Audi celebrates its five millionth quattro all-wheel-drive-equipped model - the A6 allroad. The air-suspended Avant is just one of over 140 variants across the Audi range currently sporting the all-weather grip enhancer, which made the pioneering transition from rally stage to road.'SUCCESSFUL HISTORY'Rupert Stadler, chairman of the board of management of Audi said: “Quattro is one of the key pillars of our brand and has been a critical factor in our successful history. The quattro permanent all-wheel drive system makes it possible to directly experience our ‘Vorsprung durch Technik.’”An instant hit at the 1980 Geneva motor show, the 147kW Audi quattro showcased the first mainstream four-wheel-drive system. It triggered the development that has culminated in today’s all-wheel-drive technology.In 2012, more than 43% of the automaker’s customers opted for quattro, and the Neckarsulm plant in Germany - home of quattro GmbH - equipped one car in every two with the technology. A rear axle-mounted multi-plate clutch version of the quattro system currently safely contains models with transversely mounted engines from the Q3 through to the TT, and a further developed version will soon return to the new A3 and 220kW S3. Models with longitudinally mounted engines, including the A4, A5, A6, A7, A8 and Q7, divide their torque across all four wheels by way of a self-locking centre differential, the most advanced version of which is the crown gear differential used by RS 4 Avant, RS 5 Coupe and RS 5 Cabriolet models, and soon to feature in the forthcoming RS 6 Avant and RS 7 Sportback models.The advanced crown gear differential channels power to where it is needed, and away from where it isn’t, for optimal handling composure and agility. Up to 70% of torque can be apportioned to the front axle, and up to 85% channelled to the rear, where in the RS models a sports differential is also on hand to divide power between the rear wheels in the most effective proportions possible.The latest generation R8 incorporates another variation - a viscous coupling capable of diverting between 15 and 30% of torque to the front wheels, depending on conditions. Located on the front axle, the coupling is powered through a cardan shaft that runs to the front from the transmission through the engine’s crankcase.