What Causes Breaks to Squeak?
Don’t expect brakes to be totally quiet—sometimes, they’re going to make some noises. Don’t panic, because a squeaky brake can stop a car just like a silent one can.
What causes the squeal? Modern brakes use a cast-iron disc squeezed between two brake pads lined with friction material. Under the right conditions, the disc, the pads and the caliper they’re mounted in can start to vibrate—in exactly the same way a violin’s string vibrates when stroked by the horsehairs on the bow. Most brake squeals occur at a single discrete frequency. The speed of the vehicle and how hard you press down on the left pedal will only change the volume of noise because the pitch is controlled by the stiffness and mass of the pad and disc.
Normal Brake Pad Noises
Many brake pad compositions will make a swishing or grinding noise for the first few stops in the morning until the pads warm up and drive off any moisture they’ve accumulated overnight. Ever notice a hissing or grinding noise on some rainy or dewy mornings? It’s the pads sweeping a thin film of rust that’s formed on the iron discs, and it’s perfectly normal.
In the past, brake pad friction material relied heavily on asbestos. Unfortunately, asbestos tended to give asbestos workers and brake mechanics lung cancer, so the industry has almost completely changed over to less dangerous alternatives. Kevlar is one material that’s seen a lot of use, but it tends to be dusty. Improved brake performance is more important nowadays because of increased safety requirements and equipment—and the extra road-hugging weight that comes along with these. That leads to the increased use of metallics and ceramics in the brake pad friction material. And this stuff can make the brakes hiss or even grind a little as you slow down. It’s a small price to pay for increased performance.
Many brake pads have a small finger of spring steel that will scrape on the disc as the pad reaches its wear limit. This tells you that it’s time to change pads for fresh, thicker ones before the friction material wears completely away, and you’re trying to slow down on the metal backing plates. It’s a sound not easily confused with brake squeal—it’s more of a ripping-sheet-metal noise, not a single, high-pitched note.
Stop the Squeak
Go into any auto parts store and you’ll see a shelf full of potions and widgets claiming to cure squeaks. One class of products I’m leery of is simple aerosols that you spray onto the pad’s friction material. I have no idea if they actually make the squeak go away because I’m unwilling to try anything that changes the friction characteristics of the pad. Let’s not forget, the first reason your brake system exists is, in fact, to make your car slow down. Anything that could reduce that system’s effectiveness in any way is probably not a good idea.
Okay, let’s dig in and silence our brake noise. One fix is to simply change pads to a different type of friction material. It’s usually hard to beat the original-equipment pads for a good compromise of pad life, noise, grip, dust creation and price, but changing to an aftermarket premium metallic or ceramic pad just might change the interaction that affects the resonant frequency of the pad and disc and, literally, change its tune.
Questions, or if we can be of help in any way with service on your Audi, BMW, Mercedes, Mini Cooper or other European (and now Japanese) import, please contact us. Our team of Service Specialists are here to help, for even the newest autos! (207) 882-9969.Knowing, not just “doing”, that’s the Atlantic Motorcar Center way of life.
The Atlantic Motorcar Center Service Team