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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 11-14-10, 10:20 PM   #1
jnewlin
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Rear brake makes a "thump" 1x per revolution

So I got this new wheel. When braking if the rim is cold it seems fine.

If I'm on a descent and using the brake after some time (100ft+) of braking, I start feeling this "thump" every time the wheel goes around. I assume the rim is heating up, and causing some deformation, does that sound correct? It's sort of hard to find since on the work stand it seems perfectly smooth.

Any ideas?

-john
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Old 11-15-10, 02:31 PM   #2
kwoodbury
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Have you checked the rim for abnormalities? Possibly a bulge on the braking surface. Is the wheel true?
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Old 11-15-10, 02:41 PM   #3
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Make sure the pads are low enough to be ONLY in contact with the rim--if one is impinging some very small amount on the tire, a warmer tire could be bulging out just enough and touching the rim for you to be feeling it. As Kwoodbury mentioned above, check both surfaces of the rim. Try running your finger over the surfaces, and also, shine a light down from above; look to see if the rim isn't reflecting light back with the same shine in some spot.
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Old 11-15-10, 03:23 PM   #4
Al1943
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This sort of thump is often associated with the rim joint, sometimes cured with a flat file.
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Old 11-15-10, 03:43 PM   #5
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Quote:
Originally Posted by Al1943 View Post
This sort of thump is often associated with the rim joint, sometimes cured with a flat file.
+1
You'll also probably see some rubber transfer on one side or the other of whatever is causing the thump. Look for the Telltale black (or salmon) transfer and feel that area for any distortion.

You might also wait until it happens, then get off the bike and spin the wheel with the brakes closed just enough to work as feeler gauges, and possibly see the exact cause.
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