wanna911

Strange occurrence this weekend at CMP. Ran Sat on some old A6's. Track conditions were typical CMP (marbles everywhere, little grip) Probably 13-14 Heat Cycles on the tires. Sunday morning, decided to strap on a set of sticker tires to go a bit faster as CMP was kicking the old tires butts. However no other issues. Put on a different set of wheels with the same size tires only A6 in rear and A7 in front. Went out for slow 45 mph out lap (Time Trials). Did some scrubbing to get the release mold off for most of the lap. Towards the last 1/3 rd of the lap started warming brakes with accel/decel at which point even stops from 40 mph were causing a burnt rubber smell and faint smoke in the cabin, but no sound of rubbing. I thought it may have been lingering from the previous session or some slight tire rub as some have reported wider shoulders on the A7.

With the session starting and 30 cars behind me I decided to just run a 8/10ths lap to see what was up. Started picking up speed and into turn 14 I hit the brakes pretty hard at which point there was significant smoke but the car slowed reasonably well. Held back a bit on the front straight and started braking a little early when........

https://www.facebook.com/rosszor/vid...1711147907324/






Destroyed the fronts but the rears were unharmed, my first thought was ABS, so I bolted on my two old A6's on the front and went out to the main road. Did a couple of stops from 50 and no problem. ABS worked fine on the slippery road surface. Then got a pass to go out with DE2. Only took one lap to realize there was no longer a problem. Then did a TT session with new rears and old fronts. Ran my fastest, albeit horribly under steering lap of the weekend.

I realize the common sense answer is ABS, but I've tracked this car for 8 years now and I know when I have ABS issues and what they feel like, this was not one of those times. I always get an ABS light if my ABS is having issue, usually connected with a boost leak. The tires locked every single time I touched the brake with the A7's on front and not a single time with the A6's on front. That doesn't suggest a off again on again ABS issue. It seems like one tire had almost no grip compared to the other. In the video from the start stand you can see from the skid marks the car immediately locks the front right, it's only further down you see skid from the left when there is no where to transfer grip to.



Any suggestions?

wanna911

BTW I've locked up and burnt tires due to ABS issue and none of them smoked nearly like this, even going from 160+ MPH into 10a At Road Atlanta.

coryf

We had a customer in a 996 gt3 have a similar situation with abs failure at Sebring this year. The front tires locked up and unfortunately resulted in a crash. No indication there were any faults in the system before or after. He was on R7 front and R6 rear if I remember correctly. It was impossible to do any more diagnosis after since the car was destroyed.

Have you tried switching back to the front r7 just to test if its consistently a problem? (I know they are flat spotted but just to test at low speed around the shop). Normally with an abs fault the light will come on. If its a tire diameter difference issue the light may stay off but the abs could be out of its programmed range and cause the system to malfunction. A Bosch motorsport ABS engineer we talked to said that without knowing how the system control software is written it isn't possible to know how a unit would react. That is a problem with street abs.

Veloce Raptor

This is exactly what I thought when I read this thread...

wanna911

They are REALLY flat spotted, as in undriveablen. I have more sets so I will give them a try either in 2 weeks at Road Atlanta or (gulp) at VIR.

Coochas

I think it happened because you have "Obama 2012" in your sig. Sorry.

wanna911

Would that also explain my 10 wins and 5 lap records then? Ill take those odds.

LuigiVampa

Much like a horse will dump a rider it doesn't like your Porsche was trying to dump you and get someone who didn't vote for obummer.

85Gold

Dez

Haven't run the R7 yet but is it possible the different compounds front to rear caused the problem. I know the ABS should deal with it but??

Peter

85Gold

Peter

Veloce Raptor

I still believe it is a diameter issue messing up the ABS.

wanna911

Didn't vote for Obama. Either time. But sorry you are too butthurt to talk about cars without bringing politics into it.

Dont choke on your shoes bud......

wanna911

That does make the most sense to me too. Sent Hoosier an email detailing what happened will see if they have any input. Going to run them conjointly F/R from now on.

Coochas

Ok ok ok here. The Obama comment was meant as a joke.


JEEZ!

Veloce Raptor

So here's another thing to consider: given that the R7 seems to come in a bit wider than R6 of same size, given the same aspect ratio of sidewall to width, the R7 may be thus slightly larger in diameter...