23
E61 Sports Wagon
Tire Tolerance Logic
The tire tolerance logic detects different tread circumferences on the front and rear axles.
This occurs when:
• Mixed tires are used
• Space saving spare tire is installed
• Tires are used that have been worn down to different levels
Normally, tire circumference deviations result in drivetrain torque bias (unwanted variations).
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-con-
trol 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.
For maximum xDrive performance, tires (and wheels) ofthe same diameter should be
installed on the vehicle.
Index Explanation Index Explanation
nVA
Wheel speed at front axle
2
Identical rolling circumference on both axles
nHA
Wheel speed at rear axle
3
Front axle circumference less than rear axle
1
Different rolling circumferences on front axle
4
Rear axle circumference less than front axle
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