atari_st

I just realized that I have worn out a pair of Pilot Sports in about 900 miles on the rear of my 86. What is really strange is the whole tire is worn down to the wear markers with the inner portion worn just a tad more. The wear marks look like the car was dragged sideways across concrete on all thread patterns on the tires. The fronts look brand new (as the really are). The car was aligned when i had new shocks installed on all four corners at a local shop. It goes to the dealer in two weeks for an alignment (my dealer has one of the best techs in the southeast for early water cooled cars). I assume that has to be the issue as I certainly have not burned them off.

Any ideas?

Mark Anderson

The rear of my car always has a black mist which I call fun dust. ( Michelin dust) But I also look at my mileage and when it's below 12 mpg I figure I had a good time .

PHIL928

Yeh my rear tires also don't last very long, but then again I do spend alot of time going sideways

RKD in OKC

toe

Jehu Gearloose

Hi Jeff

Who is the dealer/tech?

mike77

According to what I read once the car has been lifted the suspension needs to settle before the alignment can be set properly. This apparently takes upto 100 miles driving to achieve. If this isn't done then the when the suspension does settle the alignment will be off and the tyres will wear out. Perhaps thats what happened here?

Jfrahm

Yes I recommend an IR Thermometer ($20 at Harbor Freight) to check those tire temps across the tread. Gross alignment issues can also be seen by eye. They did probably align the car with the suspension unsettled and wiped out those tires. I hope they sell tires because they owe you a couple.

hacker-pschorr

https://rennlist.com/forums/11159541-post25.html


I'm sure aligning the rear with the front unsettled will cause the rear to be slightly off. I would think he would also be able to see a wear issue with the front tires since doing a 928 alignment with the suspension unsettled has a much more pronounced effect on the front tires.

The rear toe is most likely off. It doesn't take much to ruin a set of tires.

KenRudd

^^^^ This FTW!!^^^^

littleball_s4

To do a perfect alignment one needs a lot of hardware. But the shops with more hardware tend to be the ones that destroy the alignment more on a half decent 928.

To do a good alignment (to avoid eating up tires, for example) one needs very little. With a flat floor, a big square ruler, metric tape, a sharp pencil and a friend you can tell the toe with 2mm accuracy. If the toe is wrong, it's time to change the alignment shop.

Myself, I have yet to find a good alignment shop in spain, I've tested 5 of the best. They all have the hardware, they all screw the 928 up big time. I think it's because of the peculiar behaviour of the car when you lift it. You need to align without lifting and with turntables in all 4. But the time they did it like this was not ok either. Go figure.

I ended up doing it at home with fishing thread and stands. Immediately car felt ok and tire wear was in fact perfect. Steering wheel straight, no pulls. My advice is to do the same unless a 928 owner recommends a specific shop from first hand experience.

atari_st

I am sure the car was aligned after being on the lift without having time to settle. New alignment and tires should do the trick.

As far as the tech at the local dealer: Jim Burton. He hates 928's from a workload perspective (they are just hard to deal with) but he knows what he is doing and I am glad he has a wrench in his hands again. That's his passion!!

dr bob

I do my own alignments with a set of laser fixtures, digital angle gauges, and a tape measure.


Rear toe settings have a significant effect on the feel of the car while cornering. You'd notice the difference pretty quickly. At the same time, tire wear from incorrect toe settings in the rear will cause uneven wear similar to what it does to front tires. If the wear does not include scrubbing the inner or outer edges, you may need to look further than just rear toe and camber. Depending slightly on the ride height, worn rear shocks will allow the tire to scrub sideways with vertical displacement.


Regardless, I know that I could pretty easily scrub a set of rears with some aggressive throttle use. If you regularly take advantage of the throttle, this may be part of the problem too. I drive the throttle fairly conservatively, and avoid using the brakes if I can. So the fronts take the brunt of the load on my car. I can squeeze 15-20k out of a set of front PS-2's if I'm careful, a little more out of the rears. More typical is 12-15k fronts, 15-20k rears. Spirited (for me) driving can easily cut that life in half though.

Chalkboss

In my experience working as a service advisor Long Ago rear cupping or scrubbing was usually due to worn shocks or struts, as Dr Bob mentions. Toe angle typically scrubs the edge, depending on whether toed out or in. You should investigate the condition of the shocks as well...

Bilal928S4

What do you use as slip plates under each wheel? Plastic bags and grease or have you found an alternate to the cheap Harbor Freight ones that are NLA?

littleball_s4

Couple of alu plates with grease between them do the job.

But if you roll to the alignment place after 100km or more from the last wheels up, it works even better. I think slip plates (I thought the name was turntables but probably that's the thing in chinese restaurants, sorry) only solve part of the problem you make when you lift the chassis.

I measure, lift, correct and go, and check/iterate the next week. When you know the mms to turns ratio, you seldom need to iterate. I know it is not very professional, but it is the best amateur way, I think.