Paralever and cardan
Sporty design with consistent lightweight construction
For opposed twin-engine and K-models, BMW Motorrad consistently relies on the cardan for power transmission. The cardan shaft is a secondary drive between manual gearbox and rear wheel. The cardan shaft is guided on the inside of the paralever system. The paralever with overhead torque support combines sporty design with a consistent lightweight construction. At the same time the diminished weight ensures extremely sensitive response characteristics for the rear wheel suspension due to the low unsuspended masses.
The paralever system, which combines the tasks of the rear suspension and power transmission, eliminates the forces induced by the cardan shaft drive to a great degree, which otherwise lead to the suspension stiffening during acceleration. This upward moment (known as the "rubber cow" effect) is effectively compensated - whereas a conventional swing arm would need a length of more than 1.4 m for this. Essentially it constitutes a parallelogram-type moment strut between the rear axis gear and frame.
With the paralever on the current opposed-twin engine and K-generation, the strut is connected via a joint above the swing arm. One benefit of this design is significantly higher ground clearance in the rear wheel area.
The light alloy housing of the rear wheel gearbox is connected via a joint with the single-arm paralever swing arm, which is also cast from light alloy. This wing arm retains the cardan shaft. An additional universal cardan joint at the pivot point of this link transfers power to the rear-wheel transmission, the housing of which is supported on the frame via a leading link. The cardan shaft is likewise connected with the output shaft of the gearbox via a universal joint.
The paralever kinematics is designed so that no length change occurs, meaning no shift piece is necessary. A torsional vibration damper is integrated into the cardan shaft for greater comfort. At the pivot point on the frame - both on the light alloy frame with the K models and on the steel chassis of the opposed-twin engines, the paralever swing arm is borne on adjustable tapered roller bearings. Depending on the motorcycle type, the bevel gear on the rear axis transfers the drive torque in an appropriate fashion. The larger bevel gear - known as the crown wheel - is borne in a grooved ball bearing on the inside and on the outside in a roll core. The entire drivetrain is supported against the frame via a central suspension strut borne on the paralever arm.
Together with the weight-optimized design of the swing arm and the 50 mm large bore hole in the axle pipe of the drive housing, this results in a significant reduction in unsuspended masses on the rear wheel. The chassis therefore reacts quickly and sensitively to unevenness. The bore hole in the axle pipe also ensures effective heat discharge out of the rear axle housing.
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