ENGINE / DRIVE
For the first time in the history of BMW Motorrad, the gearbox and clutch are integrated in the engine housing of a boxer engine. For the first time in a BMW Motorrad boxer engine, the cylinders are no longer separately bolted to the crankcase, but are integrated directly. The dual-section die-cast cylinder crankcase with vertical split level at the centre of the crankshaft is made of highly rigid aluminium alloy. The two case halves form a highly rigid composite consisting of the cylinder and the bearing block for the crankshaft. The cylinder with cooling jacket is designed in open deck form. The liners are coated with a low-wear and low-friction iron-carbon alloy using the new wire arc spray process (LDS) already used for BMW automobile engines.
This provides benefits in particular in terms of weight balance but also in terms of the torsional response. In addition to saving space and weight, the new gearbox means there is no longer any need for the additional transmission oil volume previously required for lubrication. As before, a high-tensile helical gearing system is used for the gearwheels which ensures low running noise and a high level of running smoothness due to smooth tooth engagement. As before, the gear shafts of the new engine are mounted on anti-friction bearings and the gearwheels (idlers) run smoothly on needle roller bearings. Shifting of the gearwheels and therefore of the gears is effected by means of a ball-bearing mounted selector drum made of steel, three case-hardened gearshift forks with hard chrome-plated ends and sliding sleeves with force-fitting connections between the gearwheel pairs.
As the first serial production boxer engine from BMW Motorrad, the drivetrain first used in the R 1200 GS has a multiplate wet clutch with eight friction discs instead of the previous single-disc dry clutch. The benefits of this solution lie in the reduced moment of inertia, on the one hand due to the significantly reduced external diameter of 147 millimetres (R 1200 GS previously: 180 mm), but also in its more compact size. For the first time in conjunction with a boxer engine, the clutch system is fitted with an anti-hopping mechanism. The brake torque of the engine is now passed to the rear wheel at a reduced level during coasting, so that the motorcycle remains stable and safely controllable in the braking phase.