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M xDrive explained, by the guys who made it work

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Miramas, France – Since the confirmation last week that the sixth-generation BMW M5 would be the first with all-wheel drive, some purists in the cybergarage have complained that an M car with xDrive is a contradiction in terms, that the essence of the brand is its rear-wheel drive.

Well as this video shows, the petrol-heads at the M skunk works in Munich feel exactly the same. The brief from head of M Division Frank van Meel was that the new M5 should drive like a rear-wheel car – only with more traction.

The key to achieving that, according to M driving dynamics development engineer (and motorsport enthusiast) Jorg Weidinger, is that all the systems involved are controlled through a single processor, which collates sensor data from the accelerator pedal position, steering angle, road speed, slip at front and rear axles, longitudinal and transverse acceleration, friction coefficient and yaw rate.

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The processor analyses the data, compares the results with the ‘target response’ – what the car should be doing under these precise circumstances – and then adjusts the multiplate clutch in the transfer case and the M active rear differential to make the car do exactly that.

Which includes transferring just a little too much power to the rear axle under acceleration out of a corner, so that it’s still possible to ‘steer the car with the throttle’.

“This is not necessarily drifting, it’s really about predictability,” Weidinger said. “The idea is that the driver should always be able to clearly feel and control the rear axle.”

Many all-wheel drive cars switch suddenly and unpredictably from oversteer to understeer right at the limit, he said, which doesn’t inspire a lot of confidence. Early versions of the M xDrive also did it, until they set it up to do without that last little bit of traction under some circumstances, to ensure utterly predictably response to the limit and beyond.

What all-wheel drive does is to lay down the maximum available power from the 441kW V8, especially in the lower gears, on wet or loose surfaces – or on very bumpy roads, which have the effect of unloading the driven wheels and breaking traction. By programming the M xDrive to take care of that, the chassis engineers were able to set the suspension up for pinpoint handling without inducing chatter in bumpy corners.

Weidinger explained that when accelerating hard in a straight line, you want all the traction you can get (especially with 700 tyre-shredding Newton-metres under your right foot!), just as you want all the grip you can get under braking for the next corner – that’s why cars have brakes on all four wheels.

Going into a corner you don’t want, or need, any drive to the front wheels – but as you steer out of the corner with your right foot planted, you can transfer longitudinal forces to the front wheels without understeering, as the transverse acceleration gradually falls away – and if you do it gradually you can still get the tail a little hung out, with minimum counter-steering and without having to lift off the accelerator again.

The early versions, he said, battled with understeer under acceleration until they learned that working off throttle position alone wasn’t enough; the torque distribution software also had to take into account the attitude of the car in relation to the direction it was travelling in, and the transverse acceleration, in order to balance the car on the throttle.

IOL Motoring

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