Susp Setup Question for the Gurus
 

Haven't posted here in awhile since I sold my last S2k... but we just picked up a GPW '06 for my wife so I'm back. The question I have is not for the S2k - it's for my M3 race car. I have Moton double adjustable shocks, adj spring perches, and adj sway bars. Need to dial in suspension a little more and I was hoping some of the resident racers may help.

Here is the problem - the car has hard time putting power down on the exit of slower corners and seems to hop somewhat. It has been the case all the time, but last weekend at Road America I was leading by a mile and had a chance to really analyze the car's handling. It seems that the rear suspension gets loaded up in the corner and the shocks are not stiff enough to control the spring. So it seems I should increase the shocks damping, the question is should I increase compression or rebound?

Too stiff compression and I'll have hard time putting down the power - which is what my problem is now. The body lean is not excessive by any means, but something is causing the car to bounce on the loaded rear corner and loose traction. Should I bump up the rebound damping?

Anyway, I'll play with the settings at the next race but I just wanted to gather some info ahead of time on susp tuning so I don't start completely in the dark.

TIA,
Drew

Sounds to me like the root cause of your problem lies else where and adjustment of the shocks would only serve to mask the problem.

More likely you have a rear diff that is locking up more than it should or
maybe a rear sway bar that is too stiff and causing inside rear wheel lift.

A better description of "hop somewhat" would help at this point.

I see... it's probably not the diff only because the car is doing it with both of my diffs and one just got rebuilt.

so a stiff rar sway bar would cause inside rear wheel lift? that could be it - at mid ohio I was basically driving on 2 wheels around certain corners.

i'll try to describe the "hop" better. say it's a right hand turn, then what happens once I start feeding in the power at apex or a little later the loaded left rear corner feels like it's bouncing - going up and down and the car looses traction. I can hear the revs go up and down as well. Bumpy corners are the worst.

tight corners are not hairpins, just not fast sweepers. for example, at road america it's noticeable at canada corner and T14. At Mid O it was bad in many places (keyhole, carousel, and whatever the turn numbers are after the back straight), but part of the problem there was some shock problems. I had them rebuilt since so they should be operating properly.

drew

p.s. one diff is 40% lock that's the one I used at RA, the other I'm not sure but could be something higher like maybe 60% or so.

Thats sounds quite a bit different that what i have ever experienced in a Torsen diff S2K.

In a Torsen diff S2K when the rear inside wheel lifts the diff revert to open diff operation. The inside wheel then takes most of the power and spins up in speed.

The sensation for the driver is that the car looses power but the RPMs rise. As the car slows a little bit or the driver unwinds the steering wheel to compensate the lifted wheel will regain traction and the car will again start to accelerate.

This process can be repeated many times in a single turn in a S2K.

However, for the driver the sensation is not one of rear outside wheel hopping, its more like that of a slipping clutch that comes and goes quickly.

At any rate, regardless of whats causing the problem...

If you can keep the inside rear wheel on the ground that should prevent drastic changes in applied torque between the two rear wheel.

In the S2K we remove the rear swaybar and that allows the inside rear to droop enough to always stay in contact with the ground. Of coarse you need a coilover setup with enough droop to accomplish this.