The common causes of squealing disc brake pads are:
1. The leading edges of the linings weren't prepared properly. Do-it-yourselfers typically just slap in the new pads and bolt everything back together. The leading edges need to be ground down a little if they didn't come that way already. All that's needed is to rub the edges on a concrete floor or round them off with a flat file. That removes the "fingernails-on-the-blackboard" type of squeal. If that squeal can be prevented from occurring during the break-in period, they usually won't squeal for the rest of their life.
2. That flat file must be run over the piston and the calipers' fingers that make contact with the pad backing plates. That's to remove any hint of rust, dirt, or other debris that would prevent the pads from seating solidly against them. If a speck of dirt holds the pads away, it will be easier for the pad to vibrate and set up a squeal.
3. Old rotors must be machined when they're reused. The first reason is to remove any grooves and high spots so they will mate evenly with the new linings. The second reason is it is very common for the new linings to be bonded slightly off-center on their backing plates compared to the old ones, and the ridge of rust near the center of the rotor must be ground off, otherwise the new linings will ride on it and make noise. That rust will wear off on its own very quickly, but not before some of it becomes embedded in the lining where it will scrape on the rotor and make a squeal.
4. A special high-temperature brake grease must be used between the pad and caliper piston, the pad and caliper fingers, and on whatever the caliper mounts to. Pads are going to vibrate. That can't be stopped, but the grease will allow them to slide freely. Without that lubricant the pads' vibrations will be transmitted into the calipers where the squeal will be amplified. Brake grease contains molybdenum disulphide and will not migrate around to other places, and it won't be affected by the heat normally generated by brake systems.
5. There must be no hint of contamination on the brake rotors or linings. It's usually acceptable to wash off any grease but that has to be done before the parts go through the first warm-up cycle. If that grease is on there when the brakes get hot, it will soak in and never come out. The only fix at that point is to replace all the affected parts. Experienced brake mechanics will even wash their hands with soap and water before handling new linings, and they'll hold the rotors only by their edges to avoid getting fingerprint grease on them.
6. All manufacturers specify the desired "surface finish" on machined rotors. That is hard to describe. Most mechanics learn, sometimes from trial and error, how much material to remove on each pass on the brake lathe, and how fast to move the cutting bit, (speed of cut). A cut that's too course can cause a grinding or howling noise. A cut that's too fine will result in a longer break-in period. Either condition can reduce the amount of friction, and that means you have to apply more pressure than normal to the brake pedal. That additional pressure can be hard to notice, but it can easily result in overheating the linings, and that can result in one type of brake fade. Overheating the linings can melt the "binders", meaning the glue that holds the lining material together. Melted binders results in glazing. A glazed finish has lower friction, and it can cause a squeal.
7. When rotors are machined, the mechanic will measure their thickness first. There is a published legal limit to how thin a rotor can be machined to, and another legal limit that the rotor can be allowed to wear down to while in service. Typically the "discard" spec. Is.030" thinner than the "machine to" spec. But that can vary by manufacturer and model. A rotor that is machined below the "machine to" spec. Can easily land a mechanic in court if that is found during a crash investigation, even if the crash was the other guy's fault. No reputable mechanic will risk his job and reputation by reusing a rotor that is too thin. Among other things, with less mass, normal braking, especially in city driving, will cause the rotor to heat up too fast and too hot. Heating up unevenly side-to-side causes uneven friction and can cause a brake pull, and heating up too much can cause brake fade. The brake fade can include glazed pads which will squeal.
There are other types of noises caused by the braking system, but these are the most common ones related to squealing. Also be aware that some of the higher-quality linings are prone to squealing when they're cold, and in humid conditions. That squealing usually isn't real obnoxious and doesn't occur all the time.
Thursday, January 15th, 2015 AT 2:25 PM