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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 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, which is used in the R 1200 GS and K 1200 R for example, 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-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, located underneath the swing arm in the longitudinal 4-cylinder engine of the K 1200 LT. The paralever of the new flat twin 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, effectively preventing bottom-out.

The light alloy casing of the rear axle differential has a jointed connection to the light-alloy-cast single-sided paralever swing arm. It bears the drive shaft. At the rotational centre of this joint, an additional universal joint of 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 designed in such a way that there is no change in length and no 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 axle 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 is supported against the frame by means of a central strut on the swing arm. The drive train in all BMW motorcycles – flat twin models and K 4-cylinder models - is maintenance-free, except for the prescribed oil changes, which are carried out as part of regular servicing. For the first time the paralever features a lifetime oil filling for the final drive gearbox in the new flat twin and 4-cylinder models.

Apart from the paralever itself, the remaining drive train has also been revised and 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. In conjunction with the weight-optimised 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.