Chassis Dynamics -- Overview: Specifications BMW X6 E71

Distribution of the Drive Torque (xDrive w/o DPC)

The BMW xDrive all-wheel-drive system regulates the variable distribution of the drive torque between the front and rear axles, resulting in a longitudinal distribution of the drive torque. In the current xDrive generation, the distribution is performed by an adjustable multi-plate clutch. xDrive helps to improve the traction and stability of the vehicle.

In the situation shown, the distribution of the drive torque by xDrive is the standard distribution. 40% is distributed to the front axle and 60% to the rear axle.

The front and rear axle transmissions distribute the drive torque equally to both sides. In terms of driving force, 20% is available at each front wheel and 30% at each rear wheel.

This distribution is appropriate for driving in a straight line.

Because the drive torque is equal at the left and right wheels, any differing friction coefficients on the left and right are not initially factored in (of course, ASC+T will intervene if one or more wheels are threatening to spin as a result).

Scheme 26

LONGITUDINAL DISTRIBUTION OF DRIVE TORQUE BY XDRIVE EXPLANATION CHART

Distribution of the Drive Torque (xDrive w/DPC)

Dynamic Performance Control now also enables infinitely variable distribution of the drive torque between the wheels on the rear axle. Instead of a longitudinal distribution, as provided by xDrive, Dynamic Performance Control provides a lateral distribution of the drive torque. That is why, in technical documentation, the term "rear axle lateral torque distribution" often appears as a synonym for the sales designation "Dynamic Performance Control".

A new rear differential developed on the basis of a conventional differential allows the torque flow to and from the wheels to be controlled.

This means that not only can stability and traction be further increased, but the vehicle's agility can also be improved.

The combination of the xDrive and Dynamic Performance Control systems is able to further improve the vehicle behavior under acceleration and on surfaces with different friction coefficients on the left and right. Lets assume that the right rear wheel is on dry asphalt and the left rear wheel is on snow. In addition to the longitudinal distribution of the drive torque by xDrive, Dynamic Performance Control allows the drive torque to be distributed between the rear wheels. In this situation, the majority of the drive torque will be transmitted to the right rear wheel, because the left rear wheel can only generate relatively little driving force due to the low friction coefficient. This can avoid ASC+T having to intervene by applying the brakes.

The uneven distribution of the torque at the rear wheels has a second effect that is also used by Dynamic Performance Control.

Torque (M) is generated about the vertical axis of the vehicle. This means that the rotational motion of the vehicle about the vertical axis when cornering can be intentionally influenced. You can use this effect to turn the vehicle harder into a corner or to dampen the rotational movement.

Scheme 27

LONGITUDINAL DISTRIBUTION BY XDRIVE, LATERAL DISTRIBUTION EXPLANATION CHART

Fixed Distribution of the Drive Torque with Conventional Rear Differential

A conventional rear differential consists of an angle drive and a differential gear and always distributes the drive torque equally (50:50) to both sides. Different rotational speeds are balanced out.

The different gear ratios for the various vehicle models are achieved by the different number of teeth on the drive pinion and crown gear.

Distribution of the Drive Torque by the Rear Differential with Mechanical Locks (limited-slip differential)

A open/conventional differential has two beneficial features

1. The speed of the drive wheels can differ from each other because of the different distances they cover when cornering.
2. The drive torque is always distributed equally to both drive wheels and does not therefore generate any yawing.

These benefits are counterbalanced by a major disadvantage if the tire-road adhesion is different at the two wheels. The propelling forces that are to be transferred to the road are then limited to the lower of the two potential adhesion levels at the drive wheels.

If the adhesion ratio is unfavorable on one side, it means that a vehicle (without electronic dynamic driving system) would not be able to move off. The drive torque would be converted into useless rotational acceleration for the wheel with the lower potential adhesion level, while the higher potential adhesion of the second drive wheel remains unexploited.

Selectable and self-locking rear differentials are fitted in order to eliminate this disadvantage of the differential gear.

Variable Distribution of the Drive Torque in the Rear Differential with Superimposing Gear Units

The new rear differential with superimposing gear units abolishes this fixed torque distribution by the differential gear. The differential gear is supplemented by a superimposing gear unit on each side. These provide a second additional path along which the drive torque can be transmitted.

In all three load situations

1. traction
2. coasting and
3. overrun

the BMW Dynamic Performance Control system allows the torque to be ideally distributed between the two rear wheels.

A - Torque Transfer Under Traction

The traction load situation means that the engine is generating a positive drive torque. In this respect, the functions of the BMW Dynamic Performance Control hardly differ from those of the competition. The BMW system can transfer up to 1,800 Nm of drive torque from one wheel to the other.

Scheme 28

TORQUE TRANSFER UNDER TRACTION EXPLANATION CHART

The competitive advantage of the BMW system becomes clear in other load situations.

B - Torque Transfer During Coasting

During coasting , there is no drive torque (0 Nm).

When there is power being transmitted between the engine and transmission, this occurs when the engine torque is exactly equal to the loss torques. It also occurs if the driver disconnects the power transmission between the engine and the gearbox (selector lever in the neutral position).

There is then zero torque at the input to the rear differential. In contrast to the competitors' systems, Dynamic Performance Control can also bring about torque transfer in this load situation. If a wheel is to receive positive torque, an equal amount of negative torque will occur at the other wheel.

Scheme 29

TORQUE TRANSFER DURING COASTING EXPLANATION CHART

C - Overrunning: Negative Torque in the Propeller Shaft

If the engine is delivering negative torque, e.g. during overrun fuel cut-off, this is known as overrunning. Even in this case, Dynamic Performance Control allows torque transfer, i.e. a negative torque can also be distributed asymmetrically to the two wheels. The torque transfer can even be increased to such an extent that there is an extremely large negative torque at one wheel and a slight positive torque at the other wheel.

Scheme 30

NEGATIVE TORQUE IN PROPELLER SHAFT EXPLANATION CHART