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Every morning, when I ride my bicycle, there is a very loud squeak coming from the brakes. I have an aluminum rim (I think that they are of the sprint type) and rubber pad brakes. A couple of brake events afterward end that and after that the breaks are silent. I suspect it has something to do with the morning plumpness but yet:

  1. Why would a damp rim give a noise under friction?
  2. Why is it so loud? There is no amplification mechanism I can think of in the braking system.

Some photos:

A sketch of what I believe is my rim is.
A braking mechanism, Not mine but quite similar. The rim also looks similar.
A closeup on a brake pad

Photos 1 and 2 are taken from wikipedia, photo 3 is taken from a reparto corse

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A couple bicycle-focused reference links: sheldonbrown.com/rim-brakes.html#noises sheldonbrown.com/brandt/brake-squeal.html –  freiheit Nov 3 '12 at 6:10
    
This question is anecdotal. It is not universally observed that bicycle brakes vibrate or squeak in the morning. Temperature effects could be at play here. Also, surface quality change: suppose that when the bike is not in use overnight, the oxidation layer on the rim thickens by a few angstroms, but is then literally erased by the brake pads. Perhaps, also, the rubber debris which sticks to the rim undergoes some change when it is undisturbed for some hours. –  Kaz Apr 16 '13 at 0:34
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2 Answers

up vote 1 down vote accepted

It's because your brake pads are parallel to the rim. When you brake your pads starts oscillating (forward and backward) and those vibrations are amplified by the rim. Therefore ideal brake setup is to have small angle between pads and the rim.

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Some car brakes also squeak. But with higher sound because smaller disc have higher resonate frequency. –  Crowley Jun 14 '11 at 10:56
    
I think that both this and user599884 answers are the solution to my question, is there any way to give them both a credit? –  Yotam Jun 14 '11 at 11:28
    
This explanation is plainly wrong. –  Georg Jun 14 '11 at 13:02
    
Yotam: You can always vote up other fitting answers to distribute further credit. See physics.stackexchange.com/faq –  Christoph Jun 14 '11 at 17:55
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@Georg: Why plainly wrong? –  Crowley Jun 15 '11 at 7:08
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1.: I figure that the dampness on the rim changes the friction coefficients in such a way that they are right for stick-slip to occur. When the rims are dry after a few braking actions, the parameters change, stick-slip disappears.

2.: The whole rim is your body of resonance.

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Stick-slip is the right direction, only the wiki page lacks the background of negative resistance oscillators. This is main reason for not to "see" the "amplification". –  Georg Jun 14 '11 at 13:10
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