Greg H.

I have been told by several people that rather than looking planted, my rear end "bobs" around turns when I am at or near the limit on the track. It's like the tires grab and release and grab and release. I just had it aligned and it did it before and after. It did it with MPSC and I was just told it's doing it with Nittos, which feel like they are sliding more progressively. I've taken tire temps and they are pretty even. I have no idea where to even start looking for a problem. I see plenty of 993s on track and they don't all do this. In fact I can't say I've noticed it on anyone else.

Greg H.

Martin S.

Have you had your alignment, specifically kinematic toe checked out? I would get in touch a local shop that can deal with 993 alignment...one of the challenges is that the factory kinematic toe gauge doesn't work well if the car is slammed low....remember this is hearsay! I have heard this on Rennlist...dont know much more....I am not a mechanic.

I had my kinematic toe done by Jae Lee at Mirage International in San Diego. Lots of San Diego 993 guys have had the alignment "treatment" at Jae's shop.

My 993, running on NT-01 is very stable....was also the case when I ran RA-1 tires. Good luck with the fix.

chris walrod

Have a look for shock oil that has leaked out of one or both of the rear dampers. Even if they are new, doesnt matter. Then have a good look at the rear bar and its mounts.

Greg H.

Chris -

No shock oil. PSS9 set full still, though. What am I looking for on the shock mounts? I have TRG bars which have some sort of very hard plastic bushing. They are not loose - they might be tight if anything.

Martin -

Who knows? I used Brandon Kraus who used Jim Morton's kinematic tool and who Jim used to use, so I assume he knows what's going on. Then I used John Holleran who does a lot of Porsche race cars around here.

I should add that I can't feel this. It's a visual thing people behind me are telling me about.

Greg H.

Barney1

I was going to say shocks also, but Chris beat me too it...What setting do you use on the PSS9's for the track?

Greg H.

1 (stiffest) on the rear and 3 in front. My rear bar is in the middle.

Greg H.

mongrelcat

Rear toe control arms, good?

Dr. No

I think someone (David in LA?) posted recently that they had a similar problem that was cured by new motor mounts.

trophy

I have experienced the same sensations when I had PSS9's and TRG's installed in my car. I have found that when set on full stiff, they are actually too stiff (shocks not springs) causing the feeling you have experiences. Literally the tires loose and gain contact with the road surface.

Two things I would check or try.
1: Soften your rear shocks and see if that improves.
2: You may want to lube your hard bushings, After speaking to Steve W he suggested I check these and lube at least 2 or three times a year if you regularily track the car.

Additional Questions:
What do you have your sway bars set at?
What tire pressures do you run on the MPSC and the Nitto's?
What are your alignment settings?

Regards,

User 122821

Check to see if the sway bar bushings are binding. I've had a couple of cars with a similar problems solved with bushing bracket shims.

anto1150

IMHO looks like you have too much grip on rear axel.

Try lower rear-tyres pressures.

ciao

anto1150

are you sure 1 is stiffest? I have PSS10 and 1 is softest...

Barney1

With the PSS9's the lower the number the stiffer the shock. Figure that out.

anto1150

didn't know...

Once you checked the engine-mounts, why not trying to softem the rear axel a bit (especially the sway-bar)? Normally that produces understeer, but on 911s things are a little different... it coult help the front-end to work better and stay planted, allowing the rear to slide a little.
This is my opinion... more experienced people could see it differently.

bart1

Barney was making a joke about Bilstein's logic there, Antonio - not getting smart toward you.

I would also soften the rear and see if it still did it. Does the car have understeer or oversteer on steady throttle? Can you find a big parking lot to drive in circles at the limit of grip and play with the shock/sway settings?