Crowning glory of Audi’s all-wheel-drive system
AUDI’S PERMANENT ALL-WHEEL-DRIVE systems were, for many years, built around the Torsen (torque sensing) centre differential. It was the defining quattro system.
When the first RS5 launched in 2010, it introduced a different device for distributing torque, called the Crown Gear Differential. This is a compact unit built into the output end of the gearbox housing, where it splits the drive 40:60 front:rear, but this can automatically adjust to 15:85 or 70:30.
The unit comprises two crown wheels, one taking drive to the front axle and one to the rear, with a set of smaller differential gears between them. Drive from the engine comes into the centre of the unit and is transmitted to the front and rear crown gears via the differential gears. The diameter of the mating faces of each crown wheel to the differential gears gives the steady state-torque split, the rear being the bigger gear by a ratio of 60:40.
On the back of the rear crown wheel is a multi-plate wet clutch, and on the front crown wheel a slimmer version. When one axle loses traction and spins, there will be a difference in the speeds of the two crown wheels and this will cause the differential gears to rotate. This in turn will activate the clutch plates, which will try to match the speeds of the crown wheels, transferring torque to the non-slipping axle.
While the maximum split the unit can give the front or rear is less than 100 per cent, the RS4 can also use its stability control to slow the wheels on the slipping axle, sending even more drive to the axle with grip and increasing the drive on that axle to 100 per cent. Stability control-induced torque vectoring also takes place across the axles in cornering.
Engine V6, 2894cc, twin-turbo Power 444bhp @ 5700-6700rpm Torque 442lb ft @ 1900-5000rpm 0-62mph 4.1sec (claimed) Top speed 155mph (limited) Weight 1790kg (252bhp/ton) Basic price £62,175 evo rating