T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
FRONT WHEEL BEARING REPLACEMENT TYPE /6 /7HUB
IF YOU TOOK THE BRAKE ROTORS OFF THE WHEEL READ THIS FIRST!
When the front wheel bearings need a change or a check out, it seems handy to take the brake
rotor(s) from the hub first, to have better access for removing the screwed on dust seal home
rings ( type /6 hub ). But you better leave the rotors in place bolted on tight, or make
absolutely sure to remount and torque 'm down before setting the right amount
of preload on the bearings, because the wheel hub is a hollow two piece
assembled body and varies in width at least 0.15+ mm depending
if the rotor bolts are torqued down ... or not. This variation can
ruin the preload setting on the bearings... and will cause
a very loose fit or even worse, free play at the wheel
axle. This is not good for a tactile feel over the
bike's handling and will shorten bearing life
dramatically as well. So be warned !
Only adjust the front wheel bearing preload after tightening the brake rotor bolts first!
Nowhere did I come across info about this fact yet, I have checked
a few hubs. They all showed the same flaw.
Above drawing in which: Light Blue is the wheel axis, Red the axis nut, Dark Blue the
pressure sleeve, Dark Yellow the distance spacers, Light Yellow the bearings,
Grey the hub, White the brake rotor bolts, Brown the brake rotors, and
last of all, Purple.... the wedding ring for setting the amount
of preload on the bearings. Now, grey is hollow !
'' Help tool '' some junkware, for measuring axial play
After installing the above right against the bearing outer rings I was able
to take a reading about of the amount of '' LIFE '' in the hub. An M8
nut travels 1.25 mm over a full rotation. So how much life shows
this hollow hub exactly? It looks solid........... but yet there is
WARNING! A SUBSTANTIAL AMOUNT OF LIFE !
Brake rotor bolts still loose ( this is a / 7 type hub ) Central measuring nut
just touching the '' help tool ''
Bolts tightened to the specified torque. Now the wheel hub becomes about 0.15mm
narrower in width ( over a 12th rotation of the central m8 nut ). Note: the
five brake rotor bolts in this photograph are just for show
and way to short to engage with the nylock nuts.
In the photograph you see the type /7 hub that has the pressed in dust seal caps.
This construction is probably cheaper but also makes it less seductive
to remove the brake rotors first before working on the bearings.
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