If your crank arm just keeps on falling off even after you ve asked your muscle building neighbour to tighten that nut as if his life depended on it there is one simple reason.
Pedal arm fell off bike.
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.
If you are using an allen wrench insert the head of the wrench into socket located on the pedal arm.
If you are using a smart wrench place the jaws on the nut located between the pedal and pedal arm.
Remove the pedal from the socket.
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Hi if the pedals arent fully tightened they can fall off.
Loosen the pedals by turning the right pedal counterclockwise and the left pedal clockwise.
The first thing to do is to try and re tighten the crank arm on to the bottom bracket axle.
When the crank arm is loose and you pedal with it you damage the four corners of it s socket that rest in the axle.