cause of uneven tire thread wear?

[rnln]

On each single tread of the rear tire, one edge is higher, one tread is lower. Is it called tire cubbing? Not excessive but starting a little.
I heard this causes by 2 reasons, on normal cars. One caused is dead/worn shock/strut and the other reason is if the car is lowered excessively. What about our car? My shocks feel normal, harsh but not dead. I had several people look at it (pushing on the shock) and all said it's ok. My car is very lowed, a lot lower than euro setup. Does this cause tire cubbing?
Thanks.

[3.2 CAB]

Cupping. It can be caused by wheel hop, whether it is from bad suspension parts, or the tire/wheel being off balance.

[Palum6o]

My car isn't lowered and I have a cupped tire in the back pass side. I was thinking its due to alignment/corner balancing. I plan on having this done with new tires in the spring. I heard Porsche highly recommends using an N rated tire, which has a stiffer side wall?? Don't know much about the N rated tires... but I did see that the Continental tires have that rating (seen on tirerack "N Porsche" or something..

[3.2 CAB]

Are you talking about an "N" speed rated tire? If so, that is only rated for 87mph/140km/h. I have only used a "Z" or "W" rated tire for my car.

[tonythetarga]

Originally Posted by rnln

My shocks feel normal, harsh but not dead. I had several people look at it (pushing on the shock) and all said it's ok.
Thanks.

Porsche's are not like other cars. This 'bounce the bumper' method does not work. Your torsion bars are what you were trying to compress and that is why they felt stiff, the rears are almost an inch thick! The shocks only control the bounce (compression and rebound). Your shocks could still be worn.

[rnln]

mike,
mine is also the rear passenger side, the cupping. A bad alignment can cause this? umm...

Tony,
Is there a method to test them without taking them apart?

Thanks.

[jmohn]

Originally Posted by rnln

On each single tread of the rear tire, one edge is higher, one tread is lower. Is it called tire cubbing? Not excessive but starting a little.
I heard this causes by 2 reasons, on normal cars. One caused is dead/worn shock/strut and the other reason is if the car is lowered excessively. What about our car? My shocks feel normal, harsh but not dead. I had several people look at it (pushing on the shock) and all said it's ok. My car is very lowed, a lot lower than euro setup. Does this cause tire cubbing?
Thanks.

From your description, this is known as "feathering" (not cupping); feathering is usually due to improper toe. In my opinion, a small (very small) amount of this is normal with our cars since they have an independant rear suspension, the rear tires are set with some camber and are slightly "toed" in. However, if this "feathering" is pronounced on your tires, it could be due to improper toe-in especially if the "lowering" of your car was done without correcting the alignment.

Good luck,

Jerry M
'78 SC

[rnln]

Jerry,
I am not sure, I might call it wrong, but I think I have cupping. Let me reassure my terminologies...
1- feathering: is when you have rubber feather around each tread do to the road surface grinding on the tire surface.
2- cupping: on normal cars (non-P) when the shock is bad, it doesn't bounce correctly. This make each tread wear unevenly, looking at the high of the tread. For example, the 1/2 front of the tread will wear more than the 1/2 rear of the tread, or vice versa.

If the definitions above are correct, then I have cupping, and it is more on the rear passenger tire.

So, as you are saying, lowering correctly, won't be harmful to this area, alignment does. Am I correct?

[jmohn]

Originally Posted by rnln

Jerry,
This make each tread wear unevenly, looking at the high of the tread. For example, the 1/2 front of the tread will wear more than the 1/2 rear of the tread, or vice versa.

Yup, that sounds like "cupping".

Jerry M
'78 SC

[jmohn]

Originally Posted by rnln

Jerry,
This make each tread wear unevenly, looking at the high of the tread. For example, the 1/2 front of the tread will wear more than the 1/2 rear of the tread, or vice versa.

Yup, that sounds like "cupping".

Jerry M
'78 SC

[tonythetarga]

Originally Posted by rnln

Tony,
Is there a method to test them without taking them apart?
Thanks.

You'll have to inspect them to look for oil seepage. If the gas has leaked out it would also cause them to underperform. Best guess is to find out how long they have been on the car. If they have 100k on them, and they are 20-25 years old, its probably time.

[rnln]

Tony, I am not sure how long they are on there, and there is no leakage at all. I check them every time the car is up in the air. I have read somewhere to look under the cover to see the strut color but never can be successed. The whole thing is black. Does the color from the out side say anything?

[tonythetarga]

The color will only tell you what make the shocks are for replacement. For the fronts, you will need to verify what strut you have as that stays on the car when you replace the insert (shock). The strut will be stamped with manufacturer info. Probably Boge if they are black. You could (and probably should) move to Bilstien when you are ready to replace. Do all 4 at the same time when you change them.

