Brake Squeal - Stop That Squealing!
edited by Greg Mierz
Zundfolge, Dec. '97 - Publication of BMW Automobile Club Of America
This month I've edited an article on brake noise from the internet that deals with squeal and how to try and eliminate it. I believe that it is accurate and I've personally done the "sanding" thing with success and agree with the ideas to eliminate the squeal noise.
Brake squeal is probably one to the most common and frustrating problems that a car owner must face. Assuming the friction components are in acceptable condition and operating properly, squeal noise doesn't affect braking operation at all. To most owners, any perceptible brake system noise is cause for major concern. Even though you may be convinced that the noise is nothing to worry about, you can take the appropriate measures to eliminate the noise.
Brake squeal (again, providing the pads and rotors are in good condition and the noise is not being created by pad wear indicators or badly worn pads/rotors) is simply the result of pad or rotor vibration that occurs when the pads meet the rotor surfaces. Too many owners are quick to blame a specific set of pads for a squealing problem. In some cases, a vehicle with freshly-installed pads of one brand may have a squealing noise. After installing a different-branded set of pads, the noise may disappear, resulting in quick blame on the initial brand. The result; the owner assumes the first brand is substandard, while the second brand is superior.
In the vast majority of cases the "blamed" pads are unjustly accused. One aspect of pad design that we need to understand is that not all pads are created equal. Just as tire engineers may create one tire with excellent wet traction capabilities, that same tire may only offer mediocre tread life. That doesn't mean it's a bad tire. It only means that the design priority for that tire was to achieve the best wet traction possible. To gain performance in one parameter, the tire engineer may have been forced to lose a bit in another design parameter.
Similarly, a certain set of pads may excel in some areas while seeming deficient in others. For example, one set of pads may have been designed to offer incredible fade resistance, but may be noisy and may provide lousy cold braking performance. Another set may be very quiet, but may provide a relatively short pad life under heavy-duty use. You probably know that there's no such thing as perfect tire that does everything well on every vehicle. By the same token, there's no such thing as a perfect brake pad that does everything at 100% efficiency on every vehicle. As is the case in so many areas of design engineering, if you want to gain in one performance area, you may have to compromise in another area. The best approach is to use the type of pad that provides the best performance for the application at hand. If noise is created, work at eliminating the vibration-caused noise through one of the methods described in this article. In other words, don't be too quick to blame the pad for the noise problem.
In order to dampen brake system vibration, you have several options to consider: Chemical insulating compound on pad backs, shim insulators between pad backs and the caliper, and high-temperature grease on caliper pins.
Application of anti-squeal compound on the backside of the pad backing plate can serve to prevent pad vibration, although some owners consider this "damper in a tube" a temporary measure at best. If this compound is used, it's critical to allow sufficient curing time before the brake system is used. All too often, installers apply this compound, install the pads, and immediately send the vehicle out of the shop. If the compound is not thoroughly dry and cured, it will simply squeeze out from behind the backing plate on the first brake application, rendering it useless. Apply a heavy bead on the backing plate, and allow it to completely dry before pad installation. This forms an elastic cushion that absorbs pad vibration.
Insulating shims (vastly preferable solutions to anti-squeal compound) install on the backside of the pads, between pad and caliper. Shims provide a permanent vibration damper. Shims are available for a variety of models. If their use is recommended, install them! One aspect that's often overlooked and one that can easily contribute to brake noise, involves the caliper pins or sliding surfaces. If these metal-to metal friction surfaces are dry, they can temporarily bind and chatter during operation, resulting in a squealing noise. Clean and lubricate the sides and pins with a high-temperature silicon grease. Naturally, exercise care when applying the grease to prevent contamination of the rotor and pads.
That's right, sanding. A common source of brake squeal is due to the vibrations that are generated when the pads make contact with a rotor surface that is not smooth. I know what you're thinking--if the rotor has just been resurfaced, it must be smooth. However, most brake lathes machine-finish the rotor surfaces with a cutting bit that leaves a series of peaks and valleys. A nicely machined swirl finish may look great, but this surface finish can contribute to minuscule pad chatter. Once the chatter starts, a continuing resonance is created (a tiny series of vibrations that build to a harmonic pitch). Once the rotor has been resurfaced on the lathe, sand the surface with 120 to 150-grit sandpaper (you can use a flat sanding block or a DA sander). Perform the sanding while the rotor turns on the lathe to assure a uniform sanding, in the effort to avoid isolated pressure spots. Don't try to achieve a "swirl" pattern, or any distinct pattern, for that matter. As the rotor turns on the lathe, with cutting bit disengaged, apply the sandpaper from the outer circumference of the disc towards the center. Continue this until the distinct machined finish that was created by the cutting bit is no longer identifiable (about 60 seconds per disc side).
This sanding action serves to smooth the rotor surface, eliminating the severe differences between the machined peaks and valleys. "Noise" typically is generated by a series of vibrations. That applies to the human voice, records, and brake rotors. In other words, if you want the brakes to hush, you have to eliminate the vibrations that are created during the pad-to-rotor contact.
Most of us love to use spray cans of bake cleaning solvent. It cleans quickly and cuts through nasty grime with ease. And while it certainly has its place as part of any brake job, it probably isn't the best way to perfectly clean the rotor surface after machining and sanding, a thin film of microscopic dust and grit may remain in the tiny valleys on the rotor surface. To assure a clean rotor after sanding, try washing the rotor in a mixture of detergent and hot water. This will do a great job of cleaning the rotor surface. A thin film of fine grit can cause a chatter of squealing problem, so cleaning should be taken very seriously. The detergent and hot water will cut through the fine embedded layer of grit that might remain otherwise. In addition to serving as one of the steps toward reducing of eliminating brake noise, detergent washing can also improve the performance of the brake system. In fact, brake manufacturer tests have shown a gain of as much as 8% in braking torque with sanded and detergent-washed rotors.