krazik

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jzr

Mike Schuster posted a similar issue recently after having lots of wheelspin problems after taking all that weight out of the rear.

The textbook diagnosis of this problem is an imbalance in the LLTD (Lateral Load Transfer Distribution) to the rear-

The body rolls about an axis defined by the front and rear suspension geometries. Besides raising/lowering each end of the car, there's not much you can do with this. For a given roll axis, CG height, track width, and cornering g value, the car is going to roll a certain amount, transfering a given amount of weight to the outside tires. The LLTD determines how this weight transfer is distributed between the front and rear axles. In this case, the rear tires are taking an amount sufficient to unload the rear tire beyond its tractive capabilities in the lower gears.

The goal then is to increase the LLTD to the front wheels, so they taking more of the cornering burden and keep some weight on the inside rear. There's lots of ways of doing this, the hard part is doing it in a way that doesn't mess up the way the car feels and handles overall. When running full race slicks on a production car, things are always going to be compromised. The centre of gravity is just too high for that level of grip.

Every change that would traditionally balance a car towards understeer you could try. What you do with your shocks is going to depend on where the problem exists - entry, mid-corner, or exit. The problem is fixing wheelspin on exit may kill the balance in other places. Ahhh, shock tuning.
A big adjustable front sway bar would be effective without killing ride quality; stiffer front springs would also work. And I'd run more front tire if you could - I've heard tires as large as 255 fit on 17x9's up front. Whatever sort of understeer balance the bigger front bar or springs add would be offset by the additional grip of the big front tires.

Of course, the next problem you're going to find is the car picking up both inside tires.

Another (not so cheap) possibility outside of suspenion work would be a clutch-based or locked differential. With these, the car will put the power down regardless of what the inside tire is going.

Disclaimer: I have no idea what I'm talking about from a real-world perspective, but I do have some cool books!

davepk

Jason, The problem only occurs mid-corner. Once i start to unwind the steering the wheel comes down. I dont know about you but doing anything to the diff scares the crap out of me. All I see are dollar signs where the diff is concerned.

cdelena

On the surface it would seem that rear wheel lift is caused by: 1) High COG promoting body roll, 2) amount of roll due to front spring (including bar) compression, 3) suspension geometry that limits the problem corner tracking (not much can be done to change easily).

If that is true the attack may be to lower the max amount without negative impact and stiffen the front springs.. I suspect a larger front bar is the first change I would attempt. Changing the damper setting can help by changing the rate of spring compression and rebound, but settings at an extreme to address this problem could certainly have an impact elsewhere on the course.

What is the aversion to a larger front bar?

jzr

In that case I wouldn't touch the shocks, at least not first. Theoretically in mid-corner the shocks aren't doing anything since the car is already settled. I think a big adjustable bar like the Comptech is a good choice, for even if you don't like it on max stiffness, you can play around with different settings to find a good compromise.

Or you could just leave it alone, and go back to running DOT tires. We're going to have a hard enough time keeping up with you at Laguna!

krazik

[QUOTE]Originally posted by jzr
[B]Or you could just leave it alone, and go back to running DOT tires.

GEARHEAD

I have been fighting the same issue in auto-x situations. It is a frustrating condition because once you are in it, you can't get any traction to transfer weight back on the rear. From your description and from my own experience, it seems to occur during a steady state cornering force or when slightly decelerating mid corner. Damping has no effect once the vehicle has reached a steady state (ie dampers have stopped moving up down), therefore, changing the valving will not cure this condition. I would suggest maintaining the same spring rate ratio front to rear while increasing the rates. This should maintain the balance that you currently enjoy. Putting the rear springs on the front and buying stiffer rears would be a good start. This will limit weight transfer and and limit the amount of compression on the outside suspension. Part of the reason that the inside tire lifts is a lot of compression in the outside rear combined with a fairly stiff bar. This picks up the inside rear tire. A larger front bar can help but may induce understeer especially at corner exit.
I am currently putting together some coilovers and I am starting with 425 lb./in. front springs and 600 lb./in. rear springs. I'll let you know how it works.

If it is occurring during light deceleration mid corner, adding some low speed compression to the fronts or low speed rebound to the rears will definitely help.

Good Luck.

davepk

Chris, My aversion to a front bar is two fold. First its monetary and secondly its probably some stupid idea I got stuck in my head that springs should be chosen over bar when addressing body roll. At this point the first is trumping the second wether the second is a correct assumption or not.

Jason, I have this problem even when running dot tires though obviously not to the extent i do with slicks. I'm quite sure it cost me at least 2 seconds at Sears Point.

GearHead, I don't think we are describing the same problem. Corner entry and traction are not my problems unless like Jason you think I have to much I'm also a little confused that you don't advocate stiffer front springs in an effort to change the spring ratio. Either way I am curious to find out how your changes work out.

pellisS2k

Sorry.. that phrase made me laugh. It's a little late to worry about that aspect.

s2k2fast4me

Dave,

If you want to try a "cheap" swaybar approach, you can borrow either my Comptech adjustable (the newer, stiffer version) or the 1.25" Gendron bar, whichever I'm not using at the moment. If you try it out for an event you can at least answer the question on whether that does the trick or not. If it does, somebody like Saner could make a bar pretty cheaply for you.

-Andy