Z3 Roadster (E36/7) wrong size tires on front

I just found out the 2001 Z3 3.0 roadster I bought in December has the wrong tires on the front. I haven't driven it too much, but I did have it aligned, as the toe was out.

I was looking at it today and noticed it has the same size tires, Kumho Ecsta ASX's on the front and the back. They're the right size for the back. The backs are more worn than the fronts so I'm going to save the fronts when I take them off. I was able to find the right size for the front in the same tire model from a place called Performance tread, at a decent price. I've never used them before, so I'm hoping their service is good. Tire Rack didn't have the same tire and I'd like to keep the same type front and back.

I wonder now if the toe will need to be adjusted again. I had it aligned at a chain place, and I could see the guy was working pretty hard to get it right. It's too bad he didn't notice it was the wrong size tire, and tell me then.

I know it's not too common to have two different sizes on a car, but it is a weird mistake to make. I think they're supposed to be 225/45-17 front and 245/40-17 rear. The wheels are the right size, I double checked that. I guess I should have looked more closely when I bought it.

I don't see the issue. As long as the tires don't rub when you make turns or bottom out when u go over bumps. You don't need to get it realigned just because U are putting on new tires. That is if you r installing new tires soon. I would suggest to anyone that is buying new tires to have the alignment checked but u just had it done.

Thanks. I hope you're right. I'm just wondering if the alignment was wrong because it was done with the wrong size tires on the front. I guess if the sidewall profile height is the same for the 225/50 and the 245/45 it shouldn't make any difference.

Originally Posted by JackDuggan

I'm just wondering if the alignment was wrong because it was done with the wrong size tires on the front. I guess if the sidewall profile height is the same for the 225/50 and the 245/45 it shouldn't make any difference.

Your toe setting is unaffected by tire size. I doubt that the sidewall height will be the same for the two sizes.

Not a big deal. Larger tires in the front will reduce understeer. I run same size tires all around on my E46 M3.

I can't find a Kumho tire that is actually available in those two sizes on Tire Rack (not looking that much), but:

Going from a 225/50-16 to a 245/50- 16, the tire gain 0.8" in diameter.

The 245/45-17 is 25.7" and the 225/45-17 is 25.0" diameter. So I would suspect that a 225/50-17 to be about 25.8" diameter, or pretty close to the same size at the rears.

I've run a square setup for 245 or 255's for years using 4 rear Style 32's. My signature picture is out of date but is a pic of ACS 17x8.5 square setup. Very common. As said previously, toe is nothing more the relative distance from the outside front and rear of the respective tire and such an alignment may be as a result of alignment being purposefully set that way or as a result of other reasons. Rubbing is the only concern which sounds like a non-issue here.

When you do a square set-up you use the same size wheels on all four, right? In my case the front wheels are still the 7 1/2 J front and 8 1/2 J rear.

When I run a square setup all four wheels are the same size. Some folks fudge this but having an oversized street tire on a skinny wheel, while doable, is not ideal as the tire cups. The answer is to get 225/45-17's up front and put the 245's in the rear.

Originally Posted by Z3Papa

When I run a square setup all four wheels are the same size. Some folks fudge this but having an oversized street tire on a skinny wheel, while doable, is not ideal as the tire cups. The answer is to get 225/45-17's up front and put the 245's in the rear.

You know that's interesting, because I had the car looked at by a garage before I bought it. The guy who looked at it said the front tires were cupping and that it meant the struts were worn out. I don't think the struts are bad, so maybe the tire wear is because they're too big for the rims. They're a little worn on the inside, due to the car being toed-in, but they're good enough to keep and put on the rear.

Originally Posted by JackDuggan

They're a little worn on the inside, due to the car being toed-in,

Toe-in doesn't cause the tires to wear on the inside. It causes them to scuff or leave a feathered edge on the tread resulting in poor handling and premature tire wear.

Well the problem was that they were actually "toed-out" before the alignment.

Toe (In or Out) causes the tires to wear. Camber causes the tire to wear more on one edge than the other.

Toe causes the tire to have some scrub. With toe in, the tires are slightly pointed in towards each other, so they are trying to roll together, but the suspension keeps them apart, so there is some sideways scrub of the tread. The more toe, the more scrub.

Personally, I like a tiny bit of toe OUT. It makes the car turn in more crisply. BUT, you have to DRIVE the car all the time, as it wants to change direction. Toe IN is more stable.

As to needing the same size wheels front and rear to run square tire sizes, it depends on the tires, tire sizes, and wheel width. Both size rims might be within the acceptable range of rim widths. But if you put a wider tire on a narrower rim, the tendency is for the tire to hump up in the middle, wearing it out sooner. A tire pressure adjustment might be needed. Unfortunately, the only way most people can tell is to wait until the tire starts to wear unevenly. A pyrometer to measure tire temperatures at each edge and the middle would help.

In my experience, cupping comes from out of balance, bad shocks (struts), or looseness (bushings, rod ends, etc). The front control are bushings are one thing to check.

I'll have to check the LCA bushings again. I asked the mechanic who checked it out to look at them. He said they were OK, but I do notice a little vibration at times when I'm above 60 mph. The car has 94,000 miles on it. I replaced the bushings and control arms together on my 318ti a couple of years ago and it wasn't too difficult doing them together. I don't know how it would be just doing the bushings. I see a kit at BAVAuto with two Meyle arms and powerflex bushings for $250. Any recommendation on bushing material? I see stock, solid rubber M3, and urethane available.

What's the wear out point for these things? My ti has 230k miles, but I think the balljoints/bushings were done once before I did them.

The cupping I was referencing comes from over-tiring a wheel where the middle bulges a bit. The converse of a stretched tire effect. Not indicative of worn stuts.

I went ahead and ordered control arms and bushings for the front end. If it doesn't need them now, it will soon I think.

Cupping is where the tread has high and low spots around the circumference. A too wide tire on a narrow rim, with the wrong tire pressure will wear out the center of the tread before the edges.

Cupping:



Cupping on the far right, notice the "cause" listed: