Disc brakes have become widely used for mountain biking because they provide effective stopping power in wet conditions. Unlike rubber pad brakes that provide friction against the rim of the wheel, disc brakes press ceramic or metal pads against a hardened-steel rotor to stop, which prevents brake slippage. Disc brakes may need adjustment so the rotor does not rub against the pads while riding.
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Mount your bicycle in a workstation so you can turn the wheels freely while you adjust the brakes. If you do not have a workstation, turn the bicycle over and balance it on its handlebars and seat.
Locate the two bolts that attach the brake mechanism to the bicycle fork. One is above the calipers and one below. Insert a hex driver into one of the bolts and turn it to the left to loosen it. Loosen the other bolt as well until you can move the calipers by hand. Do not remove the bolts from the braking mechanism.
Spin the wheel slowly and watch the rotor as it spins. Note how the rotor is contacting the brake pads. Turn the calipers by hand to find the correct angle where the rotor no longer drags against the brake pads.
Insert the hex driver into one of the bolts and tighten it securely, ensuring that the calipers do not move from the adjusted position. Tighten the other bolt.
Spin the wheel and squeeze the brake lever to check the brake tension and alignment of the pads. While the brake lever is depressed, insert the hex driver into the brake pad adjustment bolt and adjust each pad to the desired tension against the rotor.
Spin the wheel of your mounted or upside-down bicycle slowly. Watch the rotor as it spins. If the rotor passes through the calipers without touching the brake pads for most of the rotation, then drags suddenly at one point, the rotor is bent.
Place a mark on the rotor at the location that is bent. Turn the wheel so the mark is 180 degrees from the calipers.
Place a small, adjustable wrench on the location of the bend and tighten it around the rotor. Gently bend the rotor with the wrench in the direction of the brake pad it was not rubbing against.
Spin the wheel slowly and monitor how the rotor passes through the calipers. Gently bend the rotor more, if necessary, until it completely clears the brake pads on a full rotation.