I've done some research on the forums on this subject and it sounds like the PSM doesn't like the diameter difference compared to my fronts. Here's what I have. 2002 C4S with Pirelli PZero Rosso 295/30/19 in the rear and 235/35/19 in the front. bought the car used with this setup on it. The fronts have easily 50% of the tread depth left. I had to replace the rears as they were smooth on the inner 40% of the tires. My guess is I'm outside the 3% by about .5% or maybe a bit more.

My question is has anyone else had this happen recently and is the only solution shaving the rears down a bit to get within spec and or replacing fronts as well. I know this can't be good for the center diff either.

Thanks for any advice on this.

 

Change you front tires to 225's ASAP!
Or run 235's upfront and 305's in back.
You can dive with that setup without any problems.

But since the previous owner drove it with 235's/295's chances are it's to late...
If so I'm sorry to say your viscous coupling is probably toast!!

I'd have it checked at P. dealer.

Good luck.

Sid.

 

That's what I figured. The car has been into a few very good shops in the area according to records so I'm surprised this combo made it on to the car.

I'll probably get fronts on it this week and hope that the 10K miles on this combo has not done too much damage.

 

Front size is Correct for 19" Wheels... Rear is way off!!! should be 305/25 or 315/25... Rears should be Swapped not the Fronts to Correct the Issue...

 

Hopefully the rear's are at least an 11" or a 305 or 315's going to be stretch

 

The closest Pirelli makes is a 305/25, but it's not N rated. Not sure if that really matters at this point. The 305 only gets me about 2/10th of an inch closer in OD. If I go to a 225/40 the diameter is exact to the 295/30, but these are both about 1 inch bigger that the OE sizes.

I hate to put 225's on as the car pushes a bit too much as it is, but it will probably be the easiest solution. Other than that, I bite the bullet and get the PS2s in a 315 / 235 setup and see if the shop will take back the rear tires considering I would be getting a full set put on. The tires only have about 50 miles on them. The rears are 11.5" wide. Quite the puzzle! LOL.

 

235/35 & 305/25 is the Best Combo, but only Pirelli has the 305/25 Size... Many Brands carry 315/25

 

What size rims are up front, 8' or 8.5?
If you've got 8.5's a 225 will work but look a little funny (tire will look a bit too small) but be alot less expensive to change than doing all 4 tires.

Re; the overall 1" diameter increase, I wouldn't worry I haven't had any problems and know lots of guys who are running the 19" 997 turbo rims on a 996 (235's/305's but diameter is the same 1" extra as what you've got).

Because of the crappy roads up here I have 10 pounds less tire pressure all around, this also drops the car by half an inch, ad some Swedish bikini models and your down the other half inch.

Cheers,

Sid.

 

Front wheels are 8.5" The Pirellis also have a round side wall so the stretch might really look strange. I'd rather try to get the 305's, but I'm worried that if I do it not be small enough and I still have the same issue. But... if my math is right the 305 is .3" smaller (according to Pirelli) than the 295 and if there is 10/32 of tread depth (.325") on a new tire, a worn out tire should be .3" smaller. The worn out 295 had no issues with the PSM. So the 305 "should" be the fix.

I'll have to see what the shop says tomorrow. Thanks for the suggestions. Especially the bikini models. I like that one the best!

 

I had a set of 19's with 235/35 and 315/25 PS2's, and mine freaked out badly when pushed at all.

I think it was the additional weight of the wheels that contributed most.

I put my Champion RG5 18's on and it is rock solid. PSM almost never engages.

Does it feel like one of the front brakes locks up for a second? Like right in the middle of a sweeper? It sucked!

 

That's what is exactly happening in a sweeper. The car becomes "self aware" and engages the front brakes and sends me towards the outside barrier. And this is at a pretty low speed.. maybe 40mph. The wheels and tires were on the car when I got it and everything has been fine. The new rears has started this PSM problem which points to the difference in diameter.

I measured all the circumference of tires on the car and the math works out to a 2.5% difference in roll-out and diameter, which from what I understand is within the Porsche spec. I'm pretty puzzled.

 

Try having it do it when you are really hot in a corner say around 80 mph.

I had to drive with the PSM off to keep from dying.

After trying 4 sets of wheels, I decided my 996 C4S likes 18" wheels, and the lighter the better.

 

The 19's look pretty cool, but I'm with you when it comes to light wheels. These are made by Forgeline and I'm sure their pretty light for 19's but I'm sure the package is a few pounds a wheel heavier over all.

 

mmmm wierd, i have had the same Set up on my 19" RG19's for the last 4 Summers... Mine has never Acted Wierd

 

Prob cause....your 19 RG's are very light.

My 19" RH Turbo P's were 10lbs heavier rear and 7.5 lbs heavier front.

It made a huge difference. I don't know why, but it did.

I also think that the 315's had so much grip, it threw the balance off.