Disc brakes fall into two categories: hydraulic and mechanical. Squeezing the brake lever of a hydraulic disc brake system pushes a piston that pressurizes brake fluid running through a hose to a brake caliper mounted to your bike’s frame or fork. Once the increased pressure reaches the caliper, it presses the disc brake pads against a brake rotor. Mechanical disc brakes do the same thing, except instead of using brake fluid pressurized in a hose, a cable from the brake lever to the brake caliper actuates the brake pads by rotating some type of cam.
START WITH THE OBVIOUS
Before you mess with replacing brake pads, truing rotors or stacking shims, make sure your wheels are properly installed in the fork and frame. A paint chip, dirt or a wayward quick-release spring will wreak havoc on braking performance. Pull the wheel out, inspect the dropout, replace the wheel and snug the quick release. This will fix 37 percent of all brake noise problems.
Warning:Disc brakes work by converting the speed and energy of the bicycle into heat. When you have been on your brakes hard, they will generate a significant amount of heat (in the rotor, pads, caliper and even brake fluid). So, before touching any part of your brake system, make sure it has cooled down.
The gloves come off: Clean the brake rotors with a damp cloth (moistened with isopropyl-alcohol). The gloves were removed to prevent oil or grease residue from ending up on the disc. Reduce the chance of injury by removing the wheel for cleaning.
Clean surface: Look for obvious problems before you start pulling brake pads to cure noise. Make sure the dropouts are clean. Look for chipped paint or a damaged surface that could cause the wheel to set improperly. This dropout is perfect.
We highly recommend that your tool box contain a few spare brake pads at all times. Brake pads wear differently depending on the material they are made from, the conditions you ride in and the way you apply your brakes. Some riders can get a year’s use out of a single set of brake pads, while a more aggressive rider in nasty conditions might go through brake pads monthly. Either way, when brake pads finally reach their limit, it happens quickly. Having that spare set in the box might be the difference between riding tomorrow and waiting for pads to show up in the mail. Order a set today.
Snug it up: Another culprit of brake rubbing could be loose mounting hardware. Most disc brakes require a Torx T-25 wrench (available at any hardware store) to snug the hardware. It is a good idea to keep an eye on these bolts, even if rubbing is not an issue.
Brake pads for both mechanical and hydraulic systems are available in various compounds. Generally, a softer organic (or resin) material will tend to make less noise during operation and will offer more modulation. These guys wear out the fastest. Metallic (or semi-metallic) pads will last longer, especially in wet and muddy conditions. Mixing the pads (one metallic and one organic) can have favorable braking results but will require more attention (because the organic pad will wear faster than the metallic pad).
You know the rotor is clean, true and secure, but there is still a scraping noise when you spin the wheel. It is time to take a look at the brake pads.
1) Pull the wheel out and inspect the brake pads for dirt, damage and wear. Comparing them to your new set makes it easier to identify problems.
2) You may need to remove the brake pads for proper inspection. You will need the owner’s manual to properly remove and replace them. A rule of thumb is if the pads are less than three millimeters thick (this includes the backing plate), replace them.
3) We have traced brake noise to a damaged brake pad spring (that little cage that surrounds the brake pads when they clip into the brake caliper), so don’t overlook it.
Open-system hydraulic brakes self-adjust for wear. Occasionally, the pads will move too close to the rotors and drag against them. Many times, the caliper needs to be reset because somebody squeezed the brake lever when a wheel was out of the frame. Here’s what to do:
1) If the pads are in good shape, carefully insert the blade of a clean, wide, flat screwdriver (with a relatively sharp tip) between the pads. Work the blade past the center of the caliper pistons and pry the pads apart by twisting the handle until you have retracted the caliper pistons well past the thickness of the brake rotor.
2) Reinstall the wheel. Take two business cards and slide one between the brake pad and the rotor on each side of the rotor.
3) Make sure your bike is upright. Squeeze the brake lever until the master cylinder pumps enough fluid into the calipers to return the pads to their correct position (the levers feel firm when you squeeze them).
4) Pull the cards out and pump the brake lever lightly a few more times.
If you use a mechanical disc brake, adjusting the cable tension will accomplish pretty much what we just did with the hydraulic brake.
Push’em apart: If the pad clearance is very narrow (Photo A above), take a screwdriver, carefully slide it between the pads and twist the handle (Photo B). Never force the screwdriver, because you could gouge the pad surface, reducing its life. After you do this, the pads are fully seated and ready for the disc (Photo C).
Bend it back: Eyeball the rotor spinning through the caliper. If the rotor is bent, use a tool like the Park Tools DT-2 Rotor Truing Fork to correct the wayward wobble. The disc is fairly flexible, so don’t overdo it. A little pressure does a lot.
Wear them down: Once brake pads wear to less than three millimeters thick (including the backing plate), it is time to slap a new set in.
Take my card: Hold a business card on each side of the rotor and carefully rotate them into the brake caliper. Squeeze the brake a few times. Pull the cards out. This trick has been used to silence millions of rubbing brakes.
Pad inspection: If necessary, pull the brake pads (follow your owner’s manual instructions) and look for excessive or uneven wear. Inspect for damage or contamination to the pad surface. Check the spider spring (they can produce unwanted noise if damaged or worn).
NOT YET CURED
1) With the wheel in the bike and the bike on a stand, rotate the wheel and look down the line where the rotor enters the caliper. Is the caliper pretty well centered over the rotor?
2) If not, you need to add or remove shims or loosen the caliper’s mounting hardware to position the caliper correctly. This takes trial and error and patience. Sometimes, you may get the caliper perfect, but snugging the hardware throws it off. Again, take your time.
3) If you have to use more than two or three shims to position the caliper correctly, you may have mounted the caliper or mounting hardware incorrectly. The frame or fork may also need facing where the hardware or caliper mounts. A bike shop will have to help you with this.
Mountain Bike Action is a monthly magazine from Hi-Torque Publishing in Valencia, California. It has been published since 1986. You can start a subscription by clicking here or calling (800) 767-0345.