Transfer Case- Service Techniques: Specifications BMW X3 E83 рестайлинг

Control of the locking torque for the multi-plate clutch

The locking torque for the multi-plate clutch in the transfer case can be regulated. This means that the front axle can be smoothly coupled to the drive train. The drive torque at the front axle can be increased or reduced in response to the road situation and conditions.

The DSC control unit calculates the locking torque for the multi-plate clutch as follows

1. Pre-activation = driver's command
2. Driving dynamic control
3. Detection of different tire rolling circumferences

Pre-activation

The pre-activation circuit reflects the driver's wishes. In other words. the pre-activation circuit is used to calculate the necessary locking torque.

The evaluation criteria below are taken into account to in determining the driver's command

1. Accelerator-pedal value
2. Engine torque
3. Engine speed
4. Vehicle road speed
5. Gear engaged
6. Steering angle

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 stabilize it.

The following evaluation criteria are taken into account by the monitoring system

1. Wheel speeds
2. Yaw rate
3. Lateral acceleration
4. Steering angle

The drive torque is distributed as follows in normal driving with all-wheel drive

1. 40 % to the front axle
2. 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 minimizes 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 tire rolling circumferences

If the tire 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

1. When tires of different makes and types are fitted: The tire rolling circumference may fluctuate by up to 1 % due to tires of different makes and types being fitted or the tires are worn very differently.
2. When an emergency wheel is fitted
3. When the tires are worn very differently

Slip in the multi-plate clutch may compensate for differences in rotary speed when different tire rolling circumferences are detected.

Compensation is produced by reducing the locking torque in situations that do not have great driving dynamic control.