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Although disc brakes have been safely stopping automobiles for decades, they are a relatively new feature on bicycles, mountain bikes in particular. Bicycle disc brakes slow and stop the wheels by squeezing a rotor disc connected to the axle between two metal-backed pads. The pads are held in place by a caliper mechanism. If the pads on your disc brakes are rubbing the rotor, you can adjust the caliper mechanism yourself to fix the problem.
Place your bike securely in a bike mount or turn it upside down so the wheels spin freely.
Spin the wheel with the rubbing disc brakes and check the wheel for wobble. If it wobbles, flip the wheel quick-release lever and reseat the wheel in the fork or frame. Center the wheel and tighten the quick release again. Check for wobble now. If the wheel is seated correctly on the bike but the wheel still wobbles, the rim could be bent, the wheel bearings could be damaged or the entire frame could be bent. Get the rim replaced, the bearings repaired or the frame straightened before addressing the disc brakes for problems.
Inspect the disc brake rotor, the round metal disc that spins with the wheel. If the rotor is loose, which can happen with normal mountain bike use, check the specifications in the user manual for your particular disc brakes and tighten the bolts with a wrench to the recommended tightness.
Observe the edge of the rotor from an angle where you can see it as a thin wafer between the brake pads. Spin the wheel slowly. You will be able to see if the rotor is bent from this angle. Straighten a bent rotor by first cleaning the grips of an adjustable wrench and then clamping it to the bend in the rotor. Gently bend the rotor slightly back to true. A slight pressure is all that is necessary here; otherwise you risk breaking the rotor. Alternatively, use straightening tools designed especially for the purpose, such as Morningstar's Drumstix. In this case, follow the tool manufacturer's directions. If your disc brakes still rub or pulse after you straighten the rotor, take the bicycle to an experienced professional to get the rotor replaced.
Observe the rotor again from the same angle as Step 4 and spin the wheel. If the rotor isn't centered exactly between the two brake pads, rubbing can occur. First, replace worn brake pads because uneven wear can cause the brakes to rub. Adjust new or unworn brake pads by loosening the hex bolts on each pad with a hex wrench. Squeeze the handlebar lever to apply the brakes, which will center the pads. Keep the brakes applied while you tighten the bolts securely. If your brake lever doesn't have a lock, get a helper to keep the brakes applied.
Adjust the brake pads, if possible, by turning the pad adjustment knobs located on the side of the pads opposite the rotor. Leave the smallest amount of space possible between the rotor and each pad that still allows the rotor to move freely when the brakes are not applied.
- If you are unable to fix a rubbing disc on your bike with these steps, the brake mounting brackets may need to have a bit of metal sheared off, thus squaring the mounting surface. For this you must take your bike to an experienced professional.
- Keep any tools or fingers that touch the rotor clean. The rotor must be spotless to assure a good grip for your brakes.