If everything is set up correctly, a well-tuned mountain bike should not make any noise. If your brakes are making any out-of-the-ordinary noises, they may just need a simple fix to solve.
The most common noise that might occur is a metallic whirring sound. That’s the sound of a disc brake rotor rubbing against the pads. You can tell that the rotor is rubbing if you spin the wheel and it slows down drastically without you engaging the brake lever. You may be able to actually feel the brake rub when you ride, depending on how severe it is.
Diagnosing the noise
Adjusting disc brakes is a simple process, but let’s first review how your bike’s hydraulic disc brake system works. The brake lever configuration on the handlebars has a reservoir for the hydraulic brake fluid that travels through the brake lines. When you engage the lever, the fluid pushes through the lines to the caliper.
The caliper houses pistons and brake pads. The fluid pushes the pistons, which in turn pushes the brake pads together. The pads squeeze the rotor, which is how you slow down.
Firstly you need to determine what is causing your rotor to rub on the pads. Spin the wheel and look at the gap inside the caliper between pad and rotor, if the rotor is predominantly rubbing then you will need to realign the caliper. If the disk has a distinctive bend or wobble then you may need to straighten the disk.
Realign the caliper
To realign the caliper and stop the brake from rubbing, first, loosen the two caliper mounting bolts. Then, apply pressure to the brake lever. Since the bolts are loose, this will center the caliper. While holding the lever, re-tighten the mounting bolts.
Give the wheel a spin to see if there is still rubbing. If there is, you can fine-tune the caliper alignment by loosening one bolt at a time. Spin the wheel and gently adjust the caliper by hand (push the caliper in the direction that will eliminate rubbing), and retighten the bolt.
If there is still rubbing after completing this process, there may be a bigger issue. Make sure the rotor’s mounting bolts are properly tightened and that there is no play in the wheel’s hub. If either of these is loose, that may contribute to rubbing.
Straightening a rotor
Disk rotors are designed to be very strong in a vertical plane, however even light impacts horizontally can bend a rotor enough to rub. Crashing is often the primary cause although impacts from trail debris or a simply bump when the bike is leaned outside a cafe or in a bike rack will do it.
A rotor truing tool will make the job a lot easier, an adjustable spanner works well too and if you are in a squeeze a rotor can be carefully bent back by hand. Whether you use a tool or not, the most important thing to remember when dealing with a rotors is that whatever touches the braking surface should be as clean as possible.
Truing a rotor is simple case of locating the point that rubs and determining the direction it needs to be trued to stop it rubbing. Once identified, gently bend the rotor back. Repeat this process until the rotor no longer rubs. Repeated gentle bends should stop you bending the rotor too far in the opposite direction and creating more problems.
Finally, the pistons might be sticky, which requires the calipers to be cleaned.
In most cases, pesky brake rub can be fixed with a simple caliper adjustment. So now you can get back to riding without the horrid noise of a rubbing brake.