# Xdrive - check your staggered rim width



## gjohnsen (Nov 28, 2011)

I have tried several online calculators when deciding what kind of tires to buy. I bought a set of 21" wheels with 9" in front and 10" in the rear. The tires where 245/30-21 and 295/25-21.

The tire/wheel calculators online all showed it to be a perfect match. Excactly the same diameter front and rear. 100% safe for the xdrive in other words. But in reality the difference in diameter where almost 1,3%. Way over the 1% tolerance limit for Xdrive. I searched a lot and found out consensus online was that rim width does not influence tire diameter. In my case this was not true.

I found a almost hidden away wheel calc online. It showed that my front tires should have been mounted on a 8.5" to have the perfect setup. By mounting them on a 9" wheel the calc removes 0.2" / 5mm because of the stretch. So it rides lower.

The rear tires should have been mounted on a 10.5" wheel to have the perfect setup. By mounting them on a 10" wheel the calc adds 0.2"/ 5mm because of tire bulging. So it rides higher. The calc showed a difference that excactly matched my tire diameter. 10mm/1cm difference front to rear. This would probably caused severe problems with Xdrive over time.

Couriosly the original staggered setup for the f10/f06/f12 etc is 8.5" with 245/35-20 in the front and 9" 275/30-20 in the rear. The rear tire bulging because of the smaller than ideal rim width makes the diameter excatly the same. Front and rear. That is why BMW not is using 285/30-20 in the rear that should have been a perfect match.

The link to the calc;

http://paws.kettering.edu/~amazzei/tire_calculator.html


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## wcr3d (Jul 17, 2012)

You do realize that the first number in tire size is width?
Meaning as wheel width goes up so should tire width proportionally.
Otherwise yes, you are distorting the tire.
One of the favorite calculators here is
www.willtheyfit.com


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## Jay Arras (Nov 22, 2002)

gjohnsen said:


> The tire/wheel calculators online all showed it to be a perfect match. Excactly the same diameter front and rear. 100% safe for the xdrive in other words. But in reality the difference in diameter where almost 1,3%. Way over the 1% tolerance limit for Xdrive.


The consensus I've seen is that you're probably okay +/- 3%.


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## gjohnsen (Nov 28, 2011)

wcr3d said:


> You do realize that the first number in tire size is width?
> Meaning as wheel width goes up so should tire width proportionally.
> Otherwise yes, you are distorting the tire.
> One of the favorite calculators here is
> www.willtheyfit.com


Thank you for your reply. The calculator you are referring to also shows that my original setup is OK, and is a perfect match. This calculator shows no change in diameter on a 245/30-21 tire whatever rim width you put on. I testet 8" to 11". Same diameter?

The one Im referring to, is the only calculator Ive seen that adds and retracts with different rim width sizes?


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## gjohnsen (Nov 28, 2011)

Jay Arras said:


> The consensus I've seen is that you're probably okay +/- 3%.


Thank you for your reply. I believe i to be 1%. But Im no expert. Maybe others can chime in and share they knowledge.


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## Autoputzer (Mar 16, 2014)

That original link is interesting, and it cites the origin of the formulas it uses. But, I'm not sure changing the wheel width by a half-inch would change the rolling diameter by almost a half-inch.

Audi specifies a limit of 4/32nds inch (3.2mm) difference in tread depths between tires. That's about a 1% difference in rolling diameter. Subaru specifies 2/32nds inch (1.6mm). Looking through the manual on NewTIS.com, I couldn't find a published limit for BMW. 

The rear tires are going to wear faster. So, with staggered tires and no rotation there's always going to be some difference in the rolling diameters between the front and rear. It looks like the "error" BMW introduced (making the rear tires have a larger rolling diameter) would cancel out the effect of accelerated rear tire wear. The varying weight on the rear axle would also affect rolling diameter.

I guess the moral of this story is to have a square set-up for xDrive and rotate the tires regularly. What's important for xDrive clutch wear is minimizing the difference between the average rear rolling diameter and the average front rolling diameter. 

I measure my tread depths just prior to rotation, using the data to optimize tire pressures for even wear and to identify misalignment early, before significant tire damage occurs. I've started measuring tread depths with a digital machinist's depth gauge. At 10.4k miles, two rotation stints, my difference in average front and rear rolling diameters is 0.05/32nds or 0.4mm. Hubba, hubba. :bigpimp:

The OE Bridgestone's on the X3 are rounded, with the tread depths when new being 9/32nds inch in the inner and outer channels and 10/32nds inch in the two middle channels. I've achieved almost perfectly even tread wear across the tires, at two PSI over what the door jamb decal recommends (measured in the early morning). But, I'm gong to raise the pressures another two PSI to gradually square the tires up, ending up with ideally 3/32nds inch all the way across all four tires when I'm done with them. I'm expecting to get about 55k miles out of the original tires. One more set would get me to 100k miles when I plan to sell the car/truck.


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## gjohnsen (Nov 28, 2011)

I changed the front tires to 255.30/21. According to most tire calculators it should be .7% percent change in diameter between front and rear. A very close match. But since the rear wheels are mounted on a 10" wheel and are bulging, the calculator i mentioned above calculated a identical diameter. Front and rear.

And yes, it was spot on this time also. The wheels have the excact same radius and matched perfectly. Im beginning to believe that this calculator is giving the excact right numbers..

Any thoughts?


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## u.nanimous (Nov 1, 2014)

When choosing tires how did you even come up with putting 295's on 10 inch rims? Obviously there are variations in manufacturers of rims and of tires but generally a 10" rim should use 255-285. Since 9" should use 235-265 and you picked 245 you should have tried finding a match around 265 for the rear (2nd in the recommended range for both).

Interesting that even the calculator you linked says your front tires are too narrow and the rears are too wide for the rim sizes entered. Using that calculator it looks like you should try 245/30-21 fronts and 275/25-21 rears and the radius difference is only 2mm (note: discounttire shows the difference to be 0.36"D=4.6mm).

When dealing with larger wheels (WIDE and low profile) there isn't a lot of room for adjustments and a profile change of just 5 makes a big difference, sometimes too big.

BTW, great calculator find. All our xdrive's have square setups but it will be interesting to see the results we have on our other vehicles with staggered setups.

Sent from the future using Tapatalk downloaded to my neural implant


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## gjohnsen (Nov 28, 2011)

u.nanimous said:


> When choosing tires how did you even come up with putting 295's on 10 inch rims? Obviously there are variations in manufacturers of rims and of tires but generally a 10" rim should use 255-285. Since 9" should use 235-265 and you picked 245 you should have tried finding a match around 265 for the rear (2nd in the recommended range for both).
> 
> Interesting that even the calculator you linked says your front tires are too narrow and the rears are too wide for the rim sizes entered. Using that calculator it looks like you should try 245/30-21 fronts and 275/25-21 rears and the radius difference is only 2mm (note: discounttire shows the difference to be 0.36"D=4.6mm).
> 
> ...


The wheels are Ac Schnitzer 10" wide rims that where delivered with 295.25-21 original. I know that max tire should be 285/25-21 when googling. But Michelin states that this dimension can be mounted on rims ranging from 10" to 11"....So nothing seems written in ink..

BMW has 275/35-20 tire mounted on 9" wide rim on the staggered xdrive setup in the rear original. This is also over the limit of the rim. The widest tire should be 255 on a 9" rim if you Google..But Michelin states that 275/35-20 can be mounted on rims ranging from 9" to 11"..

The perfekt match in my case was to switch the 245 to 255 in front in regard of the width of my rims. They know match perfectly


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