My rear disc brake makes noise when I ride fast

Question

I have a full suspension mountain bike with disk brakes. When I ride fast, my rear disk brake starts to make a noise similar to the one you would hear when sharpening a knife. It happens when I don't brake. If I brake, the noise disappears, I only hear the smooth braking sound.

I presume something is wrong with the rear tire - visual inspection when it rotates fast shows that it wobbles. I tried to demount it and mount it back again two times, but no success - still wobbles.

Can the non-aligned rear tire cause by disk brake to be noisy when I ride fast?

Shall I simply buy a new tire, or could the solution be elsewhere?

This is an update - with videos showing the wobbling tire.

https://youtu.be/_9Wor0KFL34

https://youtu.be/Bju-LyKCOi8

although it seems to be redundant now, as Jackson and Rider_X already suggested other solutions. Thanks guys :-)

Answer 1

With disk brakes the disk rotor is attached to the hub and not to the rim so a wobble in the tyre is unlikely to be the cause of the noise.

What I'd be looking at is the following:

1. Are the rotor bolts tight?
2. Is the rotor warped?
3. Are the brake pads worn?
4. Is one or both of the pistons sticking?
5. Is the calipar aligned properly?

1 and 2 you can check easily.

For 3 remove the pads and make sure that there is at least a couple of mill of braking compound left and that the springs (if present) are in good shape.

For 4, with the pads removed check that both pistons are extended by an equal amount; alternatively with the pads still removed push the pistons back (carefully with a soft implement not a screwdriver) and then squeeze the brake lever, do both pistons move in and out evenly? Be very careful not to push the pistons out to far! How you sort out a sticky piston is going to depend on the type of brake you have.

For 5 look at the disk rotor as you press the lever, does the rotor displace to one side? If it does this indicates that the pistons aren't applying equal force to the rotor - this could be a mis-aligned caliper or it could be a sticking piston so you'll need to check 4.

Answer 2

when there is a problem with noise and visual inspection, it's best if you could make a video and post it to, say, youtube.

But if the wheel is wobbling, you need to true it (if the rim is wobbling, too).

Also check if the wheel wobble when at rest, simply by using hand to push and pull the wheel side-way. If the wheel also wobbles, the cone might need to be tightened.

If you are sure the problem is coming from the brake, check if the disc rotor is straight by simply looks at the gap between the brake rotor and the disc brake.

Answer 3

The "sharpening a knife" sound you describe is likely the rotor lightly touching the brake pad (as mentioned in @Jackson's answer). This can happen if the rotor slightly is out-of-true (i.e., slightly warped) and the caliper isn't perfectly aligned. The sound can also come and go depending on how warm the rotor is. As you add heat the rotor (e.g., from braking) this can cause any out-of-true deflection to be more pronounced causing it to touch the brake pad with more force and hence a more pronounced sound. In terms of you only hearing the noise at speed, you may have braked earlier adding heat which increased the rotor deflection. When you hear the sound, stop, spin your rear wheel and look to see if the rotor is running true or has a deflection.

Look to @Jackson's answer for some remedies (i.e., caliper alignment, are the pads fully retracting, getting the rotor trued).

Having a rim and or tire that are not running true should not impact the operation of your disc brakes.