FARMINGTON HILLS, Mich. – Eric Denys, General Manager of Meneta North America, posted the following (the second of two parts) on the Brake Academy website addressing a variety of strategies for original-equipment manufacturers (OEMs) to combat the noise, vibration and harshness (NVH) of brake systems.
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!!!
About Eric Denys
Eric Denys is currently the General Manager of Meneta North America in Farmington Hills, Mich. His career spans 25 years at Federal-Mogul, Wolverine and Material Sciences Corporation with technical specialization in brake NVH. Denys has led teams to achieve best-in-class in brake squeal on numerous vehicle lines and is the recipient of the 2001 Ford Global Customer Satisfaction Award for his work on high mileage brake squeal reduction. His work has been published in numerous national and international papers, and in an SAE book on Disc Brake Squeal. He is the chairman of the SAE Brake NVH Standards Committee.