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Crank-Arm seems "bumpy"

Old 11-02-11, 09:48 AM
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Crank-Arm seems "bumpy"

Hi, so I don't know if this is the correct place to post this, but here it goes.

So my crank arm "RPM by FSA ALUMINUM triple 52/42/30T polished black finish" seems bumpy when I am peddling. mostly just on down motions of peddling I was wondering if there would be a fix to this problem or if i just have a low quality crank arm that just does this. thanks.
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Old 11-02-11, 10:40 AM
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bump
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Old 11-02-11, 10:44 AM
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I would guess you need new crank bearings (i.e. bottom bracket), and that is why you get a rough feel when you move the crankarms.
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Old 11-02-11, 11:19 AM
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You need to diagnose it better. Probably one crank arm is loose, or the bottom bracket is loose. Or the crankarm could be cracked near the bolt allowing it to wiggle loose (happened to me once).
Hold the crank at the 3:00 position. Does it wiggle when you try to move it with your hand ? Side to side ? If you feel movement, do you see the other crank arm move too ?

edit: could be a loose pedal too
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Old 11-02-11, 12:38 PM
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moved here from Road
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Old 11-04-11, 02:08 PM
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No it does not wiggle at all, I've checked that thuroughly. I have not checked if the bottom bracket is loose or not though. I hope nothing is cracked as this bike is brand new (got it a few days ago)

p.s thanks for moving it to the right place

Last edited by Nakien; 11-04-11 at 02:09 PM. Reason: A thanks.
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Old 11-04-11, 09:36 PM
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+1 for Homebrew01. It sounds like you have a loose crank arm (either side or both), or a loose pedal. A lot of assemblers don't check the crank bolts, and even if they're tight to begin with, they often wear in a little and become loose in the first few weeks of riding. It's common that you can only feel the "bump" or movement of the crank/pedal when you put a lot of pressure (with your leg on the downstroke, or only when out of the saddle).

Don't ignore this - it will quickly get worse and can destroy the aluminum part pretty quickly.

If it's brand new hopefully you got it at a LBS. Take it to them and describe the problem. If it's a good place they'll tighten it up in a few minutes and you'll be set for a long time.
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