Loose Bike Crank Arms
Crank arms connect the pedals to the bottom bracket axle and are part of the crankset.
A loose crank arm causes major damage. Crank arms are usually aluminum, and the bottom bracket axle is usually steel. The crank arm is pressed down onto the axle by the axle bolt (or nut). When pedaling, the rider exerts great force on the crank arms. Any looseness in the connection will wallow-out the weaker metal of the crank arm. The only remedy is crank arm replacement.
It is very common for a crank arm to come loose, especially on new bikes. Test often.
Test for Loose Crank Arms
The best test for loose crank arms is to tighten them with a crank arm wrench.
To test for loose crank arms without tools:
1. Hold the bike steady, preferably in a work stand.
2. Take each crank arm in your fingers and try to move it parallel to the plane of the bike. If the crank arm wiggles or moves in any direction other than its intended rotation, either the crank arm is loose, or the bottom bracket is loose.
3. If something is loose, while wiggling the crank arm, observe the bottom bracket axle. Is it staying absolutely still? If there is play in the bottom bracket axle, the bottom bracket needs adjustment or replacement. If there is no play in the bottom bracket axle, the problem is a loose crank arm.
Tighten Loose Crank Arms
To tighten a loose crank arm, you need a crank wrench. Look at the crank bolt or nut. If the bolt fitting is for a hex wrench, the correct tool is a long 8 mm hex wrench--it has to be long because you need leverage. If the bolt fitting is for a socket wrench, you need a thin-walled 14 mm or 15 mm socket wrench.
If you have an integrated-spindle crank, see the section on bottom brackets.
The procedure to tighten a crank arm is:
1. Put the bike in a work stand, or hold it firmly.
2. Pop off the dust covers (if any) over the crank arm bolt or nut.
3. Tighten the crank arm bolt or nut clockwise. Put a lot of torque on it. It is possible to strip these, but the bolt tends to come loose, and it pays to snug it really hard.
4. Test to see if the crank arm wiggles. If it wiggles, and the bottom bracket is not the problem, you've wallowed-out the crank arm surface at the axle, and the crank arm has to be replaced.
5. Replace the dust caps. Check the adjustment of the front derailleur.