BMW get seriously dirty - Page Two
By, Trevor Hedge

The engine crankcases are incredibly short. BMW fitted the clutch directly onto the right hand side of the crankshaft and the engine itself has a quite extreme forward tilt.

These two design features alone have allowed BMW to construct a frame where the distance between the swingarm pivot and steering head is as straight and as short as possible.

The frame tubing is constructed from premium steel tubing designed for crash protection structures and rarely used in motorcycles. There is no cradle forward of the engine which gives the appearance of the engine virtually hanging in mid-air from the upper frame mounts.

As the clutch assembly is mounted directly on the crankshaft it is not subjected to the torque multiplying speed reduction effects of the primary drive and thus can be significantly smaller and lighter. An intermediary shaft transfers the power from the crank to the gearbox.

A swingarm that pivots in the crankcases is certainly not new but BMW’s take on the layout is certainly very different and potentially opens up many benefits. Not only does the swingarm attach to the frame but also directly to the engine and in a very novel twist the swingarm pivots directly through a hollow gearbox output. This eliminates changes in chain length through the compression and rebound movements of the swingarm. This means that chain tensioning devices and routing aids can be eliminated as the required chain sag of the machine is virtually nil. Another positive side effect is reduced stress on the chain as changes in loading through the arc of the swingarm are ruled moot. It also opens up the possibility for the chain drive to be sealed from the elements in some conditions if BMW so desires.

It also allows the engine to reside in a position closer to the centreline of the machine while maintaining the traction benefits of a longer swingarm without the drawbacks of a longer wheelbase. Obviously the technical benefits of the design are theoretically huge, however it is yet to be seen if BMW can capitalise on those benefits without any reliability or servicing drawbacks. This is where the blooding of the prototypes in World Enduro competition will bring unparalleled development benefits.

The forward tilt of the cylinder also opens up a lot of space for very straight inlet tracts and a dual throttle valve system to help with coming exhaust emissions compliance issues in Europe and around the world. It also allows the fuel tank to be placed directly beneath the rider for centralisation of mass and the airbox to reside where the fuel tank normally would with its intake much higher in the machine than generally seen on enduro machines.

BMW claim that their new double overhead cam single is making comparable power to the established stars of the category and also weighs in at competitive numbers.

It is remarkably brave of BMW to test the waters of the prototypes in such a high profile competition as the World Enduro Championship. If the machines fail dismally it will hurt the public image of the brand, prototypes or not. And with proven riders at the helm all performance deficiencies will be immediately attributed to the shortcomings of the machine rather than the pilots. Something tells me they must be quietly confident that they will not be left with egg on their faces and if top ten success is forthcoming in these early stages of development the opposition will be studying their designs with great interest…

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