Rear wheel question.

[Quicksilver]

We're driving down the road in New York and someone ask me why both rear wheels of the X5 in front of us seem to be angled out rather than straight up and down like other vehicles. He said you see that what all most all BMW's.

I said it was normal but he said no it isn't. I still didn't have an expert answer to the question. So what's the real deal?

[JCL]

It is called negative camber, and is described as the wheels leaning in at the top. Positive camber would be the wheels leaning out at the top. It promotes better handling, and is designed in by BMW. This is a bit simplistic, but one way of thinking about the logic of it is to imagine a vehicle heeling out (tipping) in a continuous sharp turn. That is a natural function of the vehicle cornering. As the vehicle leans, so does the tire contact patch, thus reducing the effective contact patch on the outer tire just when you need it most. Negative camber improves the contact patch on the outer tire in corners because of the angles and forces on the tire, and because the tire has less tendency to roll under. What it feels like to the driver is a more predictable, planted feeling.

[Quicksilver]

Of course it sounds right to me but the person who asked has a 2001 X3. He has experienced what he believes is excessive tire wear and he believes the negative camber contributed to it. I don't know what shoes he had before but apparently a tire store advised him that he should buy Goodyear RSA 235 65 R17.

He's not a BMW person so it's difficult for him to understand some of the special issues and requirements for X5's So your comments' are appreciated.

[JCL]

Negative camber can contribute to tire wear, but it gets a bad rap IMO, as I don't think it is usually the problem in and of itself, assuming that the suspension isn't bent or worn out of spec. The problem is that people can see negative camber, simply from standing at the back of the vehicle, so it gets a lot of attention. What happens is that the tire deforms, and doesn't just sit on the inner edge. But people tend to think that it does just sit on the inner edge, and that that is what causes rapid wear at that location. If it did, and was in fact a design flaw, then it would be very difficult to explain the good tire life results from those of us who haven't had rapid wear on the inner edge, despite having the same designed-in negative camber. I traded my X5 in with 70,000+ km on it, and the original tires were still good. 17" in my case, not 19", but it is still telling.

What usually causes rapid wear on the inner edge of the tire is toe-in, the tires being pigeon-toed and not pointing straight ahead. This is also adjustable. If the tire is pointing inwards a few degrees, but is forced to roll straight (in line with the vehicle), then both the inner and outer edges are continually slipping (skidding), only the centreline of the tire is rolling without slipping. Both inner and outer could theoretically wear, but the inner tends to wear more, due to the negative camber. Fix the toe-in, to the minimum of the spec that BMW publishes, and you will often get much better tire life.

With wider tires, the toe-in problem is amplified. Also, sport tires often have softer compounds that wear faster anyway.

Only trouble is that you can't see the toe-in (the vehicle is in the way) so you will still get discussion about the negative camber, which you can see easily.

[Quicksilver]

His tire wear was on the inside of the back tires which is what has him concluding that the tire wear is being cause by the negative cam. I'll pass on the info in the hopes he will understand issues involved. Thanks again for the info. Still wondering about Good year tires though.

[ard]

Ditto on JCLs comments.

BMW allows huge amounts of 'toe' in their specs... like 0.08 per wheel, or 0.16 'combined' (ie the amount of pigeon toed-ness). Tires at that number will get 5000-8000 miles before they wear out (!!!). Set them for 0.01 or 0.02 and you will get 25-30,000 miles.

That simple.

And you cannot ask the dealer to "please check the alignment"... you need to get the details on what they are setting.

It is not camber- camber in spec is fine.

[Naz24]

Great post by JCL and ard as usual. I agree with them 100%. I was also misinformed..I believed that camber was the reason why the insides of my new tires were destroyed after 5k miles. But the shop that I got an alignment at informed me that my toe was so far out of whack that my front two wheels were pointing in and the back two wheels were pointing out--needless to say, my car was dragging along the street.

After the alignment, my gas mileage also improved dramatically, I was dumbfounded at what an effect it had. I gained about 1.5 more mpg. I will now do regular alignments (right after the winter time )

[Fraser]

Negative camber effects tyre wear more with wider tyres than with narrower tyres. Just another reason why wider tyres don't make much sense unless you value looks over functionality.

[dkl]

Originally Posted by JCL

What usually causes rapid wear on the inner edge of the tire is toe-in, the tires being pigeon-toed and not pointing straight ahead. This is also adjustable. If the tire is pointing inwards a few degrees, but is forced to roll straight (in line with the vehicle), then both the inner and outer edges are continually slipping (skidding), only the centreline of the tire is rolling without slipping. Both inner and outer could theoretically wear, but the inner tends to wear more, due to the negative camber. Fix the toe-in, to the minimum of the spec that BMW publishes, and you will often get much better tire life.

With wider tires, the toe-in problem is amplified. Also, sport tires often have softer compounds that wear faster anyway.

Only trouble is that you can't see the toe-in (the vehicle is in the way) so you will still get discussion about the negative camber, which you can see easily.

I believed that toe-out will cause more inner wear than toe-in. Toe-in will cause the outer edges to wear more. The factory setting does called for a smidge toe-in.

[ard]

Originally Posted by dkl

I believed that toe-out will cause more inner wear than toe-in. Toe-in will cause the outer edges to wear more. The factory setting does called for a smidge toe-in.

With flat camber, yes.

But to assert that inner edge wear CANNOT be due to toe in is wrong.