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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 08-05-09, 08:56 PM   #1
persplex
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Chain rub on front derailleur

My chain rubs against the front derailleur when I am on my large chainring and smallest cog. But it is not a consistent rub, it only rubs once with each pedal revolution. As if the chainring were bent.

If I set the derailleur with more room, then the chain derails to the outside of the chainring.

The chainring doesn't seem bent (though a few teeth have seen better days) and the problem is definitely worse when pedaling hard or sprinting.

Any advice appreciated!
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Old 08-05-09, 09:09 PM   #2
Al1943
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Hard to say but possibly bent chainring or cracked frame. Check for cracks in the bottom bracket area, especially in the chainstays.

Al
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Old 08-05-09, 11:31 PM   #3
Panthers007
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If the chain goes off the chainring, at very least the set-screw on the FD is out of adjustment. And quite possibly the position of the FD is also not right. So before we blame the chainrings, or something else very expensive to replace - here is the standard cure for an ailing FD (front-derailleur):


Take the cable off the FD. Now install it from scratch as per Park Tool Repair:

http://www.parktool.com/repair/readhowto.asp?id=75

And BicycleTutor:

http://bicycletutor.com/adjust-front-derailer/

When you have it properly trimmed and are ready for the cable - a new one wouldn't hurt - you want to pull the cable taut - not too tight, taut. Then apply 48 to 60 inch-pounds to the pinch-bolt. Then put it through it's paces.

It's always easier and faster to install a FD from square-one, than it is to make adjustments with it already attached. This tends to fix one thing - while throwing another out of kilter. Start fresh. You'll get it.

Adding a barrel-adjuster can solve many problems with the cable coming loose - or being too tight.
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Old 08-06-09, 09:44 AM   #4
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I've had this same problem. It was because the bottom bracket was loose causing the whole crankset to wobble a little. Overhauling the BB tightened things up and the problem went away.
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