Driving Dynamic Control
Driving dynamic control monitors the slip behaviour 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:
- Wheel speeds
- Yaw rate
- Lateral acceleration
- Steering angle
The input torque is distributed as follows in normal driving with all-wheel drive:
- 40 % to the front axle
- 60 % to the rear axle
Distribution of the input 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 input torque.
If, for instance, the front wheels are on a surface that has a high friction coefficient and the rear wheels are on sheet ice, for instance, (lower friction coefficient), then nearly 100 % of the available input torque is conveyed over the front axle. The rear axle is under hardly any load any can support only a low input 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 minimizes the tendency to understeer or oversteer by optimally distributing the driving power between the rear axle and front axle.
Input Torque Distribution During A Tendency To Understeer The multi-plate clutch separates fully during a tendency to understeer. This completely relocates the input torque to the rear axle, and the front axle is relieved from driving forces. Therefore a higher lateral cornering force can be conveyed to the front wheels. The tendency to understeer is reduced.
Input Torque Distribution During A Tendency To Oversteer The multi-plate clutch closes during a tendency to understeer. This relocates the input torque more to the front axle, and the rear axle is relieved from driving forces. Therefore a higher lateral cornering force can be conveyed to the rear wheels. The tendency to oversteer is reduced.