Originally Posted by
WizardOfBoz
When you say "bolt", singular, I wonder are there two pinch bolts on the crank arm? Do you gradually tighten them both by alternating? Do you use a torque wrench?
Doing all these things SHOULD keep your crank arm from loosening, but you may wish to try threadlocker (use medium, or blue, and definitely not permanent, or red) on the crank arm bolts.
If you've gone through a lot of cycles of your bolts loosening, you may have significant wear on the arm. Have a look, or have your LBS take a look.
Sorry, singular “bolt” was a typo; I just fixed it to read “bolts.” Yes, there are 2 pinch bolts on the crank arm, and I do gradually tighten them by alternating. I haven’t been using a torque wrench since it happens on the road, but can start checking that at home. My crank arm isn’t loosening at all, as far as I can tell. Rather, the preload on the bearing seems to be tightening during each ride. The bike is only 6 months old and I haven’t ridden it much at all, maybe only 200km total. Would LocTite on the preload cap threads stop the problem? I’m reticent to apply it to plastic parts.