Symptoms
- Crank arm wobbles or moves slightly when pedaling
- Clicking or knocking noise with each pedal stroke
- Crank arm bolt looks loose or missing its cap
- Problem has happened before and returned
Likely causes
- CommonLoose crank arm retaining boltThis bolt holds the crank arm onto the spindle and can loosen with use, especially on a newly built bike.
- PossibleWorn spindle interface (square taper, splined, or press-fit)Repeated loosening can round out or wear the interface between the crank arm and spindle, especially on square taper designs.
- PossibleIncorrect torque during a previous installationUnder-tightening during assembly is a common cause of early loosening.
What to do
- Remove the dust cap if present to access the crank bolt.
- Check the bolt is present and inspect it for damage or rounding.
- Tighten the crank bolt to the torque specification listed in your bike or crank's manual, since these bolts need to be quite tight.
- Check for wobble again after tightening by pedaling and by grabbing the crank arm and trying to move it side to side.
- If it loosens again quickly, remove the crank arm entirely (a crank puller may be needed) and inspect the spindle interface for wear or rounding.
- Apply fresh grease to the interface before reinstalling if it looks dry.
- If the interface is visibly worn or rounded, the crank arm or spindle likely needs replacement, which a bike shop can assess and perform.
Tools and parts
- Allen key or crank bolt tool
- Torque wrench
- Crank puller (if needed)
Crank armSearch: “bicycle crank arm replacement”
When to call a pro
A bike shop can assess whether a worn spindle interface needs crank or bottom bracket replacement.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.