BMW's xDrive system powers all the wheels in a 40:60 split between front and rear to give better grip, which in turn improves handling and cornering. It's a simple idea, but to get the best from it power needs to be able to vary between front and back axles, and between the left and right sides of the car.
An electronically controlled gearbox and multi-plate clutch system allows BMW xDrive models to vary the power between the front and back axles. If wheel slip is detected, xDrive reacts in a tenth of a second to redistribute power to the axle with the most grip.
Dynamic Stability Control.
What happens though when powered wheels start to spin? BMW models with xDrive have a range of technologies, such as ABS and Dynamic Stability Control (DSC) that detect and limit wheel slip. Dynamic Stability Control detects a spinning wheel and applies the car's brakes to limit spin.
It also activates if it detects understeer or oversteer, i.e. if the car is not going where the driver is steering, allowing the driver to regain control of the car. It's important to note that DSC doesn't give better cornering, but instead is a safety mechanism, helping the driver regain control in tricky situations.
In addition, some BMW xDrive models are fitted with Dynamic Performance Control. This works in combination with DSC, but rather than braking the spinning wheel it uses a series of electronically controlled clutch plates to speed up the slower moving wheel.
This is known as torque vectoring and give smoother turns; for example on a bend more power is given to the outer wheel, because that wheel will have a grip advantage. Or if understeer is detected, more power will be directed to the inner wheel, so that traction with the road is regained.
There is another technology in BMW’s range that features all-wheel drive. The BMW iPerformance models feature TwinPower Turbo engines driving the front wheels and an electric motor powered by batteries driving the rear wheels.
Different driving requirements get a combination of power sources; for example, short distance city driving can be completed purely electrically, while longer journeys or any speed above 78mph (where permitted) rely on the petrol engine. At lower speeds where a burst of acceleration is required, the electric motor will kick in to support the petrol engine.