Bike Boo Boos

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.

First, tighten the crank arm 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.

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Further complicating matters is the availability of several types crank arm fixings.

The most recent types of bottom brackets and crank arms have external bearings. This type is easily identified by the fact that the bottom bracket screws into the frame are wide (1cm) or wider. The bottom bracket axle may be completely hollow, with the bolts in one end.

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. Before you can loosen the clamp bolts, the crank arm must be released from the axle. This tensioning bolt is usually tightened with a special tool and does not attach permanently to the bike. If you don’t have it with you, improvise by tapping the bolt around with a screwdriver or similar. Tension bolt should not be tightened too much. This is only to move the crank arm closer to the bracket. The clamp bolts should be tightened once the tensioning bolt is tightened. This will hold the crank arms in place.

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If the tensioning bolt is not tightened to full force, ensure that no washers have fallen off the crank arm. It may be possible to make an additional washer if necessary. Find a replacement for the missing clamp bolt. You may find water bottle cage bolts or shifter clamp bolts as well as shoe cleat bolts.

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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. A dust cap may conceal the bolt. The axle is permanently connected to the crank. These crank sets can be fixed in the same way as other bolt-on types.

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. You may also find a plastic or metal dust cover over the crank arm that would allow you to see the bolt but not remove it completely.

For the first time, tighten these bolts. You may need to apply a lot of force to the tool, so you might consider using a helperbar. 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.

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You should have at least the drive side crank attached in case the crank arm falls off. You may need a helper to remove the bolt from non-drive side.

You will need to remove the bolt (and washer) from the side that is not driving. 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.

  • A self-extinguishing bolt assembly is when you find a piece of metal that covers the crank arm bolt. As it loosens, the bolt pushes against the piece of metal and forces the crank arm off the bottom bracket. In this situation, you don’t want this to happen, so remove the metal piece first. You will need either a 10mm Allen wrench, or judicious use a screwdriver to remove the metal piece from the lock ring pins.

Removing crank arms

In the process of fixing parts of the bike, you may run into the opposite problem – getting the crank arms off. This can be done with a self-extinguishing bolt or a special tool. However, if you ride gently for a while the crank arms will begin to loosen enough to be easily removed by hand. This should only be done as a last resort as it can cause damage to the crank arms or 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.

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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. Make a new pin out of wood, plastic, or an Allen tool, then hammer it into place.

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