Uneven rear tire wear on a new C4. Warning & questions.

[tomnash]

I've owned a number of sports cars over many years, but nothing as great to drive as my 2007 997 C4Cab (manual, PASM), which I picked up last May and now has 11,700 miles on it. Based on experience with soft high performance tires on an S2000 (which were gone at 10,000 miles), I have been watching the tire wear carefully. But it never dawned on me that a factory aligned car would wear the the rears very unevenly. And I never checked the inside edges of the tires, which requires getting down on the floor and shining a flashlight on them.

So ... over 100 miles each from four (!) Porsche dealers on a beautiful Porsche back road north of Paso Robles, out of cell phone range, I heard a bang and stopped before the tire flat warning turned on. Turned out that the inside edges of both rears had worn down to the steel belt, though the outsides were well above the tire wear indicator and looked like they had a few more thousand miles easily. Could the factory set alignment have gotten badly out of spec?

No. Four days later, with the new tires on, alignment showed the rears to be within range, but with camber close to the negative max. The camber was at -1°42' left and -1°41' right, with the spec range for these cars with PASM -1°45' to -1°15' .

The fronts did have the toe angle out of range (identically -10' and -11' for a spec range of 0' to +5'), but there was no sign of uneven front tire wear. A single tire hitting a pot hole would not have done this or pushed both the rears the same amount from where they had been set. So either it was some kind of common bump I hit (don't remember any), or the weight of the car changed the factory setup, or this is the way the factory set it.

I would be interested in expert opinion on what happened here. I had all the tires at within 0-1 lb of specified pressure for half load when I was driving alone with little cargo, and full load spec when driving cross country with my wife and luggage. I would guess about 7500 mile was on cross country straight roads or interstates at 80 MPH or more, and the rest on winding Northern California coastal mountain roads.

Does the factory routinely set up cars at maximum camber? Should one expect a new car to wear so unevenly in this kind of mix of driving? Was some of the cause of this keeping the tires at the half load pressure? Does alignment settle in significantly in the first 10,000 miles? Should I fuss at Porsche, even though the warranty explicitly excludes alignment issues? Should Porsche call this possibility out more clearly (along with the gazillion other warnings about tires in their manuals)? Any other suggestions or comments welcome (except snide remarks that I should have checked the inside wear).

Edited January 27, 2008 by tomnash

[PAULSPEED]

Hi,

You did not mention the tire brand or the tire size. I have run Michelin, Yokohamas, and Bridgestone on the rears. The yokohams rears wore out just like your tires very quickly. You can not find 305-30-19 Yokohamas any more. If you want your tires to last a long time, then buy Michelin pilot sport. They are very hard tires. I think you learned a lesson to look at the whole tire (front and rears). Good luck and keep it off the freeways.

Paul

[tomnash]

Hi,

You did not mention the tire brand or the tire size.

Paul

Sorry ... They were the original Michelin Pilot Sport PS2 N3 on 18" wheels (295/35ZR18(99Y)). I replaced with the same. The dealer pulled in the camber to -1°18' and -1°22' when doing the realignment. I could feel a small difference on corners but much of that could have been because of the new tires.

Edited January 28, 2008 by tomnash

[White987S]

Tomnash, what are your actual cold tire pressures?

[tomnash]

Tomnash, what are your actual cold tire pressures?

Now they are set where the dealer set them on the new tires, 37 front/ 43 rear, essentially at the Porsche recommended full load setting: 36 front/44 rear.

Usually I have them at the partial load recommended setting as I noted in the original post, which is 33/39.

I use the numbers on the dashboard display of TPS readings that indicate temperature corrected differences from nominal, displayed only when car is stopped, and these agree within reading accuracy when the tires are at ~ 20°C with a manual digital gauge I got from TireRack a few years ago. I do not use the absolute numbers displayed when the car is moving.

Edited January 28, 2008 by tomnash

[White987S]

Getting about 10,000 miles on a set of rear tires is pretty normal with our cars.

[RFM]

If the car is set with the max. negative camber you have to go to the max. toe-in to compensate for the rear and front axle as well, all in the tolerance values of course. I think that you have got the max. negative camber in combination with the max. toe-out what declare the tyre inside wear. It is necessary to top up the fuel tank for a alignment, a difference in weight of approx. 50 Kg. on the front axle gives you a totally wrong result.

[john in lb]

I've owned a number of sports cars over many years, but nothing as great to drive as my 2007 997 Cab (manual, PASM), which I picked up last May and now has 11,700 miles on it. Based on experience with soft high performance tires on an S2000 (which were gone at 10,000 miles), I have been watching the tire wear carefully. But it never dawned on me that a factory aligned car would wear the the rears very unevenly. And I never checked the inside edges of the tires, which requires getting down on the floor and shining a flashlight on them.

