If your brake pads touch against the rims while you are riding, it not only slows you down but it also wears away the pads and rims quickly.
Before adjusting anything, check that the rubbing sound isn’t the result of a larger problem. Make sure the bike is upright, then undo the quick release on the wheel, and make sure the axle is properly seated in the drop-outs before re-tightening the quick release. This makes sure the wheel is properly centered and is the first step for any subsequent adjustments. If your bike’s wheel isn’t centered when the axle is completely pushed into the drop-outs, you probably have a bent frame or bent axle. Center the wheel as best you can, and clamp the quick release closed hard.
Now, spin the wheel while watching the gap between it and the brake pad. This allows you to see if the rim is warped. If the gap changes size as the wheel spins, the rim is out of true. Adjust the spokes before continuing.
With rim problems out of the way, check that the brake releases fast and fully when the lever is released. If there is any delay, or the brake doesn’t release, it suggests that your cables are dry or dirty, or that the brake arm or brake lever mechanism is dirty or broken. While it’s normally best not to oil modern cables, doing so now may restore your braking function.
Next, check that there is no debris stuck in the pads and rubbing on the rim. Remove any chunks of stuff you find in your pads using the point of a knife or a screwdriver.
The pads may also have been knocked away from their normal location – check by holding the brake on and seeing where the pads fall. Their entire contact surface should hit the rim. If this isn’t the case, align the brake pads.
Check that each brake arm springs back when pushed – if not, the brake arm spring could be loose, broken or missing. With V-brakes, the spring often extends up the side of the brake arm – look for a piece of thick wire. It should hook behind something on the brake arm. If it is unhooked, look at the other brake for guidance and re-attach it. Cantilever and caliper brakes (on older bikes) normally have the springs recessed inside the brake arm body, as do some V-brakes. There is little you will be able to do to fix these springs but you may still be able to adjust spring tension to compensate for a broken spring.
If the arms spring back, but by different amounts, one arm can drag against the rim. Fix this by finding the spring tension adjuster – normally a small screw on the side of one or both brake arms. Experiment with turning the screw(s) in or out until the tension is strong and equal on both arms.
Tip: If you take the front wheel off to use a car rack, check that you put the front wheel in the right way round (QR lever to the left) and fully seated. Also check that you didn’t accidentally swap wheels with your riding buddy. Yes, it’s happened to me.