car. this tapers off in race suspension, as seen(like the 9/7 split) in coilover generally.

does that answer your question?-chet

[Nismo_Freak]

Another difference is that the front suspension requires a higher spring rate to reduce braking dive, and a stiffer spring rate to impeed the compression of the suspension so that excessive camber is not developed from the lowered roll center.

[Q45tech]

A perfectly neutral car would have the spring ratios equal to the weight distribution ratios............same with sway bar ratios.

Unfortunately over hang mass out board [infront of/behind] springs makes for weird polar moments of interia which can only be fixed by varing spring rates.........create understeer for safety.

The other problem is road pitch/oscillation.......you must consider the wheelbase and the necessity of trying to avoid having the front rear oscillation over highway seams/bumps become additive with the delay time. This can only be tuned at ONE highway speed.......stiffer springs raise the tuned mph.

Stock might be tuned at 55-65 mph while 1" lowering 20% stiffer tuned at 66-78 mph.

Hundreds of other things to consider when going higher than 20% above oem stiffness.

NASCAR tunes at 200 mph [actually they change springs to fit the expected track speed. Otherwise driver exhaustion would limit the race.

Real cars have from and rear suspension with camber gain curves that compensate for spring compression changes............brake dive is a function of wheelbase so on short cars it is a consideration.

[Nismo_Freak]

Real cars have from and rear suspension with camber gain curves that compensate for spring compression changes............brake dive is a function of wheelbase so on short cars it is a consideration.

Ah, but we are talking about lowering springs, which as we've discussed before initiates roll couple issues. Moreso in the front per the suspension design.

[schyawn]

good responces

From what i've gathered over the last year or so reading about suspension, is that there are a couple of conflicting issues with spring stiffness/oversteer/understeer.

Should you increase stiffness at the rear of the car, this will increase the oversteer characteristic. BUT.....should the roll stiffness be increased, reducing body roll, the camber of the tires will be more optimum for grip, and actually reduce oversteer. Generally speaking. (ie: camber curve aside)

I could then (with hesitation ) say that the modest camber curve of our front mcpherson struts as compared to the multilink rear, would gain grip moreso from a reduction in bodyroll. So, perhaps at the coilover 9/7 level, the higher stiffness at the front may actually improve front-end grip?

It seems that there is much interplay of these issues with springs/swaybars.

[Q45tech]

Grip [mu] is a function [besides camber] of actual weight vs. tire reserve weight capacity and spring essentially change nothing as to transfer weight. Obviously if you lower the CG the transfer weight will decline a tiny tiny bit.

Till you get way out of whack [with springs/bars] don't think of dry oversteer but of less dry understeer.............its the TOTAL stiffness ratio that must be maintained 45%R/55%F whether you chose oem soft or kidney damaging stiff.

Springs/bars at best can change things 2-3-4% whereas tires can affect things by 10%+.