X Series E83 X3 2.5Si (N52K) OFFRD
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Yaw rate
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Lateral acceleration
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Steering angle
The drive torque is distributed as follows in normal driving with all-wheel drive:
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40 % to the front axle
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60 % to the rear axle
Distribution of the drive torque is oriented upon the torque that can be supported by each axle. For
instance, if the car is fully accelerated in 1st gear from a standing start the distribution of the dynamic axle-
load creates a higher axle load on the rear axle. Therefore, the rear axle can convey a higher drive torque.
Example: The front wheels are on a high-traction surface. The rear wheels are on, e.g. sheet ice (low
traction). In this case, almost 100 % of the available drive torque would be transmitted to the front axle.
The rear axle is under hardly any load any can support only a low drive torque.
When driving in a curve, the lateral acceleration causes centrifugal force that forces the car to the outside.
The car leaves the stable driving condition when the centrifugal force is stronger than the maximum
possible wheel lateral guiding forces. "Understeer" is the phrase used when the car presses outwards over
the front wheels. Oversteer, on the other hand, is when the rear wheel adhesion becomes lower. The rear
of the car presses outwards.
xDrive minimises the tendency to understeer or oversteer by optimally distributing the driving power
between the rear axle and front axle.
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Detection of different tyre rolling circumferences
If the tyre rolling circumferences are not the same the drive train twists due to different rotary speeds (when
the multi-plate clutch is closed).
Differences in rotational speed can occur as a result of the following influences:
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When tyres of different makes and types are fitted:
The tyre rolling circumference may fluctuate by up to 1 % due to tyres of different makes and types
being fitted or the tyres are worn very differently.
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When an emergency wheel is fitted
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When the tyres are worn very differently
Slip in the multi-plate clutch may compensate for differences in rotary speed when different tyre rolling
circumferences are detected.
Compensation is produced by reducing the locking torque in situations that do not have great driving
dynamic control.
Emergency operation
Driving dynamic control and the ADB-X function are not possible in emergency operation.
The VTG control unit has an integral regulator for emergency control. The regulator provides redundancy for
controlling the multi-plate clutch in the DSC control units.
The regulator is used to attempt to maintain all-wheel drive for as long as possible when the DSC control unit
malfunctions or important sensor signals drop out.
Substitute values are calculated when individual sensor signals drop out. Functions are operated by using the
substitute values until it is no longer possible to control the all-wheel drive effectively. This can lead to the
complete loss of all-wheel drive.
SBT xDrive E53, E70, E83
BMW AG - TIS
12.02.2013 10:08
Issue status (12/2007) Valid only until next DVD is issued
Copyright
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