Subject: Loosening Splined Shimano Cranks
From: Jobst Brandt
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 8 mm 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.
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