Bob Rouleau

Guys, I need some advice. I am trying to deal with an evil handling 997 GT3 RS. Sorry is this is a bit long but here’s the story.

As delivered on OEM 19 inch so called Cup tires the car understeered badly. I helped this by realigning as follows:
Front zero toe, 2.5 degrees negative camber (rotated struts)
Rear 16 mins toe in per side and neg 2 camber.
Sways full soft in front and full stiff in back

The car was pleasant to drive this way with fairly neutral handling. I had less grip than guys running real cups on 996GTs (including Andrei in my previous GT3) but it was still rewarding to drive.

My 18 inch Fikse wheels arrived (same size and offset as the OEM 19s) and I fitted Hoosier R6 tires. The car oversteered a bit and I dialed it out my setting the sway bars to their middle positions.

After 7 track days the Hoosiers got hard (about 38 heat cycles) and I switched to Michelin Cups in 235/315. The car was evil – lots of oversteer. I tried dialing it out with the sway bars to no positive effect.

I went back to the OEM tires and the car handled well except for a lack of grip compared to the Hoosiers.

I figured the alignment was at fault. Racerron set up the car as per my favorite 996GT3 specs:

Front 3 mins toe in per side and neg 2.5 camber
Rear 18 mins toe in per side and 2.4 deg negative (compensate for the wider wheels in back)

He shimmed the front A arms to increase the caster. Sway bars in the mid position front and rear.

I figured the car would be a blast to drive and have the usual grip I expect from Cup tires. Well, the front end turns in like it was laser guided but he rears slide out with no provocation. I had to exit corners more slowly than on street tires! Going full soft on the rear bar did next to nothing to reduce the ugly oversteer.

So – on two different sets of 19 inch street tires the car handles well. On Hooiser R 6 with the first alignment the car handled well in spite of not having enough neg camber in back if Hoosier is to be believed. With an alignment better suited to cup tires the car was evil on the 18 inch cups. With the same alignment the (2.5/2.4 neg etc.) the car handled well on track with the OEM tires.

Am I missing something? Could the 18 inch cup tires be defective in the rear? I would welcome any advice or suggestions!

Best,

Professor Helmüt Tester

Whenever I have a schizophrenic race car on track, I start thinking "shocks". The increase in grip of the stickier tires may have you in different suspension travel ranges. Dead shock ? Locked shock ? Either that, or I look for something broken/loose in the steering or suspension locating points.

Greg Fishman

Stick with one tire and work from there. Cups and Hoosiers are as different as oil and water and like much different set ups. Work on the end that is causing the problem first, don't change things in the front if the rear is not working right.

If the rear is doing what you are saying I wonder if it is changing its toe setting as it comes under load, like maybe the toe link is moving?

And 38 heat cycles on the Hoosiers is a ton!

Veloce Raptor

Rob, contact John Gladwill at Boardwalk Porsche in Dallas. He is EXTREMELY good at setting up RS's for proper track use.

chris walrod

By swapping the struts to achieve more camber, what is it doing to caster or maybe more importantly the trail?

doc2s

bob, did you measure the ride height recently? what is it at?

Geoffrey

The RS responds well to lowering 10mm which helps the rear bump steer which is what you are experiencing. The rear toe is changing and causing oversteer.

VB997

Bob, are you running Tarrett (or equivalent) adjustable toe links?

Premier Motorsp

Bob,

Have you taken temperature readings to make sure you are getting the rear Michelins hot enough? That 315 is wide and it may take a while to heat up. Also the 315 ought to have at least a 12 inch wheel. I think 997 RS comes with 12 inch rear?

Geoffrey's suggestion about bump steer (really roll steer) may also be worth looking into. The MPSCs have much more cornering stiffness (slip angle vs. cornering force) than other tires. This means small toe changes have large effects.

Do you have rubber bushings in the rear camber arms and toe links? I think those could give enough toe change to make the car unpredictable.

I don't see a problem with the alignment specs.

Would you agree with the statement that the N-spec 19 inch MPSC is nothing like the regular MPSC?

Chris Cervelli
Spline Technologies

Bob Rouleau

Guys thanks for the input. A few comments:

1) Am suspicious of a busted shock althoough there is no evidence of same using street tires and or Hoosiers which had a lot of stick.

2) The 18 inch wheels are the same width as the OEM, i.e. 8.5 wide in front and 12 wide in the back - i.e. fine for a 315 tire.

3) I will look for a cracked bushing - no evidence of the sway bars moving or loose - no creaks or clunks.

4) Chris - I agree the 19 inch N spec 'Cups" are really a very high performance street tire and no where near the real Cup in performance. .

5) I have the OEM toe links. Racer's group version on the way. I am suspicious since on the 18 inch cup it feels like I have toe out in back in spite of having 18 mins toe in a side (static).

6) Temps on the cups were 171 174 177 going outside to inside. Looked decent to me.

Lastly I wonder why the very sticky R6 Hoosier handles well and the 18 inch cup oversteers like mad?

Best,

Larry Herman

Just curious Bob, is there a difference between the 996 & 997 rear suspensions?

doc2s

with the r6 the car is 10mm lower all around (vs. stock cups) and you maintain the stock rake. with the 18" cups the car is 10mm lower in the back and only 1mm lower in the front. you will need to adjust the ride height with the cup tires on.

Bob Rouleau

Larry - the suspension looks the same to me although the RS has some lighter bits than the 6GT3. I am not aware of major changes, although it has been said the pick up points are different but I can't eyeball those. On the Hoosier or OEM 19 inch Cups the car handles like a decently sorted 6GT3.

Doc2s - thanks, that may be a clue. There is some evidence that PASM cars are very sensitive to ride height. The OEM 19 inch tires are 0.7 inches greater in diameter in back than in front as I recall - giving the car more rake forward. You may have put your finger on the problem!

Best,

doc2s

good luck and please keep us posted.

Geoffrey

Do yourself a favor and have someone put a bump steer gauge on the rear. You'll find that it toes out at a significant rate.