Mountain bike brakes take a lot of abuse. They’re tasked with grinding a hurtling hunk of steel, aluminum, or carbon—plus a passenger—to a safe stop, keeping a ride from turning into a wreck.
If your disc brakes scream when you squeeze the levers, one of your rotors may have come out of alignment—a problem you can fix in a couple of minutes with an allen wrench.
First, spin your bike wheels one at a time to determine which is the offending brake. When you’ve found the culprit, with both wheels on the ground, undo the quick release on the squeaky brake wheel, then relock it—you are checking to make sure that the wheel is seated correctly, that the axle is firmly engaged with the dropouts.
If the brake is still squeaking, examine the rotor to see if it is bent. If you don’t have a bike stand, flip your bike upside down on the ground, balanced on its handlebars and saddle. Place your thumbnail about a quarter inch from the rotor. As you slowly spin the wheel, look for any wobble in the rotor, any change in the distance between your nail and the metal rotor. If the rotor is bent, you will need to replace it or have a mechanic replace it.
If the rotor looks straight, you likely just need to readjust your brake pad alignment.
To do this, flip your bike back on its wheels. Using an allen wrench, loosen—but don’t totally unscrew—the brake bolts on the back side of the brake, which is mounted on the left side of your bike. With the bolts loose, squeeze the brake lever several times to recenter the pads. With the brake lever compressed, tighten the bolts again, and then release the brake lever.
Give your wheel a spin, and your problem should be solved. If it is not, you may need new brake pads—ask your mechanic to take a look.