Dusky
Fixie
Karma: +1/-1
Posts: 46
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« on: March 23, 2009, 04:56:39 PM » |
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Hey guys, my bf's crank kind of just.. fell apart. I haven't looked at it, but he says he had just started pedaling on his fixed gear, paused to rock back and forth a bit, and with an explosive 'ping' the crank fell in half. It looked like someone had taken a hammer to it. He thinks the tension of the chain might have caused it (the chain didn't break or anything), since it gets loose occasionally and does this funny 'pop' up and down motion when he pedals.. what do you guys think? :|
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« Reply #1 on: March 23, 2009, 07:05:46 PM » |
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No, that's not chain tension. That's either metal fatigue and/or a faulty part.
What kind of crank/bottom bracket was he using?
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Matthew Moore
Swing bike
Karma: +6/-6
Posts: 177
"I think, therefore I am"
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« Reply #2 on: March 23, 2009, 07:40:28 PM » |
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wait, the crank broke or the chain ring?
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Dusky
Fixie
Karma: +1/-1
Posts: 46
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« Reply #3 on: March 23, 2009, 09:28:35 PM » |
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I got a look at it: it's the chain ring. It broke into shards, and the main piece that broke off looks twisted. It's a sugino chainring/crank set from Harris Cyclery.
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« Reply #4 on: March 23, 2009, 09:58:12 PM » |
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Do you know if your BF bought a new bottom bracket sized for the sugino crankset when he put the crank on?
If your bottom bracket spindle isn't the right length, it's hard to get a straight chainline, and if your chainline is at an angle, there's going to be some sideways torquing when you pedal, especially if you're skidding / going back and forth.
It's important to have a straight chainline, a) so stuff like this doesn't happen, and b) so you don't throw the chain off when you're riding in traffic. The fact that the chain was popping up and down suggests that this might be the issue.
It looks like the Sugino cranks take a bottom bracket with a 103mm spindle, which is pretty short. If he was using a 107/110/113mm spindle (not uncommon for fixed gear bikes), the chainline would be out of whack, so have him check that before he puts the new chainring on.
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Matthew Moore
Swing bike
Karma: +6/-6
Posts: 177
"I think, therefore I am"
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« Reply #5 on: March 25, 2009, 08:31:37 PM » |
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I broke my chain ring the other day also. Was he missing any chain ring bolts?
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Dusky
Fixie
Karma: +1/-1
Posts: 46
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« Reply #6 on: March 26, 2009, 07:50:46 PM » |
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I'm not sure about missing bolts in the first place, but when the chain ring shattered a bolt or two did get ripped out and go flying.
Also, I think the chainline was as straight as it could get.. he even 'inverses' the chainring to get as straight as possible, though I wonder if that compromises the structural integrity of the ring. It seems even with his new chainring (he got a solid beefy/thick messenger bicycle chain ring to replace the old one) it still does that chain popping every revolution or so. He says he got a bottom bracket specifically for the sugino cranks though he didn't mention numbers.. I wonder if the original chainring was just shoddy.
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« Last Edit: March 26, 2009, 07:55:07 PM by Dusky »
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