QUESTION: Can you shine a little light on brake squeaking?ANSWER: Great question. Brake squeal has several facets, from harmless but annoying noise, to rotor-grinding trouble.Let's start with the usual cause, which is brake-pad vibration. Your car's disc brake pads resemble large Oreo cookies, with an outer layer removed. The filling, or friction material, is pressed into contact with the brake rotor by a caliper containing a hydraulic piston. Brake pads are allowed to float slightly within the caliper's grasp, to allow uniform rotor contact, although movement is regulated by springy clips or isolator pads.Why might the pads vibrate? Brake pads come in a variety of compositions, and their condition can become degraded. Many pads contain a high percentage of metal or other hard materials to resist heat and provide long life. These pads tend to be noisier than those containing a softer "organic" friction material recipe.Pad condition and brake-rotor finish also can be a factor, as overheated brake pads can take on a glazed finish, and brake rotors sometimes develop surface glazing, grooving or hard spots. Finally, the clips or pads used to minimize vibration can deteriorate or perhaps be incorrectly installed or omitted upon replacement.Harmless vibration is generally a high-pitched squeal heard upon brake application. A squealing sound that occurs constantly, or repetitively, and goes away upon brake application, is usually the result of an active brake-pad-wear sensor. These are small metal tabs that begin to rub against the brake rotor as the pads wear thin. Applying the brakes distorts the pad slightly, often changing the sound upon brake use. In most cases, the brakes are safe to use for an additional few thousand miles.Now the bad noises. A deeper growling/grinding sound heard upon use is generally caused by direct metal-to-metal contact between the pad and rotor, as a result of the friction material wearing completely through. If caught immediately, rotor damage can be minimized; too late, and brake performance declines substantially, in addition to the need for often expensive brake rotor replacement.How can one tell the difference between harmless squeal and dangerous grinding between professional inspections? The open nature of many wheels allows the caliper and outer brake pad to be readily seen. If the pad's friction material thickness is less that of an Oreo's filling, replacement is needed soon.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.