Front and rear wheel bearings on the early airhead BMW's are of the taper roller design. This type of bearing is extremely durable when properly adjusted and maintained.
This document details the replacement of the rear wheel bearings and seals.
To work on the wheel bearings the wheel should be removed.
With the wheel on the bench the bearing cap from the left hand side of the wheel can be removed using a 6mm Allen key.
Then a top-hat spacer and inner part of the left hand bearing can be extracted.
With the wheel turned over, heat should be applied to the hub for a couple of minutes with a propane torch. It was warmed it until water drops splashed on would hub boil off.
Next, the axle can be used to drift the bearing stack out of the wheel.
Bearing parts in order.
Wheel bearing parts
About Taper Roller Bearings
Taper roller bearings work in pairs. The pre-load on them is adjustable so that they can be set to run with minimum friction but with no free play. However this pre-load is not set with the axle nut torque, it is set by selecting the correct spacers between the inner an outer bearing races.
The diagram here shows a simplified taper roller bearing set up (the final drive detail has been omitted.)
The same diagram on the right shows the parts (shown in green) which are clamped by the axle nut.
It can been seen that the axle nut just clamps the inner races to the shaft.
There is a spacer between the races and top-hat spacers on each end to prevent the seals getting squashed. This stack of parts is clamped up against a shoulder on the axle.
The outer races are also held apart by a tubular spacer. The relative lengths of the inner and outer spacers is what sets the bearing pre-load when the axle nut is fully tightened.
The diagram shows that the bearing can be made to run looser by lengthening the inner spacer. This will increase the clearance between the inner and outer bearing races. Conversely the bearing can be tightened by shortening the inner spacer.
The bearing pre-load will have originally been set by BMW and this is unlikely to need changing unless something else has changed. The small part circled here is known as the "wedding band" and this is the part that can be swapped or machined to set the pre-load. This part will have been selected by BMW to allow the bearing to run freely but with no axial play.
Bearing pre-load can be check by assembling the parts on the axle with a suitable spacer. The exact spacer length is not important, it just enables the axle nut to be used to clamp the assembly together. The nut should be torqued to 50Nm (37lbft) to replicate conditions on the bike.
If the bearing is correctly spaced it should turn freely under these conditions but with no free play axially
Check the bearings are free to spin by hand. If they aren't then the outer spacer will need shortening or the wedding band increasing in thickness to remedy the situation. If the bearing is lose and rattles then the wedding band can be reduced in thickness slightly.
A further check can be made on the bearing outer stack by using a feeler gauge to find any gap between on outer race and the spacer.
If this is possible then the wedding band can be reduced in thickness by the same amount to fix the situation.
The bearing stack can be re-installed in the wheel by re-heating the hub and tapping it into place. The axle thread should be protected by a socket.
Only light taps should be needed and it is important that the shaft stays vertical to ensure the wheel is not damaged.
New seals can be fitted before returning the wheel to the bike.
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