Left Crank arm on the Specialized Rockhopper I bought second hand after Christmas keeps working loose and eventually falls off whilst riding. Anyone had this problem before and worked out a cheap fix that doesn’t involve changing the whole set. It’s an FSA COMET crank set. There doesn’t appear to be much damage to the splined shaft, I have tightened the crank arm so you can pick bike up with it and it’s definite on. It just keeps working loose when riding.

Buy a torque wrench, tighten to specified torque (this is pretty f*cking tight usually, to the point that you’ll struggle with an allen key).

Check it after the first couple of rides and keep checking it until it’s not torquing up any more after a ride.

Whenever this happened to me it was worn splines on the crank arm and precession causing the bolts to loosen.

Put loads of threadlock on and tighten to stupidly high torques is the last ‘it’s worth a try’ fix.

Cheers, I’ll have a go at that then. Thanks for quick response.

Wwasawas, thanks also. Off to purchase said items tomorrow.

Once its happened, it’ll happen easier again.
Degrease the splines.
Set your preload
Tighten the allen bolts (i start the sequence with the outboard bolt first)
Once up to torque remove the preload bolt, load it with blue threadlock and reinstall back into the spindle and nip it down tighter than the original preload spec. As you have tighjtened the crank arm, adding more torqu to the preload bolt wont add any more preload to the BB. Careful though as it is only aluminium.
If you’ve a bunch of bagginess between the splines, you can try a shot of red threadlock on the splines themselves.

Count yourself lucky, if you’d bought the De Havilland Comet rather than the FSA one you’d be falling out of the sky.

Thanks folks. Some great advice on here. I’ll get on it tomorrow so I can get out in the afternoon.

I’d love a go in a de Havilland comet though.

Went out again, crank fell off again

You need a new crank arm, if you can’t get just the arm I believe chain reaction have the chain set and bb for £56.

That’s the reality jamesoz. I only get 50p a week spends though

That’s the reality jamesoz. I only get 50p a week spends though

Crank bolt is missing so I’ll get one this week, should sort it out.

If you dont have any luck I have a Deore chainset going spare you can have for a few quid, have chainrngs as well just maybe not all the chainting bolts

Cheers Wiggles, I’ll keep that in mind if I still have an issue after fitting crank bolt. Very kind of you.

Hooray, bike fixed, normality is restored. Going out with little Strawman tomorrow. Have a great weekend y’all.

Went out yesterday, bike held out. Brilliant to be riding steep gnarly singletrack with confidence again. Was my sons first experience of mountain biking, which he loved. Will be back out soon.

🙂