What Causes Brake Noise and Vibration?

Understanding the Causes of Brake Noise

Brake noise is a common issue that many vehicle owners encounter. It can range from slight squeaks to loud grinding sounds, each indicating different potential problems within the braking system. While some brake noises are benign, others may signal more serious issues that need immediate attention. This article explores the various causes of brake noise, the importance of diagnosing and addressing these noises, and preventive maintenance strategies to ensure a smooth and quiet braking experience.

All braking creates some amount of vibration

Whether we’re talking about car brakes or bicycle brakes, any time you apply a brake pad to a rotor or wheel, you’re going to get vibration. All normally operating brake systems generate “initial noise” that occurs during braking. Initial vibration noise is inaudible to the human ear because brake manufacturers design their friction materials, calipers, rotors, and drums with rigidity and noise-dampening features. The initial noise-dampening features are:

• Insulators like noise reduction shims or gaskets that isolate vibration and prevent it from moving from the friction material to the caliper
• Anti-rattle clips or “abutment clips” that eliminate brake pad backing plate vibration transfer to the caliper.
• High temperature synthetic brake grease applied in areas where the brake pad backing plate contacts the caliper acts as a dampening agent.

If the insulators, clips, and grease are properly installed, a braking system should not generate noise. Yet, even some “perfect” installations generate brake noise. So what’s going on—secondary noise?

What is secondary brake noise?

Secondary brake noise is vibration noise that is amplified instead of dampened. In other words, the initial vibration frequency that was inaudible is now audible. Most people blame the friction material, claiming the brake pads are “too hard” or the rotors are poor quality.

However, aside from the quality of brake parts, most secondary brake noise is caused by brake components that have deteriorated due to heat and stress, causing the parts to no longer maintain a tight fit. For example, the brake pad backing plate begins to transfer initial vibration to the caliper abutment (bracket) due to the degradation of the anti-rattle clips or deterioration of the noise reduction shim. Secondary brake noise due to degradation appears as an irritating squeal.

Rust and corrosion cause brake noise

Brakes operate in a hostile environment, subjected to water, ice, road salt, grit, and repeated hot/cold cycles. This causes steel and aluminum parts to oxidize, corrode, and lose temper and spring tension.

Why is brake noise so common now?

Car makers have reduced the mass of car components to reduce weight and increase gas mileage. The brake rotors on early RWD vehicles were massive. They contained the wheel bearings and seals designed to be machined several times. The rotors in FWD vehicles and all modern vehicles are both smaller and less massive—they’re a fraction of the weight of RWD vehicles, so they can’t dampen the vibrations nearly as well.

The brake balance has changed as well. With the engine and transaxle mounted under the hood, FWD vehicles maintain more weight in front of the vehicle than older RWD vehicles, where the transmission, driveshaft, and differential moved more weight to the rear.

So, the front brakes in FWD vehicles perform 80% to 85% of the braking, while the rear brakes provide only 15% to 20%. In contrast, the front brakes in RWD vehicles perform 55% to 60% of the braking, with the rears performing 40% to 43%. The result is much higher operating temperatures on FWD brakes, which causes glazing and rapid wear.

Technicians sometimes believe switching to a “softer” brake pad will be more forgiving and cure a noise complaint. A softer friction material changes the balance of the braking system and may reduce secondary noise. But that change may also result in more rapid wear or reduced braking performance. Rather than automatically blame the brake pad, you should first examine the noise-reduction features of the entire braking system.

Brake noise in detail

Since each type of brake noise can have different causes, I’ll list the types of noises and refer you to a more in-depth article with diagnostic tips.

• Judder—a low-frequency vibration (0 to approximately 300 Hz) in the mid-5o to 900-decibel range that may include shake or rumbling noise.

Cause: Poor tolerance or fitment—rotor to hub, abutment to the steering knuckle, caliper to bracket, brake pad backing plate to the caliper

See this post on how to eliminate brake judder and brake pedal pulsation

• Squeal— A medium frequency noise in the 300Hz to 5,000Hz range in the 0 to high 50’s dB range.

Causes: Degradation of the caliper piston or the square-cut O-ring that prevents smooth movement in/out of the caliper piston, non-planarity of the rotor face (the rotor face has high and low spots), improper brake pad fit, degraded, missing, or improper noise reduction shims, and worn friction material.

See this post on how to eliminate brake squeal

• Squeak— High-frequency vibration (5 ,000Hz to 12,000Hz ) in the 0 to high 80’s dB range

Cause: If the clips, shims, and grease are in good condition, the cause is molecular vibration within the friction material itself.

©, 2019 Rick Muscoplat