Bike pedal arm repeatedly coming off

Aubrey_Thicket

Hi all

My sons bike has a problem with the pedal arm coming off all the time. I have looked at it and nothing appears wrong. So, I remove the black circular disk and I can see that the bolt that holds the pedal arm on has worked loose. So I take out my socket set and tighten the bolt up VERY tight. The next day after a bit of ring the bolt has worked loose again and the pedal arm goes limp. Before I take it to the repair shop I wonder if anyone has a suggestion of how I can stop the bolt from working loose.

Thanks.

Aubrey_Thicket

That should say 'after a bit of riding' NOT 'after a bit of ring'

stator

I had this problem. The arm was slightly bent and had to be replaced, I can't remember what the name of the arm is, but you can buy a replacement.

Is it the side with the gears or without? It's easy to replace if it's the side without the gears. It's called the crank arm

marleyboy

When I had a similar issue, it turned out to be the crank shaft had lost some ball bearings and needed replacing, I got a Shimano crankshaft (about £30 inc installation) as it had a lifetime guarantee.

glasgowdan

There are so many different types of bottom bracket, crank and axle that it's impossible for us to offer any suggestions. You mention a black ring, which I may guess is the plastic shimano hollowtech crank preload nut. If it's HT2, then it's possible there are too many bottom bracket spacers on which doesn't leave enough of the axle for the crank arm to grip onto. Or the cranks may simply need replaced. You can get HT2 cranksets (both sides) fairly cheap.

Can you post a good close up photo?

[Deleted User]

From your description it sounds like a square taper crank: the bolt forces the crank onto a tight fitting taper which takes the drive. If you under/over tighten, or ride the bike with the crank loose, you will damage the taper and then once it's damaged it will keep coming loose no matter how many times you refit it.

If the taper is damaged the solution is a new crank, and the way to prevent damage is by tightening to the correct torque, and not riding the bike if it comes loose.

Some cranks are fitted on splines rather than taper, but the advice in that case is the same: it will keep coming loose if it becomes damaged by riding it whilst loose.

Keep_pedalling

A picture would be useful, but it is likely that riding with a lose crank has damaged both the bottom bracket and crank arm so both will need replacing.

GT60

As everyone has said riding with the crank armless will damage the arm square hole and the bottom bracket
No amount of tightening the crack arm bolt will fix the problem
A new bottom bracket and crankle set is needed sorry.
It's an easy repair to do

MX5huggy

The bottom bracket spindle is probably steel and the crank are aluminium as the aluminium is softer than steel I think you will get away with just a new crank as this is what is damaged. Take it to your Local Bike Shop (LBS) not one of the chains Halfords or Evans etc. And LBS fixed a wheel for free when Evans said I needed a new one.

PDF

+1 for if it's a square axle then the bottom bracket won't be damaged but the square taper on the cranks will be, once it works loose and is ridden the cranks arm gets damaged, the same for over tightening it.

You should just be able to get a crank arm to replace it but ensure it is torqued down correctly, maybe put a bit of blue loctite on the bolt.

(All of the above assumes it is the older bottom bracket and not the hollowtech 2)