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Old 01-24-09, 06:37 PM   #1
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Disc brake rubbing?

Hi all,

I just got a bike, Mongoose Crossway 450, and the disc brakes seem to have this light rubbing noise, front and back, which sounds kind of like rubbing a piece of paper with your finger. It doesn't seem to be rubbing significantly because as far as I can tell it's not really slowing down the bike appreciably.

Is this normal, or should I take it back to get it looked at? I know that disc brakes on cars typically do rub slightly while rolling, but it seems to me like it would be inefficient for bikes. I just thought I'd check here first before making the trip since everyone seems pretty knowledgeable.

Thanks!
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Old 01-25-09, 12:22 AM   #2
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That's normal.
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Old 01-25-09, 11:30 AM   #3
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+1. If you spin the wheel while holding it off the ground and notice no significant slowing, there is no functional issue. If its just a matter of annoyance, you can either adjust your caliper or ride fast enough that the wind muffles the sound in your ears.
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Old 01-25-09, 02:06 PM   #4
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Well, you can back off one pad or the other so that it doesn't rub against the rotor. If the rotor's a little warped, you'll have to back it off kinda far, causing you to pull more lever when it comes time to use them.

You can get them so that they're silent but with good response, though. If the alignment of the caliper is a little off, the pads will be angled, and effectively have less total clearance than if they were straight. Also, the newer wrench at my LBS used a tool to straighten the rotor, eliminating the slight warp that it had before.

Right now, that bike's disc brakes are simply excellent. Quick stopping, no extra noise, and good modulation. Not bad for mechanical Avids.
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Old 01-25-09, 02:14 PM   #5
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Originally Posted by BarracksSi View Post
Well, you can back off one pad or the other so that it doesn't rub against the rotor. If the rotor's a little warped, you'll have to back it off kinda far, causing you to pull more lever when it comes time to use them.

You can get them so that they're silent but with good response, though. If the alignment of the caliper is a little off, the pads will be angled, and effectively have less total clearance than if they were straight. Also, the newer wrench at my LBS used a tool to straighten the rotor, eliminating the slight warp that it had before.

Right now, that bike's disc brakes are simply excellent. Quick stopping, no extra noise, and good modulation. Not bad for mechanical Avids.
For mechanicals, yes. Hydros, no.
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Old 01-25-09, 02:17 PM   #6
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Yup, true; didn't think about that. The Juicys I have on another bike are even better, at least so far.
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Old 01-25-09, 03:49 PM   #7
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It is actually possible to set up the brake pads so that they're not in contact with the rotor (even with single sided piston calipers). Avids are of course the easiest to adjust, either mechanical cable or hydraulics.

Find the right angle to "peek" at the rotor & pads from the front view or rear view - with the rotor in between you and a bright background, you should be able to see some light come through in between the pads and both sides of the rotor. In a single piston actuated caliper, the "light gap" between the rotor and the fixed pad should be minimal (paper thin)... to prevent too much rotor warp during braking. On twin piston actuated calipers, the "light gap" should be evenly spaced with the rotor bang on the middle.

If the rubbing noise is intermittent, your rotor is off-true. "Peeking the light gap" should show you where the wobble is (place a chalk mark on the tire). If the wobble is small (1mm or less) place that location opposite and 180 degrees away from the caliper, and then give that rotor section a controlled firm push. Try not to touch the rotors with oily fingers, use a clean rag. Recheck with the light peek test, easy does it, little by little. Don't over do it or you could create another opposite wobble and compound your problem.

If you have a major rotor wobble (from something hitting the rotor during in a crash, while riding or while parked) take it to your LBS for assessment. It may need to be replaced.

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