Date: Fri, 13 Feb 2004 12:07:59 -0800

> My 1 year old XTR crank on XTR splined BB loosens with use.

> After each ride (an hour or so) the crank bolt looses up by about a
> quarter turn.  i.e. - when I tighten the bolt, my 8mm Allen key is
> in the 12 o'clock position, after the ride, it is in the 9 o'clock
> position.

This crank attachment was apparently designed assuming that all riders
lead with the left foot when standing on both pedals, typically over
rough terrain.  Properly tightened retaining bolts can loosen only
with weight on both pedals, right foot forward, because this is the
only condition under which the crank spindle torque reverses.

Reverse torque takes up backlash in the splines and turns the
retaining bolt slightly with each reversal.  Backlash exists because
the splines have no press fit and do not mesh snugly, so they have
backlash at the outset.  Elasticity of the spline teeth add backlash
motion, and when repeated often, can unscrew the retaining bolt.

Shimano, apparently in response to this problem, has a new design for
all its cranks.  These use a straight spline on a hollow spindle onto
which left crank having a split knuckle is secured with two pinch
bolts while the right crank is permanently attached to the spindle.
This is not an entirely new idea but the execution appears promising.
However, its BB bearings are external to the BB shell possibly
presenting a new loosening problem as does the spindle, that has a
free fit in these bearings.

http://tinyurl.com/cdxe


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