david58

Well, it didn't totally fall off - I figured out what was going on and was able to get it tightened up so I could make the last mile home.

Noticed it not shifting in the big ring / small cog well in the morning, but then it was ok and I rode on. Coming home yesterday, my left pedal felt funny. Stopped twice to check my cleat, and after restarting the second time I realized the issue was the crank. The left crank had almost worked all the way off the spindle.

What would cause this? Should I be periodically torquing those bolts? Is my bottom bracket shot? I don't know anything about most of the whats, whys, and wheres of bicycle mechanics, and am really hoping this isn't going to be an expensive repair.

HillRider

What could cause a crank arm to work its way off a square taper (I assume it's a square taper, right?) spindle is not having the fastening bolts or nuts installed with sufficient torque initially. The initial torque is very high (30-35 ft-pounds) and much higher than you would possible apply with a standard allen key or short ratchet and socket. Once the crank is installed correctly, retorquing periodically is not only unnecessary but damaging.

It is very likely your bottom bracket is ok since the hard steel spindle is not going to be damaged by the aluminum crankarm but your crank is probably toast. Riding even a short distance with a loose crank will distort the square socket in the crank arm to the point where it will never remain tight again and replacement is the only fix.

I just read your sig line and if the bike in question is a 2011 BMC SR02, it has an external bearing bottom bracket and a two-piece crank. In that case, the pinch bolts on the non-drive-side crank arm must be tightened to spec after the arm is torqued to remove all slack (very little torque) and if the pinch bolts come loose you the crank arm will indeed slip off. The multiple splines may survive this a time or two but the arm has to be inspected to see how damaged it is and, if it's usable, reinstalled properly.

david58

Thanks. This was on my Fuji Cross bike. If I have killed the left crank, I suppose I can find one someplace a bit cheaply. I will call the bike shop and see where they have landed with their assessment. Bike has about 3000 miles on it, funny it would work loose at this point. But there's certainly no way I should have been able to loosen the pinch bolts with my mini-tool.

cny-bikeman

Depending on how much a crank is stressed it may need retightening eventually. The cranks or pedals will often announce the need for retightening by clicking or creaking, especially under high pedal pressure.

HillRider

Torqued properly to spec, I've never had a crank arm (square taper or Octalink) or pedal show any signs of loosening spontaneously and I never retorque them between removals.

cny-bikeman

Lucky you. I was speaking from a mechanic's perspective. One cannot count on all cranks having been torqued properly, and it is simply not credible to me that nobody could stress an aluminum crank on a steel spindle enough to create a need for retightening. I doubt that you stress the crank arms on your bike the way I have seen some riders do.

HillRider

Riding stress isn't the issue no matter how strong the rider is. If you persist in retorquing a square taper crank you will gradually "chase" the arm up the taper until it bottoms out and/or splits. Ask any mechanical engineer about the characteristics of taper fits. Splined bbs like Octalink and ISIS don't have much of a problem with this since the fit isn't tapered nearly as much and they intentionally bottom out on the spindle.

onespeedbiker

With a two piece crank and pinch bolts (FSA Gossamer Cross Compact ?) you probably did not ruin your left crank, but you will need to know how to torque it down as yes, you can easily unscrew loosen the pinch bolts as the torque is only 95-130 inch pounds https://www.fullspeedahead.com/storag...0100128_v4.pdf

waterinfish

yeah, agree it ,Depending on how much a crank is stressed it may need retightening eventually. The cranks or pedals will often announce the need for retightening by clicking or creaking, especially under high pedal pressure.