From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Strange Hub Problem
Date: 26 Apr 2001 21:46:23 GMT

Ben Coleman writes:

>>> I have a problem with my rear hub, an XT with solid, bolt-up axle.

>>> Recently I adjusted some play out of the bearings and went for a
>>> ride.  About 5 mins into it, the hub seized up.  The freewheel had
>>> unscrewed and bound up the bearings to the point that the axle was
>>> turning and screwing out one side.

>> This sounds like a riddle without enough information to go on.
>> freewheels are notoriously hard to unscrew once they have been ridden
>> a bit.  To unscrew one on a derailleur bicycle is not possible without
>> destroying the derailleur.  You didn't mention backpedaling force or
>> any other symptom that would explain from where the freewheel
>> unscrewing force might have come.

> No riddle and no fun.

> Sorry, my mistake it is a cassette freehub, secured with a 10mm
> allen bolt.

> I had no backpedalling force and the derailleur is fine.


The hollow bolt that attaches the cassette freewheel to the hub will
unscrew when coasting if it is not securely tightened.  This screw
should have a 45 degree taper instead of a square shoulder to help
prevent loosening but Shimano assumed because they didn't come loose
with their test riders that it was not a problem.  If I recall
correctly, this bolt does not transmit torque and relies on initial
tightening only.  Precession from the rotating load tends to unscrew
it if it isn't sufficiently tight.

> The first time, I had to walk the bike home carrying the rear wheel
> as it would not roll.

Yes, when this bolt backs out it loads up the axle bearing with enormous
force, enough to damage the bearing.

> No damage...I checked the hub.  The freehub race has the silver
> plating worn off but no apparent damage.  The left race appears
> lightly worn (if at all).  The hub is 3 years old and the freehub
> about 1 year.

I suspect that under magnification you might see some damage.  In any
case, this force did not do your axle or lock nut threads any good.  I
would expect them to bind when turned by hand.

> No bonus points, just the warm fuzzy feeling of helping someone avoid
> unnecessary equipment replacement.

I haven't looked at one of these in a while so I don't know how that
attachment looks today.  It is a problem that Campagnolo dodged by
putting the hub on one pair of bearings and the freewheel on another,
thereby encouraging axle flex and water intrusion between the two
elements.

> At the moment I assume I must have made a mistake in servicing the
> hub, maybe I didn't fully tighten the freehub.  I still can't work
> out why it unscrewed.

If your roll your finger in a ring made by the thumb and forefinger of
the other hand, you will notice that it wants to rotate opposite to
the progression of the contact point.  This is precession and it
unscrews such attachments, as would left pedals with RH threads or
other such joints that bear radial loads, like the right hand cup of a
BB that works in much the same sense as your hub.

I think they thought a left hand thread would be over kill considering
that it works reasonably well the way it is.

Jobst Brandt      <jbrandt@hpl.hp.com>