BMW - Paralever and Shaft Drive


BMW Motorrad makes consistent use of the shaft drive for the flat twin and K models. Functioning as a secondary drive, the shaft drive between the manual gearbox and rear wheel is characterised by an extremely long service life and unwavering performance. This means that little servicing is required. The drive shaft rotates inside the paralever system. The paralever with upper torque rod combines sporty design with consistent lightweight construction. The weight reduction in the form of lower unsprung masses safeguards extremely sensitive rear suspension response. The paralever system, combining the functions of rear wheel guidance and power transmission, greatly reduces the forces induced by the shaft drive that would otherwise result in hard suspension during acceleration. This “wheelie” torque is effectively eliminated (a conventional swing arm would need to be more than 1.4 meters long to do the job!). Essentially it acts as a parallelogram type torque reactor strut between the rear axle differential and the frame. In the paralever of the new flat twin and K generation, the strut is pivoted above the swing arm. One advantage of this construction is to provide significantly increased ground clearance in the rear wheel area. The light alloy casing of the rear axle differential has a jointed connection to the cast light alloy, single sided paralever swing arm. It bears the drive shaft. At the rotational centre of this joint, an additional universal joint in the drive shaft transfers the power to the rear axle differential whose casing is supported by a thrust rod on the frame. The shaft itself is also connected to the output shaft of the gearbox by means of a universal joint shaft at the swing pivot. The kinematics of the paralever are such that there is no change in length and no need for a displacement piece. For additional comfort, a torsional damper is integrated in the shaft. At the rotational centre of the frame – both the light alloy frame of the K models and the steel chassis of the flat twin models – the paralever swing arm is supported by tapered roller bearings that are maintenance free and adjustable. Depending on the motorcycle type, the bevel gear at the rear axle transfers the drive torque in the appropriate manner. The larger bevel wheel – the so called crown wheel – lies between a grooved ball bearing on the inside and a roller bushing on the outside. The entire drive train is supported against the frame by means of a central strut mounted on the paralever arm. The drive train in all BMW motorcycles – flat twin models and four cylinder K models – is maintenance free except for the prescribed oil changes, which form part of regular servicing. In conjunction with the weight optimised swing arm design and the 50 mm larger bore in the drive joint housing, there is a significant reduction in the unsprung masses acting on the rear wheel. The chassis thus responds swiftly and sensitively to uneven surfaces. The bore of the axle tube also ensures effective heat dissipation from the rear axle housing.