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In the beginning: the 1977 Volkswagen Iltis, a go-anywhere Sherpa for the German military. The front and rear differentials are open bevel-gear type with a manual engagement of the front axle for snow, slick surfaces, or rough terrain.
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First appearing in 1980, the Quattro Coupe features four-wheel drive that is permanently engaged with open front, center, and rear differentials and a 50/50 front-to-rear dry-road torque split. It solves the Iltis’s problem of dry-road driveline windup and corner crabbing with a simple center differential. A novel hollow transmission-output shaft to the front axle kept the AWD system light and compact.
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For 1986, Audi simplifies the limited-slip arrangement by installing Torsen differentials at the center and rear. A Torsen diff counteracts wheelslip with worm gears that bind when there are speed differences between the input and output. Initially, the torque split is 50/50, but Audi eventually moves to a Torsen differential that allows for a 40/60 front-to-rear torque split.
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In 2010, a new “crown-gear” center differential in the RS5 finally allows variable torque splits, ranging from dry-road 40/60 front-to-rear to as much as an 85-percent rear-torque bias and 70-percent front bias. This differential uses wet clutch plates activated by worm gears to progressively shift torque between the axles.
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Also in ‘10, Audi introduces its torque-vectoring rear differential, which helps turn the car by selectively activating right or left planetary gearsets to step up the differential’s output speed to the outside wheel and “twist” the car in the desired direction. In lieu of this expensive, heavy differential—an option even on the priciest models—Audi employs a brake-based, limited-slip strategy to help induce yaw.
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