So ... over 100 miles each from four (!) Porsche dealers on a beautiful Porsche back road north of Paso Robles, out of cell phone range, I heard a bang and stopped before the tire flat warning turned on. Turned out that the inside edges of both rears had worn down to the steel belt, though the outsides were well above the tire wear indicator and looked like they had a few more thousand miles easily. Could the factory set alignment have gotten badly out of spec?

No. Four days later, with the new tires on, alignment showed the rears to be within range, but with camber close to the negative max. The camber was at -1°42' left and -1°41' right, with the spec range for these cars with PASM -1°45' to -1°15' .

The fronts did have the toe angle out of range (identically -10' and -11' for a spec range of 0' to +5'), but there was no sign of uneven front tire wear. A single tire hitting a pot hole would not have done this or pushed both the rears the same amount from where they had been set. So either it was some kind of common bump I hit (don't remember any), or the weight of the car changed the factory setup, or this is the way the factory set it.

I would be interested in expert opinion on what happened here. I had all the tires at within 0-1 lb of specified pressure for half load when I was driving alone with little cargo, and full load spec when driving cross country with my wife and luggage. I would guess about 7500 mile was on cross country straight roads or interstates at 80 MPH or more, and the rest on winding Northern California coastal mountain roads.

Does the factory routinely set up cars at maximum camber? Should one expect a new car to wear so unevenly in this kind of mix of driving? Was some of the cause of this keeping the tires at the half load pressure? Does alignment settle in significantly in the first 10,000 miles? Should I fuss at Porsche, even though the warranty explicitly excludes alignment issues? Should Porsche call this possibility out more clearly (along with the gazillion other warnings about tires in their manuals)? Any other suggestions or comments welcome (except snide remarks that I should have checked the inside wear).

I have a 2003... and with 42,000 miles. I am now on my 3rd set of rear Michelins.

I had the exact same issue that you described... although my first set lasted 15,000 miles. My Porsche mechanic out of Bakersfield predicted this issue when I first purchased the car.

He told me it relates to the car "settling" when new... and also the aggressive way that Porsche sets the car up - to maximize handling.

When the first set was replaced, he realigned the car - staying within spec, but making the rear tires more square.

He was right on. The second set wore out completely even across.. and lasted about 21,000 miles. This current set looks pretty good too. By the way, all 3 sets have been the original model Michelins.

I still have my original fronts and they are wearing fairly even. I expect to be replacing them when this set of rears need to be changed out (say 55K). I did rotate them once left to right at around 20,000 miles.

My expectation at this point is to not need another alignment going forward - unless the car / chassis are hit.

[tomnash]

My Porsche mechanic out of Bakersfield predicted this issue when I first purchased the car.

He told me it relates to the car "settling" when new... and also the aggressive way that Porsche sets the car up - to maximize handling.

When the first set was replaced, he realigned the car - staying within spec, but making the rear tires more square.

He was right on. The second set wore out completely even across.. and lasted about 21,000 miles. This current set looks pretty good too. By the way, all 3 sets have been the original model Michelins.

Was the mechanic at Family Porsche in Bakersfield? That's where I got towed to and they told me almost exactly what you said. I was looking to get some independent confirmation of this from my RennTech post. It's a plausible explanation, but it is puzzling that a factory set up car would wear so unevenly in the first 10-12,000 miles. If so, all new Porsche owners should get the same warning you got!

Tom

[john in lb]

My Porsche mechanic out of Bakersfield predicted this issue when I first purchased the car.

He told me it relates to the car "settling" when new... and also the aggressive way that Porsche sets the car up - to maximize handling.

When the first set was replaced, he realigned the car - staying within spec, but making the rear tires more square.

He was right on. The second set wore out completely even across.. and lasted about 21,000 miles. This current set looks pretty good too. By the way, all 3 sets have been the original model Michelins.

Was the mechanic at Family Porsche in Bakersfield? That's where I got towed to and they told me almost exactly what you said. I was looking to get some independent confirmation of this from my RennTech post. It's a plausible explanation, but it is puzzling that a factory set up car would wear so unevenly in the first 10-12,000 miles. If so, all new Porsche owners should get the same warning you got!

Tom

Yes it is Family Porsche. The mechanic you probably talked to is Dennis (He is their only Porsche certified mechanic). He is excellent and 100% trustworthy... He has worked on my car for the past 5 years and, when he does work, I am usually standing next to him in the service bay. Even though I have moved to the Los Angeles area - I still go back up to his shop if I need anything done.

I agree with you about Porsche telling you or even paying the cost of the first alignment... and I had asked Dennis about it... but, unfortunately, they just don't. By the way, through Dennis I have had superb warranty work done - and I was amazed at how much Porsche did cover... so, in the overall scheme of things, I thought the Porsche warranty experience was very good (even better than Mercedes).