[tgeho_e92]

XI Staggered Tire Calculation Tables https://www.e90post.com/forums/showthread.php?t=1050762

I have come from this post. The post shows a table that outlines all of the rotating speeds of all the different tire sizes.

Okay, so I bought my car with 235/40 ZR18 on the front and 245/40 ZR18 on the rear. Now, with my very minimal knowledge of AWD drivetrains I understand that different rotating speeds from front to back (if it isn't within the 1.0% threshold) can cause issues for the center differential.

Having said all of that according to the above mentioned post and table my car with the sizes I bought it at is sitting at a 1.2% which is above that threshold. Was the previous owner a dummy that chose looks over research. Is that .2% really that detrimental? Idk anything about nothing. I really don't want to find out that because I didn't reverse this my center diff is shot. Any help please!

[ewicky]

What tire pressures do you run?
Do you have xDelete?
Is the wear on the tires even or uneven? (front vs back)
When was the last time you changed your transfer case fluid? ("center diff fluid")

Buying the right size tires would be the best solution.
xDelete or drive shaft delete (at least in the summer) would be the next best, except then you don't have AWD anymore.
If the backs are well worn and the fronts aren't, then that could help change the tire size that last 0.2%.
If the backs are underinflated and the fronts are overinflated, then that could help change the tire size that last 0.2%.
Finally, and this is always a good thing, change the transfer case fluid. If the thing is going to be slipping more than normal, you definitely want it chock-full of synthetic friction modifiers in modern GL-4 fluid. The fluid in the transfer case is meant to be sacrificial. It wears out instead of the transfer case components wearing out. If the fluid is all worn out, it will start accelerating wear of the transfer case, even if you were <1%. So change the fluid anyways asap if you haven't already.

[tgeho_e92]

Originally Posted by ewicky

What tire pressures do you run?
Do you have xDelete?
Is the wear on the tires even or uneven? (front vs back)
When was the last time you changed your transfer case fluid? ("center diff fluid")

Buying the right size tires would be the best solution.
xDelete or drive shaft delete (at least in the summer) would be the next best, except then you don't have AWD anymore.
If the backs are well worn and the fronts aren't, then that could help change the tire size that last 0.2%.
If the backs are underinflated and the fronts are overinflated, then that could help change the tire size that last 0.2%.
Finally, and this is always a good thing, change the transfer case fluid. If the thing is going to be slipping more than normal, you definitely want it chock-full of synthetic friction modifiers in modern GL-4 fluid. The fluid in the transfer case is meant to be sacrificial. It wears out instead of the transfer case components wearing out. If the fluid is all worn out, it will start accelerating wear of the transfer case, even if you were <1%. So change the fluid anyways asap if you haven't already.

Nope, also still need to do a trans fluid replacement (no flush) as well. I run 32psi up front 35psi in back… I mean that's what it says on the door even though I know I'm running aftermarket Enki's that are wider and larger than those POS run flats.

[Brian86]

It'll be fine. BMW's use a wet clutch to push torque to the front axle. It's always slipping. The drivetrain can probably tolerate a much larger differential than the computer will before throwing errors.

[tgeho_e92]

Originally Posted by ewicky

What tire pressures do you run?
Do you have xDelete?
Is the wear on the tires even or uneven? (front vs back)
When was the last time you changed your transfer case fluid? ("center diff fluid")

Buying the right size tires would be the best solution.
xDelete or drive shaft delete (at least in the summer) would be the next best, except then you don't have AWD anymore.
If the backs are well worn and the fronts aren't, then that could help change the tire size that last 0.2%.
If the backs are underinflated and the fronts are overinflated, then that could help change the tire size that last 0.2%.
Finally, and this is always a good thing, change the transfer case fluid. If the thing is going to be slipping more than normal, you definitely want it chock-full of synthetic friction modifiers in modern GL-4 fluid. The fluid in the transfer case is meant to be sacrificial. It wears out instead of the transfer case components wearing out. If the fluid is all worn out, it will start accelerating wear of the transfer case, even if you were <1%. So change the fluid anyways asap if you haven't already.

Also I just checked, it seems to me (just by checking tread with my fingertips) that the rear is ever so slightly worn more than the fronts.

[tgeho_e92]

Originally Posted by Brian86

It'll be fine. BMW's use a wet clutch to push torque to the front axle. It's always slipping. The drivetrain can probably tolerate a much larger differential than the computer will before throwing errors.

Other than a fluid refresh, do you think I'll be fine running this set up? I really like my Enki XM-6s.

[Brian86]

If the computer isn't throwing big red "4x4" error messages in your face, then it's within the engineering tolerances from BMW. The T-case gears the front driveshaft a smidgen taller than the rear. Larger diameter rear tires will decrease this differential a little bit.

It could result in a non-trivial decrease in torque delivered to the front wheels... but I'd place my bets that the driveline map takes relative wheel speed into account when deciding how hard to clamp down on the wet clutch. Can't say for sure, but it wouldn't surprise me if it not only works perfectly, but doesn't add any accelerated wear.

Really I would only expect a problem if the mismatched tires were to perfectly match the front/rear T-case gear ratios. If they were too close, or went the wrong way (larger front tires) the wet clutch might start to chatter in the former, or it would actually apply reverse torque to the front wheels. That said I imagine the computer watches the wheel speed inputs and throws "4x4" errors if it's too far off.

The wet clutches are very robust. Mine had 295k miles on it and worked perfectly as far as I could tell. I swapped it for a low mileage T-case because the front output yoke developed a bunch of slop (I blame a previous owner for not changing a bad front driveshaft U-joint)

Fluid maintenance is a very good idea.

[Cyril Sneer]

Originally Posted by tgeho_e92

Other than a fluid refresh, do you think I'll be fine running this set up? I really like my Enki XM-6s.

If you want to not have to worry about it, you'd only need to replace 2 tires to match the other 2. You can keep the wheels.