Brake pads wear out. It's hard to say how long you'll get from a pair, as it will depend on where you ride, what the weather's like, and how hard and how often you brake. But sooner or later, they'll need replacing. Visually inspect the pads periodically. How much material is left? If there's only a thin sliver on top of the metal backplate, it's time for new ones. If you hear a grinding metal-on-metal noise when you apply the brake, you've worn through the pad entirely. Replace immediately!
You don't have to fit identical brake pads but you do have to use ones that are compatible. There are lots of different types, especially for disc brakes. Write down the brake model before going to shop or simply remove the old brake pads and take them to the shop. There will probably be a choice of replacement pads. The more expensive ones generally are better, as you'd expect.
For disc brakes, sintered pads are good for commuting because they wear slower. For rim brakes, overly hard pads compromise braking performance; those dearer, softer-compound pads will stop you better, especially in the wet, and they won't wear through the wheel rims as quickly. Note that carbon rims require carbon-specific pads.
To replace brake pads, you need to be able to adjust your brakes.
Rim brake pads have two basic designs: one where the pad material is permanently fixed to the metal backplate; and one where the pad slides onto the backplate. The latter is identifiable by the small screw on the back that holds the pad in place.
If you've got slide-on pads, replacement is easier. Undo the small screw on the back of the pad (not the 5mm allen bolt holding the brake to the brake arm) and slide the pad out. A screwdriver may help to push it out. Make sure the new pad is the right way around; there's usually an arrow, which should point forwards. Then slide it into place and tighten the small screw to fix it in position. Repeat for the other pad.
The new, unworn pads will sit closer to the rim. If they brush or butt up against the rim, you'll need wind in the barrel adjuster or reclamp the cable to slacken off the brake. Brake pads can creep slowly up or down the calliper over time, so check that the pads are striking the rim squarely. If not, adjust to suit. Before riding, apply the brake firmly half a dozen times to check everything is okay.
If you're replacing the whole brake pad, things are a little fiddlier. Begin by undoing a sidepull calliper's quick release or by unhooking a V-brake or cantilever brake cable from the brake arm. Then unscrew the 5mm allen bolt that holds the pad to the brake arm. As you remove the pad, note the position of any washers on the metal stalk. They'll need to go back in the same order and the same orientation.
Fit the new pad to the brake arm, taking care with washers, and loosely tighten it in position. Repeat with other pad. Close the brake's quick release (sidepull brake) or hook the cable back into position (V-brake or cantilever) to see how much room, if any, you have between pads and rim. You will need to wind in the barrel adjuster or unclamp the cable if the brake pads touch the rim.
Hold the brake calliper closed over the rim with one hand. Loosen the 5mm brake pad bolt and move the pad so it sits squarely against the rim. Still holding the calliper firmly closed by hand, tighten the 5mm brake-pad bolt. Repeat on the other side, then adjust as necessary. Before riding, apply the brake firmly half a dozen times to check everything is okay.
Start by removing the wheel from the fork or frame. If the brake's pistons can be manually adjusted, set them as far apart in the calliper as you can. (All cable disc brakes have at least one adjustable piston.) This will provide more room to get the old pads out and, more importantly, to get the new pads in.
Some brake pads have an eyelet at the top, with a screw or cotterpin to secure the pads in the calliper. Remove this. A cotterpin with flared ends will need them bending flat with pliers first.
Now look for the small metal tabs on the pads, which protrude from the calliper. Grip these by hand or with pliers and pull the pads free. Push with a screwdriver from the other side of the calliper if you're struggling.
Many disc brakes use a flat spring to spread the pads apart in the calliper and stop them rubbing against the rotor. (Those that don't use magnets to hold the pads to the pistons.) If such a spring doesn't come out with the pads, poke it free with a screwdriver.
Hold the two new pads together, with the spring (if used) fitted between them. Make sure they're oriented correctly - look for an L and R on the backs of the pads. Press the pair of pads into the calliper, the same way the old ones came out. A flat bladed screwdriver, tip pependicular to pad orientation, can help press them gently home. They should click into position (no luck? press further) and the pads will separate. The gap between the pads needs to be wider than the disc rotor. If not, gently separate the pads with a flat-bladed screwdriver.
Refit the retaining bolt or cotterpin, if used, and then refit the wheel, taking care to slot the disc rotor between the pads. Apply the brake firmly half a dozen times to bed the pads into position. You may need to adjust your brakes to prevent the new pads rubbing the rotor.
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