Mechanical Help: Left Crank Arm Falling Off

The left crank arm fell off in the middle of an xc race today. It came off about 2 weeks ago for the first time, and the mechanic at the bike shop and I agreed that I should reattach it and see what happens. Obviously something is now wrong. No creaking or noises heard just a wobble seconds before it came off.

Can I solve the issue by getting just a new crank arm, or do I need a new bottom bracket as well? What would cause this part to fail?

It’s a SRAM X01 crankset. The splines on the crank arm do seem to be rounded, and there were plenty of metal shavings inside. I don’t see any X01 replacement left side cranks available on the web. Could I get a GX crank in the same length?

Thanks for any suggestions

Clean that crank arm up and retake the pics. It is possible that the aluminum bolt inside that left crank arm is stripped. I’ve had that happen right before a race. It is replaceable. Looks like the below. I actually keep a spare on hand in case it happens again. Not sure if that is your issue but I’m guessing it is.

https://images.app.goo.gl/bjgCoiovWfMVYiRF8

Haha. If you zoom in on the second pic you can see that the receptacle is filled with aluminum shavings from stripped threads!

I removed the crank bolt and cleaned things up. It appears to me that all the shavings are from the crank bolt threads and not the splines. If I buy a new crank bolt like the one you mention, would this just happen again, or do you think that would be the long term fix? Trying to figure out what caused this in the first place.

There is probably a reason threads are gone, the crank/spindle splines could be worn putting strain on the bolt. Splines should be a fairly tight/snug fit, the bolt there is to merely hold it from sliding off. I think…

Yep that was it. Same thing that happened to me. For my crank it had to do with taking the arm on and off a number of times to re-grease the bottom bracket, change frames, etc. That bolt doesn’t hold up well to being removed and reinstalled multiple times. It is aluminum going into a steel spindle. I would replace it with the new part and ensure you use a torque wrench when installing it to about 29 foot pounds and no more. Then check it before an after your first few rides to see if it has loosened up at all.

The other thing to check is when you have tightened the bolt to the correct torque does the crank spin freely? It should since you’re running the correct bottom bracket. Then hold the other crank arm (drive side) still and see if you’re getting any play between the splines of the spindle and the left arm when the bolt is tight. You shouldn’t be as the tolerances are supposed to be tight when the left arm is pulled onto the spindle by the bolt.

Yep, it was either over/undertorqued and/or cross threaded upon installation/reinstallation. If the threads on the spindle are undamaged just replace

I have an old ZIPP carbon crank set and the left crank arm presents some movement (Play) against the aluminum bushing that goes into the bottom bracket, I have a long endurance race and can’t get the new one on time.
How risky is to loose completely the traction between the carbon and the aluminum bushing?Screen Shot 2021-11-11 at 7.03.14 AM
The movement is about 1mm up-down