BMW Airhead Wheel Bearing Replacement.


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.

 

Typical BMW Wheel Bearing

   

To work on the wheel bearings the wheel should be removed. The bike was put on the centre stand and the stand strapped in place A jack was used to lift the back wheel clear of the floor.

The swinging arm clamp bolt and axle nut were removed and axle withdrawn using a bar through the hole in the end of the shaft.

It was also necessary to remove the rear fender and light assembly to get the wheel out.

If the tyre does not fit through the gap between the brake shoes and the swinging arm it can be deflated.


Removing Rear Wheel

 

With the wheel on the bench the bearing cap from the left hand side of the wheel was removed using a 6mm Allen key.

Remove LHS Bearing Cap

 

Then a top-hat spacer and part of the left hand bearing was removed.

It is worth keeping the everything lined up in sequence during disassembly because parts are different between left and right sides.

LHS Bearing Parts Extracted

 

Next, the axle was used to drift the remaining parts out of the wheel.

With the wheel turned over, heat was applied to the hub for a couple of minutes with a propane torch. It was warmed it until water drops splashed on the hub dried off quickly.

Heating the hub

Drifting bearing parts out

 

The bearing parts were layed out on a clean surface as shown. (Click to enlarge)

 

About Taper Roller Bearings

Taper roller bearings work in pairs and are often misunderstood. 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 below shows a simplified taper roller bearing set up (the final drive detail has been omitted.)

Simplified wheel bearing diagram. Click to enlarge.

The same diagram on the right shows the parts 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.

Click to enlarge

The diagram shows that the bearing can be made to run looser by lengthening the inner spacer or shortening the out one. Either change will increase the clearance between the inner and outer bearing races. Conversely the bearing can be tightened by shortening the inner spacer or lengthening the outer one. Although this is true in theory, in practice only the length of the inner spacer is adjusted because changing the outer space changes the location of the bearings in the wheel.

 

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 above 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 was checked on the bench using a suitable spacer as shown. This spacer was an 85mm long piece of 20mm steel tube with ends squared up in the lathe.

The nut was torqued to 50Nm (37lbft).

Bearing Spacer arrowed

 

If the bearing is correctly spaced it should turn freely under these condition 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. However, as mentioned previously the pre-load is likely to be correct if the original BMW settings are still present.

Check for bearing free running

 

A further check was 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.

Checking for any clearance with thinnest feeler gauge

 

Drifting bearing stack in

The bearing stack was re-installed in the wheel by re-heating the hub and tapping them into place. The axle thread was protected by using a socket.

Only light taps should be needed and it is important that the shaft stays vertical to ensure the wheel is not damaged.

    

Both seals were removed from their locations by drifting them out with a socket of the correct size.

The new right hand seal was pushed into place with fingers after the top hat had been put into place first.

Pushing in RHS seal

 

The left hand seal was pushed into the bearing cap on the arbor press using a socket as a drift.

The seal had to be pushed in far enough so that the top hat bush could protrude through.

 

 

 

To complete the job the wheel was replaced. The axle nut was torqued to 50Nm and the clamp bolt to 17Nm to keep the axle secure.

 
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