AUDI RS4

Crown­ing glory of Audi’s all-wheel-drive sys­tem

Evo - - 4WD MEGATEST -

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

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