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Mechanical Disk Brakes

Disk brakes use two metal-backed pads held in place by magnets. There may also be a spring-loaded clip that keeps the pads in place. Pistons push the pads against a metal rotor. Activation can be via cable (mechanical) or a fluid-filled line (hydraulic). This section is for mechanical disc brakes, such as Avid.  If you have a hydraulic system without pad adjustment knobs (such as Hayes), click here.  For a hydraulic system with external adjusters (such as Coda), click here.

Proper function of the brake depends on (1) the rotor must be straight and smooth, (2) the caliper mechanism must be properly aligned with the rotor, (3) the pads must be positioned correctly, (4) there must be enough "pad" left, and (5) the lever mechanism must push the active pad tightly against the rotor, with "lever" to spare.

Check rotor, pad thickness, and caliper position. Sight along the rotor (disk) to see how much brake pad you have left. See how much "daylight" you have between rotor and pad.

Spin the wheel. Check the rotor itself for warp and damage. Look for side-to-side wobble. See if the caliper opening is centered on the rotor.

If the rotor wobbles, is severely worn, or is otherwise damaged, click here.

Aligning the caliper mechanism:

Loosen the two bolts that hold the main body of the brake onto the mounting yoke or fork. These bolts aim forward along the axis of the rotor, with one above and below the calipers. The holes for these bolts allow for some side-to-side "play" that positions the caliper mechanism. (Don't take the bolts out; just back off the tension in the bolt enough to allow the rotor to "shift" a bit on its mounting.) 

If the caliper is obviously crooked, you can nudge it a bit to center it on the rotor. Now turn the inside pad adjuster until the pad just touches the rotor.
Next, turn the outside pad adjuster until the outside pad grasps the rotor tightly. (The tight pads force the body of the caliper into alignment with the rotor.)

While the pads are tight, tighten down both retaining bolts securely.

Now back off the outside pad adjuster until it moves a tiny bit away from the rotor. Spin the wheel to be sure the rotor is clear of the pads. If necessary, also adjust the inside pad.

Adjusting cable tension:

With the cable loose, push the activating arm upward a bit until it stops moving. (At this point, it has pushed the pads against the rotor.) Now back off about 1/2 to 3/4 inch of cable length.

Tighten the cable down. Grasp the brake lever to see if the brake engages at a comfortable spot. Usually, you should have at least one inch between the handlebar and the brake lever when the brake is "full on." If necessary, readjust the position of the activating arm on the cable, until the brake engages at a reasonable point in the lever-pull. (It doesn't need to be exact -- you can fine-tune with the cable-tension adjuster on the brake lever.)
Changing the brake pads:

To change the brake pads, remove the wheel. Grasp the handle of a brake pad. Move the handle towards the inside of the slot, and pull firmly. (On Avid and Hayes, there is a retaining clip that holds the pads. You need to overcome the tension on the clip.) The pad will slide out.

Turn the pad adjusters (both the inside and the outside knobs) to widen the caliper space, making room for the thicker pad on the new brake pads.

Insert each new pad. Pads are specific for inside versus outside (and top versus bottom). Push the pad back until you feel it click into place. Make sure the pads are seated correctly into the retaining clip, and positioned over the caliper piston. If a pad is inserted incorrectly, it won't work right.

Replace the wheel, making sure it's correctly placed in the dropouts.

Dial the adjuster pads inward, until each pad very nearly touches the rotor. Spin the wheel to be sure the pads don't rub the rotor.

brk-tn10.jpg (11357 bytes) Adjusting the lever mechanism:

Tighten the brake cable with the adjusting nut at the brake lever to fine-tune the point at which the brake engages. As you pull the lever, the brakes should engage at a point that's comfortable for your hand. 

brk-tn11.jpg (13321 bytes) You should have around an inch of daylight between the lever and the handlebar when you're pulling hard on the brake. Some riders like a closer grip, leaving only about 1/2 inch, but this puts you at risk of "bottoming out" the lever with the brake only partly engaged.
braklevr.jpg (11561 bytes) If your fingers are short, or you like the levers a bit closer to your hand, you can adjust the lever's starting position. The adjustment screw opposite the lever can be used to swing the lever back towards the handlebar. (This gives you less total brake-lever motion, so the calipers need to be close to the rotor.)

If you hear a grinding or feel a rubbing sensation when you apply the brakes (while the bike is standing still), your cables are full of crud. You should perform a complete cable cleanout. If the cable or the housing is bent or damaged, you should replace the cable.

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