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Old 02-20-2011, 06:24 PM
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This is BMW explanation of the E53's xDrive system:

xDrive: Intelligent grip to match even more power
BMW’s revised X5 has the grip to match the power of its up-rated engines thanks to a new intelligently controlled infinitely variable torque splitting drive train called xDrive.
This system raises the benchmark for on and off-road capability in the luxury market and will also be shared with the 2004 X3 all-wheel drive Sports Activity Vehicle.
The xDrive system delivers improvements in stability, driving pleasure and safety, both on and off-road.
Even though critics around the world hailed the original X5 for its secure and confident handling, its muscular and agile performance on and off road, BMW engineers have devised a new system that is even more refined, more responsive and more controlled, all with the aim of enhancing the driving experience under all road conditions.
The key xDrive benefit is that it ensures drive forces are always delivered to the axle that needs it most – in an instant.
BMW’s xDrive is not simply a reactive system. Using a multitude of inputs from the Dynamic Stability Control system, xDrive is almost clairvoyant in its ability to predict vehicle behaviour and act early to enhance traction at the wheels with most grip, to maintain the vehicle’s balance and control.
The xDrive concept has two key ingredients – a centrally mounted, electronically activated, multi-plate clutch to distribute power between axles, combined with the familiar DSC system to regulate power to individual wheels.
The electronically controlled clutch is able to respond more quickly than conventional four-wheel drive systems that require the build-up of hydraulic pressure to change drive distribution.
It is also fully variable, constantly monitoring and channelling optimum drive forces from axle-to-axle.
DSC sensors constantly monitor individual wheel speed, steering angle, lateral acceleration and yaw rate angles – giving xDrive the predictive information it needs.
In extreme conditions where wheel spin is inevitable, DSC builds on xDrive, acting on individual wheels, cutting power and applying brakes if necessary. Whether the car is used on or off road, new xDrive breeds driver confidence by reducing the risk of understeer or oversteer.
Off-road, on rough tracks or slippery surfaces, xDrive responds in milliseconds, redistributing power from the slipping wheels to those with most grip, just as a rock climber, who senses his feet are about to slip, immediately puts all his energy into bracing himself with his hands.
In normal driving, the multi-plate clutch is able to distribute power from an extreme of 100 percent rear drive (with the clutch fully open) through to 50:50 front-to-rear drive with the clutch completely closed.
In theory however, if the rear wheels were sitting on ice, DSC removes power to the slipping wheels giving virtually 100 percent drive to the front axle.
The new xDrive system reacts in just 100 milliseconds. Put in context, this is just half the time it takes for the pedals to react to driver inputs.
Whenever the car threatens to oversteer, xDrive closes the multi-plate clutch, increasing power to the front wheels and ‘pulling’ the car around the corner.
Equally, where understeer threatens, xDrive reduces the power to the front wheels to a maximum of 100 per cent to the rear if necessary.
BMW did not hesitate to adopt an electronically controlled clutch rather than an electro hydraulic one (as used by some competitors).
The electronically controlled clutch provides instant power transfer in all situations, rather than an electro hydraulic system that requires movement, generating hydraulic pressure before power transfer occurs.
Hence BMW’s xDrive system is far quicker and less ‘reactive’ than competitors.
Simple four-wheel-drive systems use a viscous coupling or a Torsen differential that respond mechanically to a difference in speed and torque between the front and rear axles.
Conventional, all-electronic set-ups require at least one wheel to be spinning for power reduction. Systems using a Haldex coupling use engine control data alongside wheel speed signals.
However, BMW’s xDrive benefits from the full range of DSC information including yaw rate and lateral acceleration, allowing the car to predict the road conditions and react far sooner.
In a first for an all-wheel-drive vehicle, xDrive replicates the function and locking action of a differential with a multi-plate clutch (without resorting to a weighty central differential).
By simply using the clutch, the front and rear wheels can be powered or completely separated if circumstances require, such as in fast, dynamic situations.
On hard throttle acceleration and without any wheel slip being detected, the clutch fully engages, providing a 50:50 power distribution and maximum traction.
Under normal driving conditions, the clutch provides fully variable distribution to front or rear axles.
Transferring power to the front wheels is the responsibility of the two front drive shafts that are of equal length, assisting power distribution and helping eliminate drive train feedback through the steering.
The right hand shaft is mounted in a bearing case, passing through the sump to the opposite hub. This assists the car at parking speeds, where drive to front wheels can compromise maneuverability. In such cases, full power is transferred to the rear wheels
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