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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 11-06-08, 10:31 AM   #1
Fogghorn
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Diagnostic Help

I have taken up bike commuting this year and am slowly learning my mechanics but this has me stumped and I'm hoping someone can help me out.

The problem: When pedaling I recently/sometimes get a kunka, kunka, kunka sound and vibration through my pedals to my feet.

1. It's intermittent
2. It starts for know reason but will consistently quit (more than 50% of the time) if I drive over a significant bump. But sometimes goes away on it's own without a bump.
3. It's loudest in the small chain ring and the 3rd gear of the rear cog; or the mid and large chain ring and 4th gear of the rear cog (7 on the cog). It gets significantly quieter the further you are away from these combination's.
4. Bike is Davinici Milano
5. Happens only if I pedal
6. Is not consistent through the pedal stoke but not random either. Meaning it doesn't happen at 2 o'clock and 8 o'clock every time. It's more like 2, 7, 12, 5. Sequential but by an odd number increment that I haven't figured out.

I suspect the crank bearings but it doesn't make sense to me that it would be louder in certain gearing then others.

Thank you in advance for your thoughts.
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Old 11-06-08, 01:17 PM   #2
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Bottom bracket bearings would probably make the same noise at the same spot on every crank-revolution. I'd try a new chain first.
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Old 11-06-08, 02:13 PM   #3
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You trying standing to the side of your bike and tugging (not too hard) back and forth on the crankarm. If there is play or clunking then it is probably the BB.

Could be chain or RD but I would start at the BB.
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Old 11-06-08, 02:20 PM   #4
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Quote:
Originally Posted by DannoXYZ View Post
Bottom bracket bearings would probably make the same noise at the same spot on every crank-revolution. I'd try a new chain first.
I'd check the pedals too. Pull on the body of the pedal and see if there is play in the bearings.
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Old 11-06-08, 05:39 PM   #5
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Thanks guys for the thoughts. Appears I've found the answer, on this mornings commute the rear cog fell off. I guess that would do it :-)
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