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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 09-08-04, 03:47 PM   #1
TR-X
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Derailleur rubbing on the chain ring when I pedal hard

When pedaling lightly no problem. But when I start to put pressure by either going up hill or a sprint they rub. I was thinking it may be the cranks flexing under the stress.
The bike is a 2000 RockHopper pro. Alum frame and cranks
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Old 09-08-04, 03:51 PM   #2
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Yes..
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Old 09-08-04, 08:13 PM   #3
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How would I go about looking for a new set of cranks? What should I look for and what questions should I ask?
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Old 09-08-04, 09:24 PM   #4
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You didn't say what crank you have now! Problem is that any good step up crank for a MTB is expensive like the Shimano XTR at $450; you could try the Shimano XT unit and see if that works. Another problem is that if it's frame flex and you spend money on a new stiffer crank-your still going to have the problem. You should take the bike into your LBS and have it checked to make sure frame flex is not the issue.

But I don't know that much about MTB's so I'm just guessing.
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Old 09-08-04, 10:18 PM   #5
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Did you try adjusting the derailleur... or is it an obscene amount of rubbing?
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Old 09-08-04, 10:46 PM   #6
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Actually I think it's the bottom bracket that's flexing, not the cranks ... or maybe a combination thereof.

I found the new Shimano XT M760 cranks/bb combo set to be extremely stiff ... if there's any flex at all, it's the frame (it's an old Trek 7000 mtb frame). I also weigh about 135 lbs so I'm not that hard on equipment.

Some flex is normal. If it's really bugging you, I'd go for the new set.

I wouldn't use the XTR cranks if you have concerns about longetivity ... the chainrings are all aluminum (for weight savings), so they wear faster.
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Old 09-09-04, 12:09 AM   #7
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It's your chain length or missaligned deralier. Has nothing to do with flex.
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