Often when I go to school on my bicycle the left crank arm starts to loosen up after one or two really gentle jumps. When this happens I just tighten it with the tool I have in my backpack. It went on like this for almost two weeks after which after only one kilometer of riding it starts to creak very loudly and it also loosens up.

When I go to the repair shop, the worker just tightens it up and says it will work but after only one minute of riding it, it loosens up again. Please tell me how to fix this problem.

It is a square taper crankset where the crank is held on by a nut.

  • 1
    This question: bicycles.stackexchange.com/questions/38754/…, is not the same, but is an example of when the crank, being ridden loose, wears out the spindle, so even when it is replaced and tightened properly, it still comes loose. Note how the accepted answer recommends replacing the bottom bracket at the same time as the crank in that situation
    – Swifty
    Jul 19 '19 at 11:49

Is the crank arm made of aluminium?

A few years ago, I had a similar problem which turned out to be completely worn / stripped threads in the arm itself, resulting in the crank actually falling off on a trail.

I would first inspect the threads of the arm for any 'crushed' or flattened looking spots. It doesn't take much damage to aluminium threads for the steel bolt to start 'jumping' threads until it has completely worked loose.

  • Thanks for editing and answering my question. Jul 19 '19 at 7:21
  • You are assuming a modern crankset where the left crankarm is tightened with two small screws tangential to the crank spindle, right?
    – Michael
    Jul 19 '19 at 7:23
  • The reason I’m pointing it out is that I’ve never heard of them coming loose. The mechanic should have noticed any stripped threads when tightening to the speced torque.
    – Michael
    Jul 19 '19 at 7:29
  • No it is not a two screw crank arm. Jul 19 '19 at 9:27
  • It is a square taper crank set. Jul 19 '19 at 9:37

I’m assuming you have a square taper bottom bracket + crank arms?

See this answer: Square taper crankset develops play after changing bottom bracket

If you’ve been repeatedly riding with a loose crank arm it could be that it’s now damaged.

You could try tightening with lots of torque, grease on the square taper and threadlocker (e.g. blue Loctite, but in a pinch any kind of glue works) on the threads. A retaining compound (e.g. LOCTITE® 648) on the square taper is even better than grease.

If that doesn’t help, the easiest, surest solution is to replace the bottom bracket and crankset. A completely new Octalink crankset (which doesn’t have this problem) can be had for less than 30€ (+ bottom bracket for <10€).

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