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Tire Rack's Tires, Wheels, Brakes & Suspension
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  #1  
Old 07-08-2003, 08:40 AM
fkafka fkafka is offline
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Question Tire size difference on xi

BMW says that xi's must be equipped with 4 tires of identical diameter to avoid damaging the AWD system (presumably the differential). A couple of years ago (on another forum) one of your guys said you were investigating what sort of tolerances were acceptable, but I lost track of the issue and never heard what you decided. Currently my fronts run about 3/32nds lower than the rears, just from normal day to day wear. My interest is not in running different widths front and rear, but rather in figuring a way to a) run summer and winter tires with a common spare, and b) swap out of the awful OEM tires as they rupture (one or two at a time) rather than replacing five when one busts.

So to sum up: what's the biggest tire diameter difference the xi system can handle without undo wear or damage?

Thanks in advance, F
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  #2  
Old 07-08-2003, 08:49 AM
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Gary@Tirerack Gary@Tirerack is offline
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This is a question that can only *really* be answered correctly by BMW themselves. Mixing a 3/32nds difference in tread depths in the same tire type should not be as significant as mixing completely different treads and compounds. The only statement they tell us is to have all four tires match exactly for size/brand/model.
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Old 07-08-2003, 09:09 AM
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Kaz Kaz is offline
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IIRC, the xi system is fully open-diff, and all axle control is handled through braking using DSC-X. This theoretically means that there should be no mechanical problem with running slightly different size tires (like through normal wear, per your example). I agree with Gary. Given that the tires are all the same make/type/size (i.e. getting rid of any other possible variables), a difference of, say 3/32-5/32 from tread depth shouldn't be significant. Condsidering that we know the 2WD DSC system is made to handle the rotational difference of the factory staggered setup (from the 330) with a ~3/10" (over 9/32") tire diameter difference, and that those smaller rear tires wear faster, I think we can make the assumption that diameter differences less than that on DSC-X shouldn't cause it to trigger prematurely.
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Old 07-08-2003, 02:50 PM
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WILLIA///M WILLIA///M is offline
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You can go on the Canadian site and build one with the staggered wheel set up. As Kaz said that combo is ever so slightly different to begin with.

Edit: I didn't notice that you were posting from Prague. You have been able to order them that way in Europe since they were introduced.
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Old 07-08-2003, 05:21 PM
fkafka fkafka is offline
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Thanks for the help and ideas guys. If I learn anything juicy on this subject I'll be sure to post it back to this thread. -F
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  #6  
Old 07-09-2003, 09:00 AM
fkafka fkafka is offline
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Not very juicy, but just for the record...

1. Amazing! One *can* get an xi with staggered tires in Canada.

(aside to BMWNA: if I'd been offered the Canadian smorgasbord of options here in the US, I would have spent an extra $US 4,000 on those high-margin extras)

2. The Canadian staggered sets are 225/45-17 by 245/40-17, so I looked at a number of brands to see what variations one gets between these sizes.

3. Within tire brands, these appear to be very similar in size. Usually the difference is 0.1 to 0.2 inches diameter, and 5 to 10 revolutions/mile.

4. Across tire brands, the variation in similar size tires is somewhat larger. (e.g. compare a 205/50-17 LM-22 to a 205/50-17 Eagle RS-A) Actually that comparison is pretty close in size, but others are 0.3" and 10-15 revs/mile off.

5. Gary's comment about mixing different treads and compounds probably precludes the use of several brands on a day-to-day basis. However, it sounds ok to use a winter tire as a spare briefly if its size is within these other normal variations that BMWCanada allows.

6. Hurrah!
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