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Front derailleur rubs against chain once per rotation, bent crankset or warped BB?

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Front derailleur rubs against chain once per rotation, bent crankset or warped BB?

Old 04-30-12, 10:41 PM
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Antron
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Front derailleur rubs against chain once per rotation, bent crankset or warped BB?

Hi, there's a really annoying "screeching" sound that comes from the chain hitting the front derailleur at the same spot in the pedal rotation. When I'm riding and I look down at the gears, it's noticeable that the 3 chainrings up front sway laterally from left to right to left (and so on) while pedaling. Any ideas if it's a bent crankset or broken BB? I'd take a photo, but it's not that easy to see from a picture..

Thanks
-Antron
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Old 04-30-12, 10:45 PM
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AlphaDogg
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Do you hear clicking or feel any movement in the cranks? If so, then it's probably a BB problem. If not, then the chainrings are probably bent. You can align them with a rubber mallet, provided that they are steel.
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Old 04-30-12, 10:49 PM
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What kind of crank and BB?
How old is your setup, and did this develop recently or was it always like that?

A bent spindle would be pretty unusual, generally they just snap when they break.
More probable is a bent section on your chainring, but it's odd that they're all three bent in the same place. Any chance you've got a loose chainring bolt?
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Old 04-30-12, 10:51 PM
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Antron
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Oh really? Yeah I can definitely feel a vibration from the right pedal, plus the clicking sound. Any guesses if the bottom bracket will be noticeably warped or chewed up? I'll get my crankpuller and take a look, will post back with details.
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Old 04-30-12, 11:00 PM
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It is a steel Shimano FC TY33 crankset, nothing too special and I have no idea about the bottom bracket... The setup is under a year old, but now that I think about it, I began hearing this sound almost immediately after it was built up. Both BB and crankset were used when I got it.
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Old 05-01-12, 12:54 AM
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My guess is bent rings or the crank isn't seated properly.
Maybe the square hole is "wallowed out" from over tightening?
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Old 05-01-12, 05:17 AM
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+1 on checking the BB. If that's not the case, and other possibilities are ruled out, it's possible that the chainrings are just slightly off - chainrings are not always perfectly flat.
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Old 05-01-12, 10:45 PM
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Hmm, I actually tried tightening the bolt that holds crankset on and it actually made the problem less noticeable. However, as I tightened it made a pretty horrible high pitched clicking sound. Then as I pulled off the crankset completely, it made that same noise coming off. Maybe I overtightened and warped out the sqaure hole like you said Bill? In that case, new crankset??
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Old 05-01-12, 11:05 PM
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With something mass produced like that, I'd take it entirely apart and check it for burrs and crud that may prevent it from fitting together correctly. Is the square hole dry? There should be a small amount of grease applied there.
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Old 05-02-12, 12:20 AM
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It is most likely a bent crankset. Your crankset has flat steel four arm spider. Most likely what happened was the crankset banged into something which bent the big chainring and the spider along with it. You could probably fix this enough to use it. First remove the crank and remove the chain guard. Now lay down 2 short 2x4s (about 8-12"; you may need 3) and lay the crankset with big chainring on the 2x4's and the crank arm between them. This should allow you to see which spider arm is bent and you can take a rubber hammer and beat on the middle chainring to straighten it out.. Good Luck.
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Old 05-02-12, 12:22 AM
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Originally Posted by WickedThump View Post
Is the square hole dry? There should be a small amount of grease applied there.
Oh no! Let's all just pretend WickedThump didn't post this and move on..
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Old 05-02-12, 05:23 AM
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Originally Posted by onespeedbiker View Post
Oh no! Let's all just pretend WickedThump didn't post this and move on..

Too right! Might as well start a helmet thread.
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