quattro is a permanent all-wheel drive system. To prevent the wheels on one axle from spinning, the drive torque is redirected to the other axle. In the Audi R8, this is performed by a directly responsive multi-plate clutch with optimised control system, which permits variable distribution of the torque between front and rear axle.
Due to the mid-engine layout, quattro has been set up with a heavy rear-axle bias in the R8. The transmission is mounted behind the longitudinally installed engine and has an auxiliary drive for a prop shaft, which directs the power to the front axle. There an electrohydraulic multi-plate clutch distributes the torque. With the basic drivetrain configuration, around 15% of the power is directed to the front wheels and 85% to the rear wheels; if required, up to 30% flows to the front almost instantly. A locking differential at the rear axle further enhances the dynamics. Should a wheel on one axle spin, it is braked by the electronic differential lock (EDL).
Article source: www.audi.com