[Gunter]

Sounds like another case of looks over performance meaning: bad camber and toe settings.
People do lower away without realizing how much performance suffers.
Without knowing details about the actual alignment settings, it's all wild guessing.
Do an alignment/cornerbalance closer to Euro specs; you'll have better performance and the tires will wear much better.

[eapcpa]

Do an alignment/cornerbalance closer to Euro specs; you'll have better performance and the tires will wear much better.

I agree with Gunter. I used to be in the tire business and he is spot on. You will need to replace the tires in order to see that the corrections have indeed made a difference. Once a tire shows unusual wear it will continue for it's useful life. This is different from toe-in correction in that since your tire is cupped it will continue to setup a certain amount of vibration while driving and may even get very noisy to the point you might think a wheel bearing is going bad. Good luck with it and make sure you find a shop that is familiar with the 911 suspension not just any high volume tire store.

[96740]

My rears ( Continental Sportcontact 2's) are wearing quite a bit on the inside after 8k miles, but the car is very low with PSS9's and aligned to euro specs and I do allot of canyon driving around the Santa Monica Mtns... so I was expecting this.

I do have a related question though... What tire pressures are most of you running for daily/canyon/city socal type conditions?

[rnln]

I am so sorry guys, I might be wrong when I was first asking this question. Let me describe a little more detail this time.

The reason why I thought the tire were cupping because at some certain road, I "feel" the roaring noise. I am not sure I should decribe it as roaring or rumble. It is more like I feel it, not really I hear it. I think you know what I mean if you experienced when you start having the cupping. And it only happen at a certain road. The road doesn't have bad surface but I don't know why only on that road. And when I came home, get down to look at the rear tire, I saw 1/2 of the tread is "blacker" then the other 1/2 of the tread.

After all the discussion on this thread, I look more careful at the rear tire again and I did see the front 1/2 of each tread is "blacker" then the rear 1/2 thread. I than wiped the tire with wet tower to clean it, and look at it again, then I don't see it's cupping that much, or maybe just very very slightly, not to the point of making the rumble feeling.

I went to work today, I didn't forget to get down and look at the tire again before I went in and I did not see this (1/2 of the tread is "blacker" then the other 1/2). At this time, I didn't think about it anymore.

On the way home tonight, I didn't feel that rumble noise on that road/area. After I passed it, I then turned around and went through that road again to make sure. This time, I did feel it at the end of the road, not the whole road.

When I got home, I got down to check the tire again after I parked the car and I saw the 1/2 "blacker" tread again.

Now, let see if my analysis makes any sense. At my house, when I tried to park my car, I usually have to turn left and right, back and forth several times before I parked the car. And also, maybe because of the kind of concrete whichs make the treads bite more to the concrete. Which most of the time make 1/2 of each tread more "black" than the other 1/2. At work, at the spot in the garage, I usually don't have to go back and forth and turning left and right to get in. Also maybe different kind of concrete. Which doesn't make the treads bite to the concrete as much at my house.

If my analysis above is correct, then what is the rumble feeling that I sometimes felt at that particular road? No, it doesn't feel/hear like the wheel bearing. I had worn bearing before, on japanese car, so I kind of know how it is like. This feeling/noise feels like it is on the tire and the road surface.

Sorry for the confusion and a long description.

[Gunter]

Like I said, without more details, it's just wild guessing.
So, what are your actual alignment specs?
Fender-lip height front/rear?
Wheel and tire sizes front/rear?
Camber front and rear?
Toe front and rear?
Cornerbalance numbers?

If you don't know, get an alignment and then post the numbers.

[Gunter]

Originally Posted by 96740

My rears ( Continental Sportcontact 2's) are wearing quite a bit on the inside after 8k miles, but the car is very low with PSS9's and aligned to euro specs and I do allot of canyon driving around the Santa Monica Mtns... so I was expecting this.

I do have a related question though... What tire pressures are most of you running for daily/canyon/city socal type conditions?

If the tires are "wearing quite a bit on the inside", the car doesn't have Euro specs; more like excessive negative Camber.
Same for height:
You say:"The car is very low aligned to Euro specs".
Euro specs are not that low.
What are your figures for fender-lip height, Camber and Toe?

Tire pressure depends a little on Tires, weight, temperature and personal preference.
I like 32-33 cold in my rear 8's with Bridgestones.

[richde]

Cupping is caused by the tire carcass flexing because it's overloaded. If only one is doing this, get corner balanced.