What is this noise from rear brake disc?

[johnwards]

Hey, so my son's mtb has started making a weird noise from his brakes. He thought it might be the pads so he's changed them, but no difference.

The disc itself visually looks fine, and it has no buckles from what I can tell when I spin the wheel.

Here is the video of the sound on YT, the noise is when he pulls the brake.

 

[si_c]

Sounds like there's no braking material left to me.

Edit to add: Obviously if the pads have been changed then it's not that, should read the post properly. Just a thought - what happens to the rear wheel when the brakes are pulled - is it pulling the wheel towards the caliper? Could be spokes hitting the caliper. Probably need the bike in a stand to get a better idea.

 

[johnwards]

Yeah but it's brand new pads, and it's the same noise as it was before the pads. Would that mean it needs a new disc?

 

[twentysix by twentyfive]

Sounds like something hitting the spokes.

 

[johnwards]

> Sounds like something hitting the spokes.

Only when pulling the brake? Wheel doesn't seem to move, but I'll give it a check.

 

[Brandane]

Had a similar noise from the front disc on my Genesis Croix de Fer... Did the same as OP and changed the pads, made no difference. So changed the disc and problem solved. No idea why; maybe it was worn but still looked ok? I don't have vernier calipers to accurately measure disc thickness, so it was just a guess.

 

[sevenfourate]

Had a similar noise from the front disc on my Genesis Croix de Fer... Did the same as OP and changed the pads, made no difference. So changed the disc and problem solved. No idea why; maybe it was worn but still looked ok? I don't have vernier calipers to accurately measure disc thickness, so it was just a guess.

Not sure if pads on push-bikes run right to the very edge of the discs ? If anything like on a car - you’d expect to be able to feel a lip on the edge of the disc as it wears…..

 

[johnwards]

Yeah disc feels smooth up to the edge, feels the same as the front and the front is fine.

 

[twentysix by twentyfive]

Only when pulling the brake?

That's a possible clue. Does anything move while pulling?

 

[silva]

What about tying the brake lever enough distance towards the handlebar, then bike upside down, then turning wheel with some force, to see what happens?

 

[T4tomo]

the cable routing looks odd, it goes inside rear stay. is the cable twitching and hitting spokes as you brake?

 

[Dadam]

Are you sure it's the brake? Or even the rear wheel. The noise is not in sync with the wheel rotation. Sometimes it's when the white lettering on the tyre is at the top, sometimes when it's at the bottom.
If you say it only makes the noise when he's pulling the brake, is pulling it moving the cable so it rubs on something like the tyre?

It's hard to tell but it looked more in sync with pedal rotation, when his left foot is down, though if it is when pulling the brake he's probably stopping pedalling at those points.

You need to remove the variables, one at a time.

 

[Jody]

Pads are catching the spokes/arms on the disc.

 

[johnwards]

Okay, so what I'm taking from this is that this isn't "usual"

No "oh that will be the disc" simple answers.

It's super hard to track it down as it only happens when riding it, putting it upside down (we don't have a stand) and just pulling the brake it's fine, just stops instantly no noise etc.

So its because of the extra load (weight) it has to deal with.

If I feather it in, just a gentle pull it's fine, I can't feel anything in the handle vibrating. It's only when doing a full stop etc.

Dunno if it helps, but he's been practising his wheelies and it's only started since then. (eg not manuals, ones you kick up with cranking)

 

[SuffolkBlue]

the cable routing looks odd, it goes inside rear stay. is the cable twitching and hitting spokes as you brake?

That's a good spot - definitely something to look at. Cable tie the rear brake cable outer to the rear stay and then see what happens.