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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 01-28-08, 01:38 PM   #1
dai
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Crank Arm Loosening/Causing Resistance

I recently acquired my first fixie, and all was well until my left crank arm started coming loose after around 20 miles. It was wobbling back and forth, so I walked the bike to my LBS and picked up the 8mm allen wrench needed to tighten it up. After tightening it with considerable force, the drivetrain gave me a lot more resistance than usual on the way home. I took the left crank arm off completely and the drivetrain was completely smooth again. As I screwed the left crank arm on again, it stayed smooth until the last half-turn or so, where I really had to apply some pressure with the allen wrench. At that point, the drivetrain starts to show a lot of resistance.

Am I tightening the crank arm down too hard? If I fail to tighten it down very hard (stopping before the drivetrain shows much resistance), the crank arm just gets loose within 10-20 miles to the point of wobbling uncomfortably.

The cranks are sakae signatures on a loose-bearing square taper bottom bracket.
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Old 01-28-08, 01:44 PM   #2
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it sounds like the spindle is too short. is the crank arm hitting the bottom bracket shell and dragging on it with every rotation?
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Old 01-28-08, 01:49 PM   #3
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That certainly could be the case - there is virtually no gap between the crank arm and the adjustable cup of the BB. For kicks, I put a tiny amount of marine lube on the adjustable cup surface then tightened the crank arm down again. This seemed to lessen the resistance, so it looks like the crank arm is indeed dragging on the adjusting cup. What can I do about that?
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Old 01-28-08, 01:52 PM   #4
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you might have deformed the hole in the crankarm.
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Old 01-28-08, 01:52 PM   #5
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You need a BB with a longer spindle
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Old 01-28-08, 01:57 PM   #6
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Quite possibly the spindle as others mentioned, but are you sure the cups are installed correctly? If they're not threaded in far enough, seems to me you could get the symptom you describe. Same thing if your bearings are too large.
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Old 01-28-08, 03:42 PM   #7
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Quote:
Originally Posted by chase. View Post
you might have deformed the hole in the crankarm.
+1 ...that's possibly why you're able to crank it farther in than it's supposed to.

Last edited by roadfix; 01-28-08 at 04:15 PM.
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Old 01-28-08, 04:07 PM   #8
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After pulling the adjustable cup in a quarter turn or so, the resistance is now lower. I pulled the crank arm off and inspected it. Approx. one half of the inside of the crank is worn in a semi-circle pattern, presumably from the cup. I reinstalled the crank arm and, when tight, it is flush with the cup on one side for half of the arm rotation, while the other side has a half a millimeter or so gap. I guess it's just rubbing up ever so slightly against the cup. Is there any easy way to fix that? If not, should I just buy new cranks?
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Old 01-28-08, 05:34 PM   #9
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Your crank arm is toast.

Fortunately, it is the non-drive side, which should be easily replaceable. Ask your LBS if they have any sitting around.
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Old 01-28-08, 05:42 PM   #10
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Yeah, it seems likely that the crank arm got a little loose at some point. Then that little bit of play let the arm rock back and forth, gradually widening the opening for the spindle. You should be able to pull the drive side crank and see the difference in the openings, in which case you need a new crank arm. Keeping the crank bolted tightly in the future should prevent a relapse.
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Old 01-29-08, 04:55 PM   #11
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Thanks for the replies. The hole had gotten widened a slight bit by the previous owner. I replaced my left crankarm today with one from my LBS and voila - problem solved!
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