Most rim brakes on mountain bikes these days are v-brake style. You may have to adapt these instructions for the caliper and cantilever brakes which were used on older bikes but the general principles are the same.
Before you adjust rim brakes, make sure the wheel is sitting properly in the dropouts and the quick release skewer is closed tightly. This ensures your adjustments will be correct after you next remove the wheel.
First check that you have sufficient rubber left on the brake pads. There’s no point adjusting everything if you then have to change the pads. Most pads have grooves molded in to them which help clear dirt and water away. If those grooves have been worn away, it’s time to replace the pads.
Some pads are “cartridge” style, where the rubber part of the pad slides backwards out of a metal holder, allowing you to replace the pads without changing any other adjustments. The pads are typically held in place with a small screw or pin which must be removed before you can push them out of the metal holders.
Other pads are made in one piece, with the rubber molded on to the post which attaches them to the brake arms. If you need to replace these pads, or if your cartridge style pads are misaligned, you will have to adjust the brake pad alignment.
To adjust the pad alignment, loosen off the bolt that holds the brake pad post and push the brake arm in so that the brake pad touches the rim.
Make sure the whole surface of the pad is in contact with the rim – none of it exposed beneath the rim and none touching the tire – before tightening the bolt again. When you are finished, the pad should be level in comparison to the rim when seen from the side, and parallel to the rim surface when seen from the front. It can take some messing around to get all the adjustments right and then tighten down the fixing bolt.
At this point you may want to toe in the pads. This stops them from squeaking so much when you grab the levers. Pad toe-in means making the front of the pad touch the rim before the rear of the pad. It is achieved by folding a business card in half and placing it between the rim and the trailing edge of the pad before you tighten the pad down. As this takes more hands than the average human possesses, it is also possible to tie a knot in a rubber band and slip that over the trailing edge of the pad before you secure the pad to the brake arm.
Now check that both brake pads touch the rim when you squeeze the lever, and both arms spring back away from the rim when you release the brake lever.
If the pads only just touch the rim when the brake lever is fully squeezed, adjust the cable tension. Do this first by turning the barrel adjuster on the brake lever counter clockwise. If this adjustment doesn’t take up enough slack in the cable, turn the barrel adjuster one turn short of all the way clockwise and then take up slack at the brake arm by loosening the cable clamp bolt, pulling the cable through, and tightening the clamp bolt down again. After doing this, you may need to make fine adjustments with the brake lever barrel adjuster.
If the arms don’t spring away properly, or one stays closer to the rim than the other, try adjusting the spring tension adjuster screw for that brake arm. Typically, tightening in the adjuster screw will move the arm further away from the rim. Remember that if you reach the end of the adjustment with one arm, you can always slacken off some tension on the other arm to achieve the same effect.
Check that all the bolts are tight before you ride.