355Mono

I am happy with my Fuji 105 Tread overall, but the rear brake has been vibrating under medium to hard pressure on my road rides. The tech at my shop sanded the pads, but the vibration quickly came back. I don't mind the noise, but the vibration to my saddle is really uncomfortable. Do I need a new pad, or rotor? I would imagine the discs are not super high quality on this bike. I'll soon be on my road bike, but enjoy training on the Tread.

Skipjacks

Does the vibration go away for a while after sanding the disc?

If not, the disc could be slightly warped.

If the sanding fixes it even for a mile, it's not a warped disc.

You could try to swap the front and back pads (or the front and bask discs if they are the same size) and see if the vibration moves to the front.

If you swap the pads and the vibration moves to the front, get new pads. If the vibration stays on the rear, try a new disc. They aren't too expensive if you don't go super high end. It's worth a shot.

Swapping the pads might have some other issues associated with it. I'll defer to anyone else with more brake knowledge.

FlashBazbo

But before any of that, I would check to make sure (1) that the disk is seated properly on the wheel, (2) that there isn't any play in the rear axle / quick release, or wheel bearings, and (3) that the rear triangle doesn't have any frame cracks or other issues. Then, swapping parts is the next diagnostic step.

Skipjacks

What he said.

That's a much smarter first step than mine.

FlashBazbo

I don't know about that. I'm just a lazy bike mechanic. I do the stuff that requires no tools first. Yours will likely isolate the problem.

355Mono

Yes, the vibration went away for a month. Maybe pads. I'll re seat the rear quick release wheel too. Thanks.

Skipjacks

Nothing wrong with checking the easy thing first.

There is no point in isolating the problem if you can make the problem go away easily!

If sanding the disk resolves it temporarily, it would indicate that it's not a tightness issue as all the sanding in the world wouldn't compensate for a loose bolt temporarily.

My shot in the dark guess is that new pads would solve this. That the rotor is getting glazed due to the pad compound, so that when the brakes are applied they don't slide smoothly on the rotor. Instead they stick to the rotor, causing the pad to grab unevenly which creates a vibration. The sanding removes the glaze for a while so it all works but the glaze quickly builds back up.

Maybe the front pads cause the same problems but aren't used as much or not pulled with as much pressure so it's not noticed. Or the rear pads are just a factory error.

If you do switch to different pads, be sure to sand the rotor first or whatever is glazing them could cause the same problem.

355Mono

The tech indicated the pad was glazed, and lightly sanded it. That worked for a while, as said. I'll inquire about a new pad. The vibration feels like a hand buzzer toy in my crotch. And, that ain't good.

Skipjacks

Hey...we're not here to judge.