By: Eric Denys
Photo courtesy of Link Engineering
Increasing the system damping is the other NVH technique that can be used to reduce brake noise.
The best way to increase damping is at the brake pads level. Friction materials and pad under layers with increased damping properties, are now available. Brake shims have been successfully used for years. People have also tried to significantly increase the damping of the brake rotor (loudspeaker of the noise) with very limited success so far.
So, how are shims working?
When brake squeal is generated, the pads will vibrate and shear the shim various viscoelastic layers (rubber, adhesives…) which will increase the damping of the pads, reducing its vibrations and propensity for noise. By using different adhesives, the damping can be optimized for a certain temperature range to match the noise temperature signature. It can also be increased by designing a shim with a thicker steel layer.
The outside layer of the shim will also affect the pads vibration level, the amount of energy transmitted to the caliper (isolation), which will have an impact on the level of noise being generated.
It is also important to remember that the shims will provide damping only when the pads are vibrating, which means at frequencies at or above the 1st bending mode of the pads. This 1st resonant mode will be located between roughly 1 and 5 kHz, depending of the size of the pads.
Throughout my career, many people have asked me what the best shim is. This is a very difficult question to answer as every system requires a different type of shim, but if I had to pick one, I would recommend a bonded shim for mid/high frequencies with a clip-on cover on top to cover lower frequencies. Unfortunately, this type of shim could create some other issues (packaging, caliper drag…).
Challenging job to be a Brake NVH Engineer!!!