X-drive issues

Doug Huffman said:

The tire CIRCUMFERENCES must closely match, else the clutches are wearing while they are engaged.

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Circumference vs. Height??

SenseiJohn said:

Circumference vs. Height??

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I have created an Excel table to calculate various tire combinations, given the fact that the wheels are staggerd widths. It is almost impossible to find front and back that are really close in height. My original setup had the fronts taller, as I said. 3.5'mm taller. I have found a setup that makes the REAR taller by 2.5mm. Looking at the factory setup for 20" M wheels for X5, they have the REAR taller by 0.5mm, while the standard X5 19" wheels have them with REAR taller by 1.5mm. I just bought NEW rear tires 335/25 -22. Currently using 285/30-22 on front (with Front 3.5mm taller) and I calculated that with 275/30-22 fronts, then the REAR will be 2.5mm taller.. I am thinking that is the best way to go.

Otherwise I find one combination of front and rears that gets it to the same 0.5mm taller REAR, but to do that, I have to go to a 315/35 rear and 275/40 front, and both front and rear wheels will end up about 2" taller (1' more tire height). Not sure that I have enough clearance for that. It appears to be plenty of clearance in rear, but need to see how much up/down travel in front to be sure front tires won't rub anything.

I haven't had transfer case replaced or rebuilt yet. Just trying to plan ahead.
Am I on the right track? Thanks.

SenseiJohn said:

Circumference vs. Height??

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Both. Circumference is pi (3.14149...) times diameter (height).

Actually, the important quantity is "rolling diameter," twice the distance from the hub center to the pavement. That accounts for deformation of the tire sidewalls.

BMW SUV's take odd-ball sized tires, especially with the larger wheel diameters. Tire stores are bad for selling those customers something that's close. Check to see that you have the correct tire sizes. The door jamb decal should have the correct tire size(s) listed.

TIreRack.com says you X5 didn't even offer 22" wheels as an option. So, you have some sort of Frankenstein set-up... the joys of buying a used BMW. What size tires do you have?

Sizes To Fit Your Vehicle (tirerack.com)

The rear tires wear faster than the front ones. So, with a staggered set-up, that diameter/circumference difference can become greater as the tires wear and when you only change two tires at a time.

I ordered Frau Putzer's X3 xDrive with a square set-up and I do a four-tire rotation pattern. After 35k miles, the average front and rear tread depths only differed by 0.17/32" (0.21mm). At the next rotation, the difference will be about the same, but in the opposite direction.

The X3 spends most of its life with the back seat and cargo area empty. So, BMW's 50/50 weight balance is usually maintained. To keep the rolling diameter the same, accounting for sidewall deformation, I keep the front and rear tire pressures the same.