Don't kick me another tire query post

[wpoll]

Originally Posted by 80stech

Then what is the XDrive emblem on the the newer cars referring to ?

xDrive, the all-wheel drive system, I guess.

Quite a few sedans etc. also had/have xDrive, in addition to the SAV range.

"iDrive is an in-car communications and entertainment system, used to control most secondary vehicle systems in late-model BMW cars."

https://en.wikipedia.org/wiki/BMW_iDrive

"xDrive is the marketing name for the all-wheel drive system found on various BMW models since 2003. The system uses an electronically actuated clutch-pack differential to vary the torque between the front and rear axles."

https://en.wikipedia.org/wiki/BMW_xDrive

[Bdc101]

I've gone on a bit of a deep dive into the xDrive system now that I sold my 2003 and bought a 2012 --


The xDrive system actually monitors the rolling speed of the wheels and can detect a mismatch in tire diameter. When it is outside of a certain range, it will not lock up the clutches and send any or very much torque to the front. Remember there is no differential in the xDrive cars; the rear driveshaft is rigidly attached to the transmission output shaft, and the "transfer case" device can modulate how much clamping force to apply to the clutch pack in order to try to lock it to the same speed as the rear driveshaft. When it is 100% clamping, it is the same as a locked center differential or what we would normally call "four wheel drive." When it is partly clamped, it can slip and the amount of lockup modulated by the xDrive system controls how much torque to send to the front.



But the main takeaway, contrary to some of the advice given in this thread, is that there is NO center differential and if you have enough difference in rolling circumference from front to rear, you will put your xDrive system into a safe mode.



This link will tell you all you want to know: https://archive.org/details/BMWTechn...p?view=theater

[80stech]

Good point. Really the XDrive "system" includes the front and rear diffs and the ABS brakes, the XDrive transfer case is just part of it. Saying that 100% of the torque can be put to either axle, like some of the literature says, I think is a bit of a stretch though. The pre facelift transfer case must have a mechanical differential though ?? And what is the "safe mode" ??

[BimmerBreaker]

There isn't a safe mode and it will only disable 4WD ("it will not lock up the clutches and send any ... torque to the front") under the conditions listed below:

Originally Posted by BMW

Permanent four-wheel drive is only completely deactivated in three control situations:

• During very tight cornering with low engine torgue to allow speed compensation between the front and rear axles (e.g. parking)

• At speeds > 180 km/h

• When the vehicle dramatically understeers

Regarding tire diameter, they say this:

Originally Posted by BMW

The tire circumference can fluctuate up to 1% or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road.

If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required.

It will still engage 4WD when it detects mismatched tires. Only the specific behavior of how much/when it locks is changed. It would be a major safety issue if it disabled power to the front wheels whenever the tires were mismatched. It will always try to maintain operability of the system, ie. maximizing traction by splitting torque front/rear as required.

The BMW xDrive system also still does not respond well to large variations in tire diameter front and rear. Notice how it implies a difference of 1% is accounted for in the design, but it doesn't describe behavior for >1% variance. They do conclude the tire section with this:

Originally Posted by BMW

For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle.