I honestly bet it looks as it does, as opposed to a simple, nickel-plated shaft, to emphasize it’s rotation and thus leave no doubt that it IS the shaft itself and not a decorative cover. I know, there’s a huge knuckle of a tripod joint spinning just aft of it. Still, given the my amount of engineering which went into it, probably not a bad idea
[from motorcycle news]
For the R18, BMW have thrown away 40 years of driveshaft tech and started again. To create the classic look of the R18, BMW have fitted a cantilevered rear-end. That means the entire rear-end is fixed, as if like a hardtail, and it pivots around one point at the bottom of the frame – at the top of the cantilever is a rear shock providing 90mm of travel.
"This looks beautiful and classic like the R5," says Sepp Miritsch, Head of Air-Cooled Boxer Series. "However, the knock-on effect of the offset pivot points means that the driveshaft needs to be shorter as the wheel moves through its arc. To get around this we have used a tripod joint – a motorcycle first."
A tripod joint uses a three-pronged yoke attached to the gearbox output shaft with roller bearings on the end. As the suspension compresses and the wheel arcs upwards, the yoke spins at a constant velocity while bearings run in groves in the driveshaft, allowing the yoke to plunge down inside the driveshaft housing.
This enables the driveshaft to automatically adjust its length without any loss of power. The cantilever effect of the rear end also prevents any shaft jacking – clever stuff, eh?
[end quote]
