A Proposal to Predict the Noise Frequency of a Disc Brake Based on the Friction Pair Interface Geometry

The mode of vibration of a noisy disc brake is always diametral with a noise frequency marginally less than the free mode of vibration of the disc. Wheel speed does not affect the frequency but if brake pressure is altered then the noise frequency changes accordingly - an increasing pressure resulting in an increasing frequency over a specified range. Such observations have been made of a variety of different disc brake designs from single piston sliding fist type callipers to four piston opposed rigid callipers with it being possible to relate the noise frequency to the free mode of vibration of the disc in all cases. If the characteristics controlling this behaviour can be identified then the same principles and criteria may be used to predict the noise propensity of any brake at the design stage.

The paper proposes, and shows, that the preferred frequencies of excitation of any disc brake system may be related directly to the free mode frequency of the disc. I addition it is shown that the interface geometry of the pad, the effective rubbing diameter of the disc and the effective working length of the pad are all important features regarding the frequency generated. It is demonstrated that this relationship allows the probable instability frequencies of a disc brake to be predicted and shows that the frequency will vary over a computable range as a result of the brake pressure. The principle is successfully extended to consider a braking system arrangement employing two callipers working on one wheel. The proposal suggests that brake noise be as much a basic design problem as a material selection problem and answers questions such as why chamfers on pads changes the propensity of a noisy brake to generate noise.

A case study of a heavy vehicle using a four opposed piston calliper on the rear and a pair of four opposed piston callipers on each front wheel goes some way to validate the proposal. The study and tests show that angular positioning of the front wheel calliper pair has a significant influence on the propensity of the brake to generate noise and the frequencies that were subsequently emitted.