Brake Noises Explained

Even after several years in the bike repair business I still get frustrated working on brakes sometimes. There is a lot going on with a proper brake set-up because the pads have to be:

• perfectly symmetrical (unless the calipers are dual pivot)
• flat on the brake surface
• not too high or too low on the rim's braking surface, with respect to how the pads will wear (on linear pull brakes, for example, the pads tend to creep down the rim surface as they wear. On dual pivot calipers one pad rides up while the other slides down)
• not crooked when viewed from the side

The list goes on, and that doesn't include the parameters for spring tension, cable length, and lever pull, which can have certain special considerations, especially with cantilever brakes. 

Now even after you think you've set up the brakes perfectly, you take it for a test ride, grab a handful of lever and "SQUEEEEAL!" Every dog in the neighborhood is barking, babies are crying, people are yelling at you -but you don't notice because your eardrums have burst. This is brake squeal. 

I should stop now and mention that all brakes make noises whenever they're engaged. The job of the mechanic is to tune the pitch so that humans can't hear it. This is done first by cleaning the rim and pads with rubbing alcohol and a clean cloth. Sometimes that's enough and you can stop right there. Grease from your fingers that might have transferred to the rim alters the friction characteristics of the braking surface and is one of the biggest culprits of brake squeal, especially on disc brakes. Rubbing alcohol helps dissolve it. I like to rub the rim and pads down with sandpaper too to get rid of any thicker contaminants. 

The next move is the adjust the "toe" or "heel" of the brake pad. This means adjusting the angle of the pads so that either the front or the back of the pad hits the rim first. I haven't seen any research as to whether "toeing" is better than "heeling" anecdotally I've used both methods with success -you just have to make sure the pads are mirror images of each other. To do this it is easiest to wrap a rubber band around the back of the pad and squeeze the brakes, then tighten the bolts and you're good. 

One worry about toeing and heeling is that it will make the brakes feel mushy (excessive modulation, to use jargon). This might mean the pads are toed or heeled too much. 

If after you've toed to the maximum of comfortable modulation and cleaned all surfaces of contamination and there's still squeal, you can take out the pliers and actually bend the pads. I've found that flat or concave pads are more likely to squeal, so as a last resort I will bend the ends of the pads into an ever-so-slightly convex shape. This will lead to uneven pad wear, but it's better than noise. 

Most of what I've been writing about applies to the standard rim brake setup. But for disc brakes you don't have the ability to manipulate the pads. There are a host of chemicals  you can put on the pads and rotors that will transform the dry friction characteristics of the ceramic pads against metal into lubricated friction, thus altering the pitch, but that's more of a band aid. My advice is to just live with it. Most of the time it goes away -that's true with rim brakes too.

The last tidbit I want to share is that most brake pads have a piece of metal embedded inside them that screeches when if hits the rim. This is a sign that the pads are worn and need replacing as the metal embed is a few millimeters behind the fresh pad surface. Be sure to check that before you spend much time fussing with toe or whatever.