Bicycle brakes become less effective over time. Cables stretch and pads slowly wear away. If you can squeeze the lever fully on without the brake engaging, urgent action is required.
Test your brakes
Stand next to your bike. Squeeze the front brake lever and try to push the bike forward. The rear wheel should lift and the lever shouldn't touch the handlebar. Now repeat this test with the rear brake. The rear wheel should lock and skid as you push the bike forward. If either brake isn't working properly, it's likely to be a result of slack in the cable – unless your bike has hydraulic brakes, in which case they probably need 'bleeding' to remove air bubbles. (That's a job for the bike shop or a confident home mechanic.)
Is the brake properly set up? V-brakes have a quick release so that the wheel can be removed and refitted easily. The 'noodle', which is a J-shaped metal guide tube, can be disengaged from its cradle. In this state, the brake will not work. Sidepull brakes often have a small quick release lever on the caliper, enabling it to open wider (as shown). Ensure this lever is closed, otherwise the brake pads will be too far from the rim.
Examine the brake pads. There should be a good thickness of braking surface remaining. (If your brakes make grinding noises in use, there's no pad material left: you're applying metal to metal!) Time for new pads? Head to the bike shop.
Adjust cable tension
All cable-operated brakes should have a barrel adjuster – a hollow knurled bolt where the cable exits the lever or enters the caliper. Some bikes use 'inline' adjusters part way along the cable outer instead.
To increase cable tension, turn the barrel adjuster anti-clockwise. Try one full turn initially, then half turns, repeating the brake test periodically. If the barrel adjuster has a threaded lockring or locknut, unscrew this to enable the barrel to turn, then screw it flush to the lever or caliper to keep the barrel firmly in its new position.
If a few turns of the barrel adjuster don't solve the cable tension problem, try re-clamping the cable. First wind the barrel adjuster back in. Then undo the bolt that anchors the cable to the brake.
Use one hand to squeeze the brake mechanism together. This is easy with sidepull brakes and V-brakes: simply hold the brake blocks against the rim. With a cable disc brake, push the caliper's brake arm up to engage the brake. The brake doesn't need to be jammed on; just touching the rim or rotor is okay. Don't let go of the brake until you've re-clamped the cable.
Then with your other hand, pull more cable through the cable clamp, until the cable is just taut. Let go of the cable now and tighten the clamp bolt.
Finally, let go of the brake mechanism.
Since you weren't pulling on the cable when you re-clamped it, there should be enough slack that the brake pads don't rub the rim or rotor. If there's too much slack, use the barrel adjuster. If the cable is too tight and the brake rubs constantly, repeat the above process but don't squeeze the brake mechanism fully against the rim or rotor – just hold it tight enough that you can pull the cable through further than its previous clamping point.
Centring the brake
Sometimes just one of the brake pads will rub. In this situation, you need to centre the brake.
Sidepull brakes often have a small adjuster screw on top of the caliper, at one side. Screw this in or out – slowly, so you can watch the brake arms move.
If your bike's sidepull brakes lack this feature, slacken the fixing bolt that holds the brake to the frame or fork, move the brake, then retighten the fixing bolt.
V-brakes have a small screw at the bottom of each brake arm. These adjust the spring tension. To move the brake pad away from the rim, increase the spring tension by screwing inwards. To move it towards the rim, decrease the spring tension by unscrewing. As the brake pads' positions are determined by the spring tension on both sides, you'll often tighten one side and unscrew the other to get it right. Work in small increments – e.g. a half turn at a time.
Disc brakes, whether cable or hydraulic, are held to the frame or fork by two large (5mm) Allen bolts. Undo these two bolts enough that you can move the disc caliper side to side by hand. Then squeeze the brake lever so the brake is fully on. Without letting go of the brake lever, use the 5mm Allen key in your other hand to tighten the caliper's frame/fork bolts.
If the pistons on both sides of the caliper move – all hydraulics and some cable discs – the caliper should now be centred. If, as with most cable discs, only one piston moves you may need to adjust the position of the fixed one (see below).
The pads of sidepull brakes and V-brakes need to be in line with the braking surface on the rim. Pads set too high will touch the tyre and rub a hole in it; pads set too low will develop a lip that can hold the brake pad against the rim. To adjust pad position, undo the bolt on the pad, then carefully tighten as you hold the brake manually against the rim.
With cable disc brakes, the piston (and pad) nearest the wheel is usually fixed; it doesn't move when you squeeze the brake lever. But you can move it in or out to get it the right distance from the rotor. Reach through the spokes with the relevant Allen or Torx key, turning anticlockwise to move the pad away form the rotor and clockwise to move it towards the rotor.
The position of the moving piston/pad is usually determined only by cable tension, but on some brakes (e.g. Avid BB7) it can be moved in or out independently with a ratchet dial.
When you've adjusted your brakes, squeeze them on hard a handful of times, then repeat the brake test. If you're still not happy, go to the bike shop.
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