Analysis of Automotive Brake Noise Generation and Solutions

　　Brake noise is a significant quality concern and a persistent technical challenge in the automotive industry. During braking, noise generated at the brakes transmits through the chassis and vehicle body into the cabin, negatively impacting the driver's perceived quality and comfort.

　　Brake noise is characterized by its non-repeatability, randomness, time variance, instantaneity, intermittency, and overall uncertainty. Its generation mechanism is highly complex. Factors such as the structural design of the brake caliper, assembly precision, wear of the brake pads, and disc thickness variation (DTV) all influence noise production. Even the same brake assembly can generate noise at different frequencies under varying conditions of temperature, humidity, braking force, and vehicle speed. The primary theoretical framework for analyzing brake noise is self-excited vibration. Essentially, noise results from friction-induced coupling and system instability caused by improper modal parameter matching among brake components. This instability triggers self-excited vibration, leading to audible noise. Frequencies can range from tens to tens of thousands of Hertz, and optimization strategies vary depending on the specific frequency to improve the overall driving experience.

　　Mechanism of Brake Noise Generation

　　Disc brake systems are inherently nonlinear, time-variant, and subject to numerous uncertainties. For instance, wear on friction surfaces generates material debris and hardened surface films. The formation, alteration, and removal of these factors are unpredictable, yet they significantly affect contact stiffness and frictional characteristics, thereby influencing the system's dynamic behavior. Furthermore, material aging effects are difficult to forecast and also impact system dynamics. The primary mechanisms of brake noise generation are as follows:

　　Instantaneous interaction in the contact zone between the brake disc and pad creates impact excitation.
　　Brake squeal is generated by the coupling of strong impact excitation with the weak modal response of components, or vice-versa.
　　Improper matching of the structural dynamic parameters within the brake assembly.
　　Instability within the friction-induced closed-loop coupling system of the brake structure.

　　Brake noise can be categorized into low-frequency and high-frequency types. Low-frequency noise mainly involves the coupling of the brake disc's out-of-plane modes with the modes of components like the caliper, carrier, and pads. High-frequency noise primarily results from the coupling between the brake disc's out-of-plane modes and the pad modes. The interaction between pads and disc involves both rigid body motion and elastic vibration. This elastic vibration is the root cause of brake squeal, and the dynamic coupling between the components plays a crucial role in noise generation.

　　Classification of Brake Noise

　　Shudder/Judder (5–100 Hz): A common vibration during braking, primarily caused by resonance in the vehicle's suspension and steering systems. It originates from tire pressure variations leading to component imbalance and brake torque variation. The perceived shaking correlates with the resonance frequency and the vehicle's sensitivity (influenced by transfer paths, subsystem resonance frequencies, and damping characteristics).

　　Moan (<500 Hz): This low-frequency noise often occurs at very low speeds with minimal or no brake pressure applied, sometimes during turning. It is typically related to the stiffness of brake components, axles, and suspension; a bound state between brake and suspension mounts; pressure distribution between pads/disc and caliper/pads; and non-braking drag torque.

　　Groan (<600 Hz): Often felt by the driver as a rhythmic vibration or low-frequency sound, usually coinciding with the vehicle nose dipping during braking, and is more common in automatic transmission vehicles. This rhythmic vibration is caused by stick-slip (creep) between the pads and disc. Its occurrence is less frequent and is linked to pad thermal deformation, pressure distribution between pad/disc and pad/caliper, disc deformation, the friction-velocity relationship, and the rigidity of the caliper and bushings.

　　Squeal (1000–3000 Hz): Caused by the coupling of natural frequencies between brake components and parts of the suspension system.

　　Middle Frequency Squeal (3000–6000 Hz): Generally caused by brake system instability. It is closely related to pad formulation, disc structure and material, and has some connection to the vehicle's suspension system. This type of noise occurs frequently.

　　Brake Noise Mitigation Strategies

　　Key solution approaches include:

　　Eliminating the excitation source (e.g., adding chamfers to pads, optimizing pad material formulation, matching shims).
　　Increasing damping (e.g., adding under-layer to pad material, applying damping shims to the disc or caliper, adding tuned mass dampers to the caliper).
　　Modifying the pressure distribution on the contact surface between pad and disc.
　　Adjusting the brake disc's elastic modulus.
　　Improving brake disc thickness variation (DTV).
　　Modifying the pad friction coefficient.
　　Altering the brake disc's thermal capacity.
　　Optimizing the in-plane and out-of-plane modal properties of the brake disc.
　　Optimizing the brake disc's cooling design and managing thermal deformation.
　　Optimizing the natural frequencies of all brake system components to avoid modal coupling.
　　Reducing braking drag torque.
　　Optimizing the noise transfer path through structural modifications.
　　Optimizing the brake caliper mounting configuration.
　　Adding vehicle sound insulation materials (absorbent materials like nylon, rayon, or polyester).

　　Conclusion

　　The mechanisms behind brake noise are complex and cannot be analyzed or solved using a single model. Extensive testing and validation are required. The same brake assembly can produce noise through different mechanisms under varying operating conditions. A detailed analysis based on the noise frequency is essential to identify the root cause for effective optimization. A substantial test sample size is necessary to avoid coincidental solutions. Brake noise involves multiple disciplines such as materials science, mechanics, thermodynamics, and tribology, making it a cross-disciplinary research field with numerous influencing factors. Continuous exploration is needed to advance the understanding and resolution of brake noise issues.