Full-time all wheel drive.
Full-time all wheel drive with an epicyclic center differential paired with a Ferguson viscous coupling; Torsen-type rear differential with 5 to 1 wheel torque ratio.
Three differentials are at the heart of the system. Drive to the front wheels is linked through a free-floating differential; drive to the rear wheels is transmitted via a 56/44 front/rear torque-splitting Ferguson viscous-coupling-controlled epicyclic central differential. The real innovation as far as production cars are concerned however, lies between the rear wheels.
The Torsen (torque sensing) rear differential is similar to that found on McLaren Formula 1 cars. The result of combining these differentials in this configuration is an automatic-thinking four-wheel-drive system which requires no manual input from the driver, yet ensures maximum potential traction at any given time.
The Torsen differential is a true 'intelligent' differential in the way it distributes torque. It divides the torque between the two wheels according to the grip available and it does it without ever locking fully; maximum lockup is 70 per cent.
Standard differentials are either free-floating or self-locking. Free-floating systems are good at differentiating between wheel speeds on bends, but always supply the same amount of torque to both wheels. In this situation, however, there is a risk that the wheel with the lighter load (on an incline, for example) or less grip, will lose traction. To counteract this possibility, totally self-locking differentials ensure that both wheels rotate at the same speed but in doing this, prevent free differentiation in cornering, to the detriment of handling and stability.
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Figure: Lancia Delta HF 4wd (84KB)
Figure: Lancia Delta HF Integrale rear Torsen locking differential (lower left), viscous coupling(2) locking planetary gear center differential(1) (lower right). On the top - manually locking rear differential for Lancia Dedra Integrale. (175KB)
Figure: lancia delta hf integrale (346KB)
Figure: lancia delta hf integrale rally (1065KB)
Figure: lancia delta hf integrale rear drive shaft (157KB)