Accessories Bicycles Parts Specials Tools

Search sheldonbrown.com and sheldonbrown.org

find us on FB

More Articles by Jobst Brandt
Next: Stuck Pedal Removal
Previous: Left Hand Threads

spoke

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.

Jobst Brandt

Spoke Divider

More Articles by Jobst Brandt
Next: Stuck Pedal Removal
Previous: Left Hand Threads

Spoke Divider

Spoke Divider

Articles by Sheldon Brown and others
Harris
Home
Beginners Brakes Commuting
Lights
Cycle-
Computers
Do-It-
Yourself
Essays
Family
Cycling
Fixed Gear
Singlespeed
Frames Gears &
Drivetrain
Bicycle
Humor
Bicycle
Glossary
Bicycle
Links
Old
Bikes
Repair
Tips
Tandems Touring What's
New
Wheels Sheldon
Brown

Accessories Bicycles Parts Specials Tools

Harris Cyclery Home Page

If you would like to make a link or bookmark to this page, the URL is:
http://sheldonbrown.com/brandt/loosening-cranks.html

Last Updated: by John Allen