Larry Herman

Here's a question for all of you veteran racers. I started running the GT3 (to your left) in DE's this year. I have over 20 years of track experience and 6 years of club racing. I have never, ever had a flat, and yet in the last two events, Watkins Glen & Summit Point, I flatted the left rear after about 7 or 8 runs. There was plenty of rubber left on the tire, the temps were around 150 ~ 160 degrees and the pressures were at the recommended 33 psi. They are Pirelli P Zero Slicks. I checked them every session.

What would happen is that the left rear tire after working great, would start to feel like it was grinding in a right hand turn. Within 2 or 3 seconds it was flat; that quickly. Could I have rolled the tire off the bead due to the weight of the car? I never felt comfortable with 33 lbs, but Bob Woodman says that it is right and the tire wear and temps back this up. There were no obvious punctures, but the inside of the tire got ripped up when it deflated.

No one else in GT3s running the Pirellis is having this problem, but I am running about 2 seconds faster than the next quickest guy. Any thoughs or experiences anyone?

924RACR

The only thing that comes to mind for me might be other causes for the loss of the air - valve stem or rim sealing might be bad? Deflection of a rubber valve stem or poor sealing of rim halves? Just trying to examine all the options. I've been in cars that have even experienced excessive deflection of sidewall in extreme maneuvers to cause the rim to touch down and hit pavement, in some cases resulting in loss of beat seating and pressure (catastrophic). Your case seems like a slightly more gradual loss of pressure than just from one hit, since you describe experiencing the grinding sensation _followed_ by the loss of pressure - not practically simultaneous. Just a guess, though. I should add that when I've peeled the tires off of cars, it's been with pass car tires on 4000# cars, not race or sport tires, and I would fully expect that the sidewall of any track tire would be more than adequate to prevent this from happening.

Good luck finding an answer...

trumperZ06

Could the tire be 'slipping" on the rim?

People have had this happen "drag racing"... using low pressure ~ 26 psi !

You might mark both the tire & rim... then see if you are possibly getting "slippage".

viperbob

I have seen this with a number of guys running the Pirelli's. The tire just all of a sudden self destructs. One of my partners had this happen to his car. Both rears blew within 1 mile of each other. Both blowouts destroyed the tires right at the tread to sidewall seem. I then have seen a couple of other 993/996s with similar blow outs at speed on the track recently. One guy went through 3 tires in a weekend. He finally just gave up. I do not know the details, but i believe my partner sent his tires back to Tire Rack. Not sure what happened after that...

I would suggest trying Yokohama's. I LOVE them and they are very affordable...

Larry Herman

Thanks Bob & others. My tires also tore right at the inside tread to sidewall seam. I have also checked all of the obvious (no leaks, metal valve stems are tight, caps on, etc). I am definately not going to run Pirellis again. Could be that the rim halves are flexing and losing air, but they have held pressure for weeks before and after the blow outs, so I doubt it, but when we remove this tire we will check that carefully too.

How long do the Yok's last and how many heat cycles before they fall off?

fatbillybob

Can we assume that you are using the corect tire size for the rim. Too wide a tire makes for poor sidewall support. I had this happen on voctoracers on the rear. gt3 is tail heavy and this was known to happen on one size of hoosiers it is on their website. I went to Pirelli slicks and have never had a problem. However, that grinding noise might be you locally overheating and grinding that tire to a bloody pulp at maximum load that the 2 second slower guy just can't get. Are you giving lessons? I could always us some help. That noise was the predcitor of my victoracers demise too. Do you regualarly drive to the point of that grinding digging in sound and feel of the tire to pavement? How much negative camber are you running?

MJR911

Larry, are those the stock wheels on your gt3?

DrJupeman

I had a left rear Pirelli slick de-laminate on me at ~120 mph and sent me and my GT3 Cup into the wall. Not a pretty picture. Does Pirelli have issues?

RSRRacer

I have had this happen numerous times with Yoko slicks on my 996TT.

I was running 280/650/18 rears on the OEM wheel size, which I think is too wide of a wheel for the slick.

I have a feeling it is due to the size of your wheel, or an alignment problem in the rear. The street GT3 is not THAT much heavier in the rear than a GT3 Cup.

38D

I had a similar problem with tire failures, and mine turned out to be a bad seal between the rim halves. Since resealing, no issues. Btw, what brand of rims are you using?

Larry Herman

I am running Fikse 9 x 18 with 245/645s (width/diameter) in the front and 11 x 18 with 305/645s in the rear. They are the correct size for the rims. I am running 3 degrees negative in the front and 3.25 neg in the rear. Tire temps, wear, and now a few photos show that I have the correct amount of camber in the car, though I am considering taking out about 3/4 of a degree in the rear. I am running factory spec toe-in in the rear (3/16 total).

As far as driving, The tires feel normal, grip well, and are predictable throughout every session. I slide them around less than the street tires. I have only felt that "grinding" twice. Each time it was immediately before (or as) the tire deflated.

MJR911

Larry, make sure you get a good shop to do the alignment. A yellow gt3 was aligned by a well known shop in our area with way toooo much caster and ended up tearing up the fender lip at the Glen. PM me if you want the details.

Larry Herman

Mitch, I have the best shop to do alignments, I do them myself Smart Strings alignment system and Smart Racing camber gauge. It may be a couple of hundreths off of a laser system, provided that the laser system is setup up right & the user knows what they're doing, but it will never be f%*&#d up. And yes, I know who worked on that car. Classic mistake of not knowing the car. I flipped my strut tops and no problems with caster.

Bob Woodman is going to take the time to stop on his way to Lime Rock and carefully remove and inspect the wheels & tires. Can't get better service than that.

viperbob

I love my Yokohama race slicks (250/650 & 320/650). I see very little fall off in them with heat cycling. It seems that I get about 24-32 (6-8 DE days) full heat cycles out of them before I run them to the cord. I use the 2nd softest coumpound out of 4.

2 of the people I referenced above as having issues where using factory wheels, so no possibility of a rim seal being bad with their failures.