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Both my rim brakes --- front and back --- have started to make a periodic noise when I engage them. They will be silent for a little bit, then gradually start to make noise. As I brake, the noise will increase and eventually culminate in a very loud noise, and then be quiet again to restart the cycle. This is definitely independent of speed or how much I've been riding that day. The cycle seems to repeat itself every 1-3 days.

The noise is best described as finger nails on a chalkboard. It is definitely not a squeal.

Why does this happen? I suspect my pads are getting worn, but they are not past the indicator lines yet. The bike is a little over a year old and has gotten "medium" use.

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Can you post a recording of the sound your brakes are making? That might help diagnose the problem. –  Neil Fein Feb 7 '11 at 1:47
    
@neilfein, yeah, I can try with my phone. I will update the question when I do it (next few days). –  carl Feb 7 '11 at 4:15

3 Answers 3

up vote 7 down vote accepted

Sounds that bikes make are difficult to describe. "Fingernails on a chalk board", so sort of a high pitched dry scraping noise, not a squeal, squeak or scream?

That sounds like something ceramic, stone or metal rubbing against the metal rim to me. The last loud noise where it then goes quiet could be whatever it is getting turned just before getting entirely dislodged. Does that sound about right?

I'm not an expert, but my best guesses:

  1. Poorly made brake pads made with chunks of stuff in them.
  2. You're picking up some sort of grit that's getting stuck in the brake pad.

The consistent pattern points much more to the pads being manufactured that way. But I've had similar kinds of sounds when I picked up a bit of sand on the rims.

Brake pads aren't solid rubber/poly-whatever, the're actually a compound, typically with a sort of ceramic or metallic material embedded in the rubber-like material. Poorly made brake pads can have large chunks of that material instead of consistent fine particles.

Either way, when the noise starts, examine the brake pads. You'll probably see something embedded in the pad that you can pry out with something pointy (screwdriver, tip of a knife) or scrape out with a razorblade. It's quite possible it's something the size of a grain of sand, not a huge chunk.

And if it definitely looks like it's something that the pad was manufactured with in it, consider replacing the pads with a different brand; It's likely that your rims are getting worn faster.

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nice answer; I would add that if there are many pieces of rock/grit on a pad, it is sometimes easier to scrape off with a razor blade than to pry them out. –  David Feb 9 '11 at 6:36

I'm reasonably confident that this is just another variation of brake chatter. The pads are generally "toed in" so that the front of the pad contacts the rim first, but as they wear that "toe-in" effect is lost, and you get chatter (which can be anything from a high-pitched squeal to a rasping sound). The chatter is caused by the "heel" of the pad grabbing, rocking the brake arm forward, then releasing.

Do of course check the rims for any sign of gouging, and check the pads for any dirt, etc. (And of course check that the pads are not worn out.) But probably you will find nothing beyond ordinary wear.

You can ignore the sound, or adjust your brake pads to restore a degree of toe-in. Try to avoid too much, though, as it increases (uneven) pad wear and reduces brake effectiveness.

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A potential cause is the alignment of your brake pads. Pads needs to be adjusted and aligned so that the front-edge of the pad hits the rim slightly before the rear-edge of the pad hits--this is called the "toe-in". Improper toe-in results in your brakes very rapidly grabbing and slipping from the rim; this happens fast enough you only hear the effect--and it sounds terrible.

The trick I was shown for adjusting toe-in is to loosen the brake shoe; insert something ~0.5-1.0mm thick under the rear-edge of the brake while holding the brakes down, then re-tightening. A second set of hands can be great for this--but you can also just put a rubber band around the pad and tie a knot in it to keep it in place. Your goal is to have the back of the pad slightly further from the rim from the front.

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You can also use a "third hand". –  Daniel R Hicks Aug 2 '11 at 17:22

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