Driving Dynamic Control: Notes

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 stabilize 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.

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 input torque.

When driving in a curve, the lateral acceleration causes centrifugal force that pushes the car outwards. 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.