5 Series E60 M5 (S85) SAL
Driving dynamic control
Driving dynamic control monitors the slip on the front and rear axles. Driving dynamic control has the task of
achieving optimum traction and keeping the car stable or to stabilise it.
The following evaluation criteria are taken into account by the monitoring system:
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Wheel speeds
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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
In emergency running mode, the dynamic driving control and the function of ADB-X are not possible.
SBT xDrive E60, E61, E90, E91, E92
BMW AG - TIS
23.02.2013 19:47
Issue status (12/2007) Valid only until next DVD is issued
Copyright
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