Crank loose/falls off

If your crank arms “flop” with every pedal stroke, or if you can wobble them from side to side, fix them now before they become irreparable.

The first thing to do is to try and re-tighten the crank arm on to the bottom bracket axle. If this doesn’t work – the crank loosens after a short period of time, or the bolt is missing – there are more drastic measures.

Complicating matters further, there are several types of crank arm fixings out there.

The most recent types of bottom brackets and crank arms have external bearings. You can tell whether you have this kind if the parts of the bottom bracket that screw into the frame are quite wide (1cm or wider). Also, quite frequently the bottom bracket axle is either completely hollow (you can see all the way through it) and one of the crank arms clamps on to this axle, or the axle is hollow but with a bolt in just one side.

The clamp-on cranks with hollow bottom bracket axles have a through axle attached to the drive side crank arm, and the non- drive side crank arm has two bolts through it which clamp it on to the other end of this axle. You must loosen these clamp bolts before you can tighten the crank arm back on to the axle using a tensioning bolt – most often this tensioning bolt is tightened with a special tool that does not permanently attach to the bike. If you don’t have it with you, improvise by tapping the bolt around with a screwdriver or similar. Do not over tighten the tensioning bolt. This exists purely to bring the crank arm close to the bottom bracket. Once the tensioning bolt is tight, the clamp bolts must be re-tightened to hold the crank arm in place.

If fully tightening the tensioning bolt has no effect, check that no washers fell off with the crank arm. If necessary, it may be possible to fabricate an extra washer. If one of the clamp bolts is missing, find a replacement – water bottle cage bolts, shifter clamp bolts, shoe cleat bolts all may fit.

The three main types of crank on modern mountain bikes

The other type of hollow bottom bracket axle has a bolt on one side – typically the drive side. The bolt may be hidden behind a dust cap. The axle is permanently attached to the other crank. Fixing these crank sets is very similar to fixing other bolt-on types described below.

If the crank arm has a bolt on both sides that looks like it would thread on to the bottom bracket axle, you have a Square taper, ISIS, CODA, Octalink or similar fixing method. Again, there could well be a plastic or metal dust cap over the crank arm where the bottom bracket axle would poke through, or even another metal piece that lets you see the bolt but not totally remove it.

Initially, just try tightening these bolts. You may need quite a lot of force on the tool you are using, perhaps by using a helper bar. If tightening doesn’t work (or works, but the bolt soon comes loose again), read on…

Square taper bottom brackets have just that – a square, tapered end to the bottom bracket axle. These sometimes wear too small, so try to shim the crank-to-bottom-bracket interface with strips of aluminum can or similar.

For ISIS and other designs where the drive side crank slides on to a splined (wavy) piece of pipe, try some kind of thread lock solution between the bottom bracket and the crank arm.

If the crank arm fell off and you lost the bolt, at least make sure you have the drive side crank attached. To remove the bolt from the non-drive side, you may need a helper bar to get more leverage.

Take the bolt (and the washer behind it) from the non-drive side, and use them to re-attach the drive side crank arm. The non-drive side crank may stay put at least temporarily without a bolt through it, but don’t put pressure on it as you ride, as this will loosen it quickly.

  • If you found a piece of metal over the crank arm bolt which let you see the bolt but not remove it, this is a self extracting bolt assembly. The bolt pushes against the metal piece as it unscrews, forcing the crank arm to come off the bottom bracket axle at the same time. In this situation, you don’t want this to happen, so remove the metal piece first. The metal piece will probably unscrew clockwise (reversed thread), and will either need a 10mm Allen wrench or judicious use of a screwdriver or other pointy object in the lock ring pin holes on its face.

Removing crank arms

In the process of fixing parts of the bike, you may run into the opposite problem – getting the crank arms off. Normally this takes a special tool (or self extracting bolt), but if you remove the bolts and ride (gently) for a bit, the crank arms should loosen up sufficiently to be removed by hand. Only do this as a last resort, as it CAN DAMAGE the crank arms and BB.

A blast from the past

There’s one other type of crank attachment that we haven’t mentioned so far. If the crank arm has a nearly circular piece of metal slid through it, perpendicular to the bottom bracket axle, with a bolt on the other side, you have a Cotter Pin crank.

Old-school cranks used a cotter pin to hold them on

Since this system has not been used on new bikes for at least twenty years, it’s time to upgrade your bike. In the interim, undo the bolt, tap the pin out (leave the bolt on loosely while you start the tapping process, so you don’t hurt the threads), then try to shim the old pin with a strip of aluminum can or water bottle plastic. As a last resort, fashion a new pin from wood, plastic or an Allen wrench, bash it into place really hard, and hope it holds.