Lightweight driveshaft and Paralever swinging arm - an innovative variation on a traditional principle
An indispensable feature of the new R1200GS (as on all BMW Boxers) is the maintenance-free driveshaft extending to the rear wheel. Having proven its merits for more than 80 years, this is the only logical drive concept with the crankshaft in longitudinal arrangement. In practice, this concept means supreme customer benefits - considering that chain drive, despite many improvements over the years, is not appropriate.
Over the years, BMW has consistently improved the driveshaft principle, meeting new requirements and demands in the process. The biggest innovation came in 1988 with the introduction of the Paralever swinging arm on the most powerful enduro at the time, the BMW R100GS. Using an additional pivot between the final drive and the swinging arm, BMW's engineers were able to disconnect the unavoidable reaction of the driveshaft from the swinging arm, thus eliminating the often-criticised torque reaction effect, where the rear end moved up and down when accelerating or braking.
A further advantage provided by the Paralever is that the springs do not harden when accelerating flat-out, providing a significant improvement in traction - an important feature when riding up steep gradients on rough terrain.
Considering the significant benefits of a driveshaft, the higher weight and increase in unsprung masses are immaterial (versus chain drive).
While in the process of developing the R1200GS, BMW's engineers were not satisfied until even this feature (more noticeable when an enduro is used off-road) had ultimately been properly improved. Accordingly, the entire transmission system of the new R1200GS has been redesigned - right through to the rear wheel. The result is a drivetrain that, together with the much stiffer Paralever swinging arm, reduces weight by approximately 10 per cent. Unsprung masses are lower and provide an even more sensitive response on the rear wheel suspension that is already acknowledged for it superior qualities.
The main components of this new system are the axle drive and the swinging arm itself. Made of an ultra-strong cast aluminium alloy, the Paralever swinging arm (through its design and specific dimensions tailored to load conditions) is even lighter than before but also stiffer and stronger.
The geometric configuration of the entire swinging arm has likewise been modified, and dive compensation is now almost 100 per cent. With forces acting on the final drive housing supported above the swinging arm, ground clearance at this point is greater than before, efficiently protecting the swinging arm (even on rough terrain) against damage caused by rocks and similar objects.
The swinging arm is a two-piece forged aluminium unit. Another new feature is the re-configuration of the swinging arm mount, which has been moved from the final drive to the rear frame - again in the interest of extra strength and robustness under the most extreme off-road conditions.
The pivot point of the axle drive unit resting in the swinging arm has been moved down to provide a more favourable geometric arrangement which also incorporates greater stiffness. The bearing point, in turn, has been reinforced to match the higher load and forces from the more powerful engine.
With its improved kinematics, the driveshaft no longer changes in length as a result of geometric conditions. This also helps to save weight, since the length adjustment unit formerly fitted is no longer required. The shaft itself now comes as one single piece and - like before - runs without oil in the shaft sleeve.
The final drive housing is matched in size and dimensions precisely to the inner contours of the angle transmission, avoiding even the slightest waste of space.
The crown wheel has been re-calculated and is also lighter than before; the wheel flange is now made of aluminium in order to save weight. With its larger diameter, the flange offers the wheel better support, with additional weight-saving for the rear wheel (around the hub).
The outer sign of distinction and a visual highlight of this extremely compact and elegant lightweight structure is the 50-mm hole drilled through the axle pipe of the final drive housing. The final drive itself comes with lifetime oil filling and this means that there is no need to change the final drive fluid throughout the entire life of the motorcycle.
In re-configuring the design principles of the entire machine, assembly of the final drive has been simplified and further refinements introduced. The hollow axle sleeve helps dissipate heat from the final drive because of its greater surface area and efficient flow effects. Signals for the ABS brake system and the speedometer are read by a sensor directly in front of segment pieces on the crown wheel - making the outer emitter wheel, used on the former model, superfluous.
The increase in diameter of the drill holes on the wheel flange mean that five light wheel bolts, measuring 10 mm in diameter and with longer expansion length, are used instead of the former relatively heavy wheel bolts with a cone. This means even greater safety when bolting on the wheels and allows use of a smaller wheel bolt wrench (less weight in the toolkit).
New running gear - stiff, higher strength steel tube frame
The R1200GS retains the basic configuration of all Boxer suspension concepts - the engine/transmission unit serves as a load-bearing element and the wheel guidance components are bolted onto the frame. The Telelever longitudinal arm pivots on the engine housing, as before. The actual frame of the R1200GS employs the front subframe, which supports the Telelever at the top, and the rear frame with integrated swinging arm mount.
The engine/gearbox unit is bolted to the rear frame at four points and onto the front frame at two points. This composite structure, incorporating the drive unit, forms the motorcycle's load-bearing framework.
The two frame elements of the R1200GS are lightweight structures in what is called "truss" design: The front frame is a triangular structure made of high-strength steel tubes welded together. It offers the great advantage of highly robust support on off-road terrain, plus package benefits and greater stiffness over the previous cast aluminium front frame. Weight is unchanged.
The rear subframe is also made of straight steel tubes welded together, creating an overall low weight configuration of maximum stiffness and strength. New features are the special mounts for the side-stand and main stand and the rider's footrests, which are connected directly to the rear subframe. The side-stand folds easily and conveniently even when the rider is sitting on the machine.