Two tires or four?

[Don C]

I have cupping on the rear tires of my 1997 Z3 with Fulda 225/50ZR16 92W tires. I don't see it on the front tires.

I'm thinking to replace the two rear tires. The tread seems to be OK on the front so it would be nice to not have to replace those.

Is this a good idea, or am I asking for problems?

Reading the posts here, I see that the cupping could be from low tire pressure or from suspension (shocks?) problems. I've not noticed any excessive bouncing, so I HOPE the problem has been air pressure.

Other than the bounce and see how long it takes to settle procedure, any other tests I should do to avoid cupping problems on the new rubber?

This is the 1.9 Z3. Don't know how many miles are on the tires; car has 70,000.

[Hathaway]

Cupping usually happens because you have worn out shocks. I had one tire on my car cup because of a bent wheel, but thats unlikely for you since it is both rears. Some people recommend replacing all 4 tires at the same time, but if the fronts look good I would say only replacing the rears is fine, Ive done it many times. The only good way to tell if a shock has gone bad is to pull it off the car and compress and extend the shock by hand (this should only be done if you have two people to pull the shock apart or a vice to clamp one end in). If the shock travel feels smooth and linear (doesnt have really soft or really stiff spots) then its probably fine. Also, I dont know if you have ever actually done this so Ill try to explain, the shock should have steady resistance, it should take a reasonable amount force to push the rod in and pull it out. If you can easily hold the shock in one hand and slide the rod in and out with the other hand, the shock has gone bad. Now, after reading all that, I not even sure of how practical this method will be because I beleive Ive read that it is a royal pain to remove the shocks (at least on the e36 3 series), but have never actually done it.

[Hathaway]

Sorry for the novel by the way, just trying to explain the best I could.

[Calypsocoupe96]

Don't worry. Same tires same axle no problem.

[Don C]

Hathaway,

Thanks for the complete description of the shocks test. Being new to German Engineering, I wasn't sure that the rear suspension even is shocks, so you've made me a little more knowledgable if just for that! The car pretty much passes the bounce on the bumper test; guess I'll have the tire guys take a look while the wheels are off.

[SiGmA]

The bumper bounce test is *near worthless* so I am told. Try the over-the-speed-bump-at-20MPH one instead. See the rebound you have then. If its pretty stiff and only a couple of bounces, you're good.

[Don C]

Took 'er to the tire guys today and we decided just to do a front to back rotation for now. Still have plenty of tread left so I'll just watch the "new" back ones for signs of inappropriate wear. I suppose I'd best keep an eye on the air pressure too.

There are a couple of speed bumps near work. I'll check them out next week.

Thanks for the suggestions! I got by at no cost today; we'll see how long that holds out!

[Hummer5665]

I always heard you should use the same tires for the whole car....if thats what your talking about.