Brake squeal is often classed under ‘automotive comfort’, and noisy brakes can indeed make driving uncomfortable. Squeal is much more important than that, though, since it can mask real problems.

Sound pressure levels can in practice be greater than 80 dB, and brake squeal has been a major contributor to noise-related brake warranties. It has become steadily more noticeable over the last 30 years as vehicles have become quieter. NVH (Noise, Vibration and Harshness) is becoming a key aspect of customer satisfaction, particularly for plug-in hybrids and Electric Vehicles.

Mechanisms for NVH in rotors and disc pads are still however insufficiently understood. Lapinus has therefore been investigating NVH in a number of projects over the last few years, both internally and with external partners.

Brake noise is caused by on-surface oscillations in the rotor, most likely generated by unstable dynamic system behavior due to friction-induced vibration.

All brakes can emit noise. In the morning, they can grind as the pads warm up and clean a thin layer of rust from the rotor. They can rattle if the brake pads come loose. And they can squeal if they are hot, for example while braking harshly when going down a hill. Ideally though, the only time a brake should squeal is when there is an actual problem. A metal wear indicator for example produces a squeal when it contacts the rotor near end of life. If you continue to ignore that brake squeal then next comes a grinding noise, which accompanies rotor damage.

Different mechanisms cause the noise, each under specific driving conditions (conditions for front and rear brakes differ). Each brake pad is believed to have specific oscillation areas in which the noise (in kHz) typically occurs.
Under certain conditions rotor and brake components can vibrate at their combined resonant frequency. Squeal therefore normally occurs at only one or just a few natural frequencies. Your speed and how hard you press the brake pedal changes the volume of the noise, but not the frequency.
The brake pad itself is only responsible for one third of the noise caused by the vehicle. The rest is determined by the interaction between rotor, pad, caliper, and environment.

As a result, a pad which is completely noise-free on one vehicle can generate loud noise on another. The noise behavior of pads on a particular vehicle also depends on environmental conditions like temperature and humidity. Without in-depth knowledge of brake noise and its causes, this can leave pad manufacturers open to noise complaints that are difficult to solve. The friction material is often blamed for brake squeal, but that is not always the case. Each mechanism has its own cause and solution(s).

Noise can be caused by brake roughness following uneven rotor wear, which can be on one or both sides of the rotor. The sound is radiated from the disc rotor, which acts like a loudspeaker.

There are four major categories of brake noise:

• Brake squeal: occurs at high speed, generating noise at frequencies > 1000 Hz.
• Brake moan: occurs at moderate speed, generating noise at frequencies ~ 100 Hz.
• Creep groan: occurs at almost standstill and leads to noise at a frequency of 100 Hz.
• Judder: occurs at almost standstill and leads to noise at a frequency of 10 Hz.

Several other mechanisms may also be found in the literature. Stick-slip occurs when the apparently smooth brake motion is actually jerky. Sprag-slip occurs when there’s a locking action because of compressive and shear stiffness, which requires no variations in the coefficient of friction. Negative friction-velocity slope involves friction-force induced vibration. All of these may theoretically lead to instability of the system, but mode coupling is generally accepted to be the most important mechanism, where energy is transferred between longitudinal and transverse modes of vibration.

Key factors in rotor squeal will likely include rough surface finish, metallurgical instability, insufficient damping, deflection (curvature) and disc thickness variation.

Pad processing and curing conditions will be studied by the research partners alongside characteristics like hardness, porosity and abrasiveness. All these are likely to affect the frequency and sound level. The harder the pads the lower the porosity, and the higher the abrasive content the greater the likelihood of squeal.

Squeal can be reduced by modifying pad shape, by cutting slots and/or chamfers, and by attaching noise insulators to the pad backing plate. The shoe plate thickness can also affect squeal.

Previous Lapinus research on NVH analyzed friction forces. This research showed that the type of mineral fibre makes a great difference. Results show a strong effect of fibre type for squeal rate as well as excited frequencies. There was also some history effect due to changes in friction behavior with increasing brake noise after multiple applications.

Important for the calipers is to avoid uneven pressure. The calipers should also be stiff to avoid excessive vibration.

Environmental factors include low temperature and low or high humidity. Squeal can also happen at low speeds and decelerations; some brakes can for example generate noise at the end of the braking.

Different frequency ranges require different kinds of damping. Caliper damping using tuned absorbers can be effective between 0 and 2000 Hz. Higher frequencies, between 1000 and 5000 Hz, can call for dampened springs, slippers and other accessories. And higher still, between 2000 and 16000 Hz, can need shims on brake pads and (as noted above) slots and chamfers.