My dad has a 700c hybrid bike I gave him. It was a cheap Walmart bike that I upgraded many parts on. I didn't change the cranks or bottom bracket. A couple weeks of riding and he tells me that the right crank arm keeps falling off. He tightened the nut over and over but it came loose after a hundred yards of pedaling. He sent me a couple pics. He's not close enough for me to work on the bike myself.
Can you tell if he needs new cranks or if a new axle is needed? It's a fixed cup BB. I repacked it before giving it to him, and it all looked good. I want to order whatever parts he needs and walk him through the repair.
Thanks for any help!
by GTZaskar
4 Comments
Edit. It’s the left crank.
Lubricant the hole in the crank arm and try again. If it fails it’s because the hole shape has been modified. It will always come loose. Get a new one and grease the hole first before filling.
New crank. The axle is significantly harder than the crank. Being ridden loose at all will deform the square taper in the crank to the point where it will never again fit properly and stay in place.
When your dad gets a new crank he needs to make sure it’s the same length as the old one and the square is in the same orientation. The nut should be tightened to about 45 Nm (400 in-lbs) and then left alone.
About 29 foot-pounds of torque are required for initial installation on most square taper crank fixing bolts. That’s a lot of oomph. There’s a good chance that the slap-dash assembly job on that bike didn’t cover that. The arm’s square hole is probably wallowed out and replacing the crank are with something that has the same length and same square orientation with sufficient torque should fix it.