Paralever and shaft
BMW Motorrad makes consistent use of the shaft drive for the flat twin boxer and inline 4-cylinder engines. The shaft drive between clutch and rear wheel is characterized by an extremely long service life and unwavering performance. This means that little servicing is required. The cardan shaft rotates inside the paralever system. The EVO paralever with upper torque rod, which is for example used in the R 1200 GS and K 1200 S, combines sporty design with consistent light-weight construction. The weight reduction safeguards extremely sensitive rear suspension response due to the lower unsprung masses.
The paralever system, combining the functions of rear-wheel guidance and power transmission, greatly reduces the forces induced by the shaft drive which would otherwise result in a hardening of the suspension during acceleration. This is effectively eliminated - a conventional swing 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, located underneath the swing arm in the flat twin models up to 1150 cc and the longitudinal 4-cylinder models. In the paralever of the new boxer and 4-cylinder generation the strut is pivoted above the swing arm. One advantage of this new construction is to provide significantly increased ground clearance in the rear wheel area. The rear spring travel of the R 1200 GS for example amounts to 200 mm.
The light alloy casing of the rear axle differential has a jointed connection to the light-alloy-cast single-sided paralever swing arm. At the rotational centre of this joint, an additional universal joint 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 and is done so by means of a universal joint shaft at the swing pivot. Kinematics on the paralever are designed in such a way that there is no change in length and not displacement piece is required. For additional comfort, a torsional damper is integrated within 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 which are maintenance-free and adjustable. Depending on the motorcycle type, the bevel gear at the rear wheel transfers the drive torque in the appropriate manner. The larger bevel wheel - the so-called crown wheel - rests in a grooved ball bearing on the inside and a roller bushing on the outside. The entire drive train, supported against the frame by means of a central strut on the swing arm, is maintenance-free, except for the prescribed oil changes, which are carried out as part of the regular service. For the first time the paralever features a lifetime oil filling for the final drive gearbox in the new boxer and 4-cylinder models.
In the new boxer and 4-cylinder, not only has the paralever construction been revised, but the remaining drive train has also been reduced in weight. The entire unit is much more rigid. The bevel and crown wheel bearing of the rear axle transmission have been further reinforced for the higher performance figures of the new models. All in all, the rear axle transmission is now smaller and lighter due to the lower transmission speed. In conjunction with the weight-optimized swing-arm construction and the 50 mm larger bore of the drive-joint housing, there is a significant reduction of the unsprung masses acting on the rear wheel. The chassis thus responds more swiftly and more sensitively to uneven surfaces. The bore of the axle tube also ensures effective heat dissipation from the rear axle housing.