An epicyclic gear train served as centre differential, splitting the torque between the front and rear axle according to a fixed predetermined ratio—56% front/44% rear on the Prisma 4WD. The ring gear of the epicyclic differential meshed with the gearbox output shaft, its sun gear transmitted torque the front open differential, and its planet carrier, through a pair of bevel gears and a three-piece drive shaft, to the rear differential. A Ferguson coupling controlled the centre differential, preventing excessive relative slippage of the two axles. The epicyclic differential, the Ferguson coupling and the front differential were mounted coaxially in a single transverse unit just behind the engine.