Shaft final drive

Shaft drive replaces the chain with an enclosed drive shaft, the same idea as the propeller shaft in a rear-wheel-drive car. On a bike with a lengthways crankshaft, such as a BMW boxer or a Moto Guzzi twin, the gearbox output already points along the bike; on others a pair of bevel gears first turns the drive through 90 degrees.

• The gearbox output turns a short shaft fitted with a universal joint.
• The joint lets the shaft follow the swingarm as the suspension rises and falls while still turning.
• The shaft runs inside the swingarm tube to a housing at the rear hub.
• There, a small bevel pinion on the end of the shaft meshes with a large crown wheel bolted to the hub, turning the drive through 90 degrees again and providing the final reduction.
• The housing is filled with gear oil and sealed; the oil is changed at long intervals and that is the whole service.

Because the pinion tries to climb the crown wheel under power, a simple shaft drive makes the rear end rise under acceleration and squat on the overrun. BMW's Paralever and Moto Guzzi's CA.R.C. add an extra link to cancel that reaction.

What you notice: complete silence and cleanliness at the rear wheel, no chain lube on tour, and a little extra weight at the back of the bike that you feel only in the most committed riding. Efficiency is slightly lower than a chain and the parts are expensive, so it is found on premium machines: the BMW R 1300 GS, Honda Gold Wing, Triumph Rocket 3 and Moto Guzzi V85 TT.