A shaft drive instead of a chain is still the philosophy for most BMW models due to its reliability. Nevertheless, even this part is subject to WEAR and TEAR due to age. BMW mechanic Julie Breetzke explains the science behind the shaft drive.
The shaft and rear axle drive is constructed in a way that is maintenance free like Carbon fiber motorcycle helmets for the BMW rider. Only during regular inspection service is the oil changed. So, for their boxer and K models, BMW relies on the shaft drive to transmit power. The benefits of this drive concept become clear mile after mile of ownership. With a chain drive for example, regular cleaning and greasing is necessary after a long-ride and perhaps during the ride after a heavy rain. Every 10 to 20,000 miles the chain has to be replaced which can be costly. A shaft drive, however, can hold up for more than 60,000 miles. Performance will always be the same and higher power loads can be transmitted. For these reasons, the shaft drive’s higher weight and price are willingly accepted.
But despite the benefits, the rear axle drive is not free of wear and tear. In drive is connected with both a joint to the single-sided swingarm that accommodates the drive shaft, and parallel arm. The swingarm is mounted to the frame with adjustable and maintenance-free roller bearings. To improve comfort, the shaft drive includes a torsion damper, which cushions the load change. The larger bevel gear is embedded with ball bearings in a roll sleeve. These are the technical requirements so far.
Problems often first appear with the oil sealing rings. Over time the rubber parts of the seal harden or shrink on their sealing surface. Oil mist on the rear axle housing around the wheel indicates a defective radial shaft seal; loss of oil near the wheel hub may suggest the seal in the housing cover. A quick response is needed as the increasing loss of oil may find its way to brakes or tires. Or low oil may damage the complete drive assembly. Discovering oil on the drain plug often means the drain seal was not replaced during the last oil change. Many vehicles with a Paralever swingarms have a small buildup of oil in the rubber boot to the bevel gear. In that case, it’s often not the seal itself that’s defective but the oil finds its way past the threads of the threaded ring. A little bit of sealant can help in this situation. To install a new seal, special tools are required.
Play the final drive can be checked by the rider: Hold the wheel with both hands and try to tilt it. Continue testing the play while turning the wheel 90 degrees. If play is noticed during this test it should be measured exactly at a workshop and a cause found. It could be the final drive bearing in the swingarm. The needle bearing isn’t damaged it can be adjusted, or with four-valve boxers, the floating bearing can be adjusted. If the problem lies with the bearing in the housing cover, notice if the whole axle in the housing tilts and not just the wheel alone. In this case the bearing in the housing cover should be replaced, and possibly also the bearing bushing in the housing.— Uwe Lütjohamm
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