dwe8922

At the VIR race, I had a left rear tire blow out (thru the esses - very scary). I took it back to Michelin for them to help me figure out why, and came up with it likely being a bad valve stem. The inside was cut and had the belts protruding, which Dennis said was consistent with a deflation failure. Everything else looked ok, so I replaced it with a spare tire, and mounted the right rear (bought at the same time as the left rear) on the left. These tires have a Sebring enduro, and three sessions from Atlanta on them. I ran the Sunday sprint race at VIR on this setup.

I noticed the tire would not hold air, and took it off today to find the inside side of the left rear (tire that had been on the right rear) had begun to fail. The steel wires are sticking out of it like it a mustache - like needles poking through the rubber, and the tire doesn't look abraded. The inside of the wheel well has two spots where its worn to bare metal. The rear camber -3.5, the ride height is factory spec, and there is no evidence of any worn suspension members.

Any suggestions?

Larry Herman

Yes, too much rear camber is overheating the inside edges can causing the failure. Had the same thing happen to my GT3 running Pirelli slicks with -3.3 degrees. 2 failures in 2 weekends, both coming during the 7th session; not coincidence. I suggest reducing the camber to no more than -2.75. That's what worked for me.

J richard

Pic of the tire? Does the rubbing on the fender align with the fail point on the tire? The description of the steel belt ends poking through the tread is suggestive of an internal belt failure, either a real hard stone bruse which would usually show either tread or internal damage. Also the tire would act like it had a flatspot...

Larry has a type specific answer tho....

jrgordonsenior

I ran -4.0 in the rear for 4 years on my 03' cup without any type of failure. Almost exclusively I ran on Michelin Yellows and later Blues. If it's rubbing that would create heat and possibly the subsequent failure. I'd pull and roll the rear fenders to assure proper clearance...

Larry Herman

Wow JR, that is a lot of camber in the back of that car. I'm sure that it worked for you and was setup right, but that would be way too much based on my experience with the GT3 platform. I'll have to check into it more. What were you running in the front?

jrgordonsenior

Hi Larrry.... -5.0 up front. 6-cups like a lot of camber. The IMSA cup guys would go even further in qualifying to something like -6.0/5.0 them set it back a little for the races. They did experience failures which is why IMSA stepped in and created maximum camber limits for each course. Still they max out at 3.8 rear and 7-cups definitely don't like as much camber as the 6-cups do....

http://www.imsachallenge.com/2010/gt...GT%2010-07.pdf

dwe8922

My camber setting is ok temp wise - I've checked that setting by taking temps across the tire. The belts poking out through is what I don't get. It doesn't look like the tire rubbed the inner fender well/tub (at least from looking at the tire - no sign of abrasion really on the rubber; the wires literally look like they grew out of the rubber). It looks more like the fuzzy belts caused the abrasion on the fender well/tub. Clearance is fine on the outer fenders.

The tires could be bad; they were bought at the same time. That doesn't seem likely to me though. Both failures happening on the same left rear position seems more to do with the cause. But the suspension looks ok, and my ride height is stock.

????????

J richard

I'd get it off the rim and look at the inside, it would not surprise me if you see a similar tear, otherwise the tire would still hold air.

Is it possible that you were hammering that corner on a rumblestrip or curbing that caused the damage?

jrgordonsenior

Over the years I''ve seen mutiple failures on slicks that couldn't be rationally explained. One in particular was a Michelin blue blowout of a outside rear in front me at 165mph on the banking at Cal Speedway. It was their 2nd HC.
I've also seen Hoosiers go and I'nm sure Yokes have occasionaly blown up too.
My point is you can get a bad batch of rubber occasionally.

Just a thought... None of the above slicks like to be beat up when they're cold. If you're scrubbing them hard when cold or bouncing them off berms, etc. before they get up to temp then you could possibly be damaging the sidewalls. What temps do you start your cold tires at and what is your hot target?

38D

I would try marking inside the wheel well with a paint pen in a few place just to make sure it's not rubbing. If there are old rub marks it can be hard to tell otherwise.

dwe8922

I talked to Michelin today, and he said that if it had been caused by a rub, that it would have really noticeable wear on the tire, which it doesn't seem to have. He said it's a failure of where the belt breaks at the shoulder caused by either too much camber, or too low pressure. Basically anything that puts too much pressure on the shoulder causing the belts to break. I will check my camber, which I think is ok, and also recheck the corner weights. I did a quick test for leakage of the valve stem, and that was ok. I was starting at 20 cold, and hitting 31 hot, which are in spec. I did not check before and after the runs where they failed. The only other thing I can think is I might have a leaky rim halve gasket? He said those can test fine statically, but leak once in use.

fatbillybob

I my short lifetime racing I have "only" seen Michelin slicks do the mustache trick and its been more than once.

coryf

Is that a speedway rear? the speedway tire will do that with flat track camber settings. (anything over -2.0 or so) Have never seen a blue/green/N1 tire do that (when driven with normal tire pressure) Only the speedway tire (think its a s9b or something)

Geoffrey

S9C is the Speedway tire.

coryf

Thanks Geoffrey.

To add to my previous post. I remembered a Green rear tire doing this same thing but only after it was driven around at paddock speeds when flat. A wheel was leaking and the tire was very low. It destroyed the belts when flat and after a session on the track (at the correct pressure) showed the "hairs" through the sidewall.