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Octalink BB: Keep tightening left bolt, save a crank?
Hi all:
Without understanding the physics of the situation, I'll accept as correct the assertion that a design defect in the Shimano Octalink BB results in the left crank bolt loosening when riders stand on the pedals with the right foot forward. Eventually this loosening of the bolt can result in the trashing of the crank arm. Apparently this process is different then the "loosening" of crank bolts with square-tapered cranks: in this case the crank arms (both of them) "elbow" their way up the taper within a few miles of riding, relieving pressure on the bolt. The uninitiated say "these bolts have loosened, I'd better tighten them up" which then allows the elbowing to resume, followed by more bolt tightening in a vicious circle until the cranks are destroyed. Going back to the Octalink setup: it's not clear when sufficient relief of bolt pressure starts to result in crank arm destruction. I'd assume that this occurs when the torque falls below the minimum range specified in Shimano's literature. It's also not clear how quickly this loosening occurs, though I'm kind of assuming it doesn't typically happen within a very short time. At least I didn't find a post that said "I put new Octalink cranks on my bike and within an hour of my left crank arm was loose." Anyway, it strikes me that one strategy for getting good life out of an Octalink setup would be to check the torque of the left crank arm after every ride. As long as the bolt torque was above the Shimano minimum you would be assured that the crank arm hasn't been damaged. If the torque was reduced from the torque you initially set upon installation, you'd re-torque to the installed value. Could this work? I don't see why it wouldn't. TIA Tom Young |
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#2
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Octalink BB: Keep tightening left bolt, save a crank?
Tom Young writes:
Without understanding the physics of the situation, I'll accept as correct the assertion that a design defect in the Shimano Octalink BB results in the left crank bolt loosening when riders stand on the pedals with the right foot forward. Eventually this loosening of the bolt can result in the trashing of the crank arm. Apparently this process is different then the "loosening" of crank bolts with square-tapered cranks: in this case the crank arms (both of them) "elbow" their way up the taper within a few miles of riding, relieving pressure on the bolt. The uninitiated say "these bolts have loosened, I'd better tighten them up" which then allows the elbowing to resume, followed by more bolt tightening in a vicious circle until the cranks are destroyed. Going back to the Octalink setup: it's not clear when sufficient relief of bolt pressure starts to result in crank arm destruction. I'd assume that this occurs when the torque falls below the minimum range specified in Shimano's literature. It's also not clear how quickly this loosening occurs, though I'm kind of assuming it doesn't typically happen within a very short time. At least I didn't find a post that said "I put new Octalink cranks on my bike and within an hour of my left crank arm was loose." Crank attachment is not dependent on torque, only position of the retaining bolt. Therefore, loosening causes no damage until the crank and bolt back out far enough that there is insufficient spine engagement with the spindle and the profile in the crank shears off. That is why Shimano made a second version with a deeper spline, erroneously thinking there was insufficient strength in the interface. Anyway, it strikes me that one strategy for getting good life out of an Octalink setup would be to check the torque of the left crank arm after every ride. As long as the bolt torque was above the Shimano minimum you would be assured that the crank arm hasn't been damaged. If the torque was reduced from the torque you initially set upon installation, you'd re-torque to the installed value. Just take up any slack that occurs. Torque, as you see, has no meaning here. I find interesting that the right crank bolt doesn't also unscrew because the spindle has the same torque reversal at both ends. That it doesn't unscrew seems to indicate that its thread tends to tighten the thread similarly to the left hand thread of the right BB cup or left pedal. In Shimano's position, I would have changed the left retaining bolt thread to be opposite from what it is rather than make the spline deeper. Whether that had a positive effect would readily be demonstrated quickly if they tested it with goofy footed riders. Could this work? I don't see why it wouldn't. You could change your leg position when standing to not cause loosening. That may be asking too much but the alternative is a different crank assembly. I'm not sure you can keep after that crank bolt enough to prevent damage. I'm surprised that Shimano did not recognize the problem sooner... or at all. Of course they don't read wreck.bike so where would they discover such things. Jobst Brandt |
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Octalink BB: Keep tightening left bolt, save a crank?
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