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disc brake squeals when rolled backwards

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disc brake squeals when rolled backwards

Old 04-26-22, 05:25 PM
  #1  
linberl
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disc brake squeals when rolled backwards

and before you say just don't do that, lol, this is on a recumbent trike and I have to "walk in reverse" sometimes to fine tune parking or turn around in tight spaces. What could make the disc only squeal when run backwards but be totally silent forwards? thx
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Old 04-26-22, 09:18 PM
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That would happen on a rim brake bike of mine sometimes, just on a front cantiliever, if there was the slightest drag on the rim. It would only make noise if walking it backwards. Front cantilevers are (were?) known for being noisy--the fork would act as a resonator. The friction coefficient would change very slightly with a change in direction, and would set up a resonance, like a wet finger on a wine glass.

So I'm guessing you have something similar--a slightly dragging pad, or a slightly misadjusted wheel bearing, and there's resonance between the disc, pads, caliper and frame. You can check clearance visually, and it helps to have a white sheet of paper in the background. Hydraulic calipers may not be retracting correctly, or they may be off center. If they're mechanicals, they're easy to adjust.
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Old 04-26-22, 09:29 PM
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Originally Posted by linberl View Post
and before you say just don't do that, lol,
dang
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Old 04-26-22, 09:31 PM
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The caliper should have no way of "knowing" the direction of rotation of the disc. The only thing I can think of is that the hub or frame flexes slightly.
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Old 04-26-22, 09:54 PM
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Brake pad adjustment is indeed directional. Traditional rim brakes require proper toe-in, where the front of the pad hits the rim first, then the rear of the pad under higher braking force. Rolling the wheels backwards and then applying the brakes will cause squeal. It's the same as doing toe-in backwards. You can have it one way or the other, so obviously you adjust so the brakes don't squeal while riding.

Now I'll admit that I don't know if any toe in is designed into disc brakes. I sure haven't noticed any angle at all. And if there were any, it would quickly disappear with brake pad wear. So I assume disc brakes have zero toe in. They probably designed instead to eliminate resonance vibration frequencies that cause squeal. But I could be wrong. But brakes squealing while rolling backwards doesn't come as a surprise to me.
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Old 04-26-22, 10:04 PM
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OK good tips. it's a new trike, so first i will go thru and check the torque on everything up front (tadpole trike). Then if that doesn't solve it, I'll pull the disc brake and clean it and reassemble and check the clearance on adjustment. If that doesn't work, then it's probably beyond my tools and experience, so I'll swing by a lbs for advice. Thanks guys......
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Old 04-26-22, 11:05 PM
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Turn the pads around 180 relative to the caliper (assuming the pads are symmetric) and see if you get the squeal going forward. If so, it is the pads. If not, it is something else.

Is it SRAM? If you turn SRAM brakes through rotors backwards, it produces satanic lyrics. (If you go forward, it makes turkey mating calls.)
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Old 04-27-22, 08:22 AM
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If the pad is squealing in either direction, then by definition it is touching the disc.

I know hardly anything about disc brakes--other than I don't think the pad should be touching the disc at all when not applied.
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Old 04-27-22, 09:05 AM
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Does it squeal in "reverse" if you are going straight back or just when turning?
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Old 04-27-22, 09:26 AM
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Originally Posted by Saudadeii View Post
Does it squeal in "reverse" if you are going straight back or just when turning?
In reverse. when I walk the trike backwards.
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Old 04-27-22, 09:49 AM
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Disc brakes might rub ever so slightly on the rotor when not being activated. So the brake rotor being much like the cymbals on a drum set don't need much to set them vibrating and making noise.

They probably are more susceptible to this when rolling backwards as everything on them has worn-in for going forward. Also, I find that just letting my front wheel turn most of the way to one side while rolling my bike forward or backwards seems to make the rotors produce a harmonic noise.

I think it might be that the hydraulic hose gets a little deformed and pushes the pads out just a tad when the front wheel is turned sharply.
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Old 04-27-22, 09:54 AM
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In my case, the trike is new so nothing has worn in yet. It's had maybe 15 miles on it so far. And it wasn't do this the first few days I had it. I've got a stand coming in a week where I will be able to get it off the ground and spin the wheels and really see what is happening. until then, I think I'll have to live with it.
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Old 04-28-22, 10:24 AM
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One noisy brake or two? Mechanical brakes?
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Old 04-28-22, 06:21 PM
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Considering how much time you are going to be pushing the bike backwards, how much time and work (and risk of degraded braking) is it worth to spend silencing it?
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