marlinspike

Is it normal for these cars to wear the inside edge of the tire more than the outsides on the rear tires? The wear smoothly increases as you go from outside to inside, but the difference is huge (like 8k more miles judging by the outside of the tire but in serious need of replacement judging from the inside of the tire).

1999Porsche911

Normal. The factory camber setting is very aggressive for a street car and you ride on the inside half of the back tires. You can dial out the camber to be less negative and your tire life will double.

marlinspike

Thanks. Naw, I won't dial it out, I know that's how a good performance setup is, I just didn't know if this was supposed to be like that (never sure how much they dumb things down for us normal people). Thanks.

UCrazyKid

Normal. 1.5 deg. neg camber in the rear will do that. You just need to drive the car harder in the twisties to even out the tire wear!

marlinspike

Heh, well with my mom driving it I doubt that will happen.

wdonovan

Finally, someone touched on the subject I've been trying to get answered all over the place.

Observation: My 996 sits like my old 356 with the swing axle, and the outer edges of the rear tires are almost in the air (OK I exaggerate). This will not help tire life.

Theory: Huge negative camber on these cars to keep the outside tire flat at cornering limits (track, evading police...) when body roll is very high. When driving in a straight line or just driving mildly (like 90% of street driving), the tires wear out the inside edges. If I may take the liberty to speculate, I'd say that the suspension changes drastically with body roll, just like the old 356's, so Porsche spec'd high negative camber to neutrallize outboard camber in hard turns. Camber must be neutral then, why would they have partial inboard edge contact only when you need maximum adhesion? If my theory is correct so far, then this suspension setup probably also accounts for tire pressure specs being higher for the size tire and axle weight then other cars using similar size tires. I think the high pressure is needed to belly out the tire producing more contact in the center to keep from just wearing out the inside edge in a few hundred miles. I put a pyrometer on the tires and (in my mind) confirmed this by seeing temperature kept warm along the inner half of the tire then dropping to cold for the outer half (at 40psi). At lower pressure (34), only the inside edge makes any heat.

Proposal: Start at the cause of the problem and work through to the desired end (better tire wear without sacrificing much performance). I can give up a small amount of performance as my car will never leave the street for the racetrack.

Schedule: 1) Install a stiffer rear swaybar. 18mm now (car is 02 C4 Cab). This should reduce body roll which is now significant. 2) Reduce rear camber to the neighborhood of -40' to -45' (whatever seems practical for the amount of body roll reduction). Factory low limit is now 55'. 3) Reduce tire pressures using an I/R thermometer, taking a survey after mild or highway driving, and also after hard cornering.

Does this make sense to anyone in here? ALso... anyone know the next 2 steps up in swaybar size that will fit my car?

1999Porsche911

Simply reducing the camber will double the life of the tires and not reduce handling for most street drivers. Leave tire pressure at or near 44 psi.

UCrazyKid

wdonovan-
Your theory is an interesting one. However, I don't think adding a thick swaybar and reducing camber will give you the desired results. As with everything, there are no free rides. Having a thicker rear swaybar can reduce the ride quality. Essentially you are tieing together the rear wheels with a spring. With a thicker bar when one wheel hits a pothole the shock will also be transmitted to the other side through the bar. You can end up with a harsher ride when going to a stiffer rear bar.

With that said, there are many options for a firmer rear bar. The most popular in order of stiffness (softest first), M030, GT-3, H&R

Andy

Carrera Mike

If your car is lowered from stock, it will be worse too.

Gretch

my 2000 C4 is used for high mileage commuting, it has 97+ thousand miles on it. I took all the camber out of the rear set up and run the tire pressure at 36 psi. For my purposes, and limited ability to judge such things, the performance remains way in excess of anything I can use, and I get approximately 24,000 miles out of a set of rear tires.

Tippy

I agree, a lot of rear tire wear has to do with the camber.

chsu74

Gretch,

What tires are you wearing for the 996 C4? I will be driving 100% street during fair weather only. Have done many tire searches but have not found a concrete answer.

TIA

Chao

wdonovan

Looks like I've got a few people thinking now. I reallize that the major downside to my proposal is a harsher ride. I'm willing to put up with that. I started thinking about this whole thing because of Porsche's tire pressure recommendations. They seemed stupidly high. People said that Porsche knows more than anyone and the pressure HAS to be high. But coming from 6 C4 and C5 Corvettes that use similar size tires, I couldn't see why a 3600# car needs 30 lbs. air and the Porsche at 3200# needs 44 lbs. Something's wrong here. The Vettes tires last long and wear evenly. Not the Porsche even though the extreme compromise of running the tires right to (above, when hot) the tire maker's advice. Again, anyone know of the next one or two bigger than 18mm that will fit 02 C4 Cab?

UCrazyKid

I'm not sure what you are asking here?

The largest factor causing the rapid rear tire wear is that 60% of the cars weight is hanging off the back of the car. Not something that the Corvette has to contend with. I have also heard that the very high rear tire pressure has to do with protecting the rims from damage with a very low sidewall tire.

All this said. I feel that 12K-16K miles on a set of summer ultra high performance tires on a sports car like the Porsche is completely reasonable. If you can shell out this kind of money for the car and maintenance you should be able to buy some new shoes for the car every 12 to 18 months.

Even my Honda S2000 would go through tires at the same rate as the Porsche in the rear.

All though far from ideal, I believe that the tire life on the 996 is acceptable. I think for those using the 996 as a commuter car you really need to rethink the purpose of this vehicle. It would probably do you better to buy a honda civic for those long drives if your end goal is to spend the least amount of money on your commute.

Just my $.02