William P. Steel

Rotor asymmetry used to reduce disc brake noise

Asymmetry is applied to a heavy-duty commercial twin caliper disc brake rotor as a means to alleviate an undesirable high amplitude noise. The problematic frequency is 2400 Hz, the rotor blade exhibiting a 5- diametric mode order of vibration. The asymmetry is introduced by drilling sets of radial holes into the disc rim. Modal analysis is carried out over a range of frequencies using added masses applied magnetically to the rim of the rotor. This shows the amplitudes at set frequencies to reduce considerably when asymmetry is introduced. When a set of 5 masses is added to the rotor the vibration amplitude at the troublesome frequency is seen to be considerably reduced. Finite element analysis complements the experimental results. The analysis of a plain disc initially shows the two normal modes at very close frequencies but when asymmetry is introduced, by drilling holes in the rim of the disc, there is a noticeable frequency decoupling of the 2 normal modes. This is also accompanied by a distinct positioning of the antinodes with the antinodes of one mode positioning at the sets of drilled holes, the other mode antinodes being between the holes. When an asymmetric disc is fitted to the vehicle the noise is eradicated. Significant testing of the vehicle has since been undertaken without noise being generated

A study of brake noise and the influence of the centre of pressure at the disc/pad interface, the coefficient of friction and calliper mounting geometry

Abstract It is generally accepted that the coefficient of friction between the friction pair, the pad and the rotor plays a significant role in the propensity of a brake to generate noise. Because of the ease with which changes may be made to the pad compound material it is this that is more often the subject of change when there is a problematic noisy brake with both the rotor and the calliper tending to remain ‘isolated’ from examination. This paper investigates the propensity of a brake to generate noise over a range of temperatures and pressures under conditions that allow a mechanically induced offset centre of pressure between the pad and rotor to be varied. It is shown that for a high-volume production car brake there is an increased tendency for it to generate noise when a very specific leading offset centre of pressure is engineered. With this condition it is shown that a situation is promoted whereby the brake becomes mechanically unstable, with system changes such as brake pressure and temperature variations having little influence on the brake to alter its tendency to generate noise. Furthermore, it is shown that the critical offset centre of pressure may be related to the coefficient of friction between the friction pair and the mounting geometry of the calliper. Confirmation of the findings are supported by a repeat investigation of the vehicles alternative brake calliper. It is suggested that the source of a noisy brake may lie as much in basic mechanical design as inappropriate material choice. Further confirmation is demonstrated through the study of a commercial sliding fist-type brake, where the brake is significantly noisier when the centre of pressure and calliper mounting arrangement provide an appropriate spragging angle related to the friction coefficient.

The measurement of the absolute displacement of a noisy disc brake

Abstract The investigation of in-plane vibration of a noisy disc brake is problematic because it is difficult both to measure and to verify. Because of the disc structure and the inability to visualize disc in-plane vibration, there has been reluctance by researchers to accept the contribution of a displacement parallel to the surface of the object, or in-plane displacement, to noise generation. In addition to measuring absolute displacement, it has been difficult to isolate the in-plane and out-of-plane components of displacement using either non-contact or conventional displacement measurement techniques. This paper investigates absolute displacement of a brake disc during noise generation. Double-pulsed holographic interferometry is used to record a series of time-related images of the brake head from three different angles of observation. Because each image views the brake head from a different perspective, each of them records a different degree of in-plane and out-of-plane displacement. By careful analysis of the three images, it is possible to isolate the in-plane displacement from the out-of-plane displacement. The time-related series allows the displacement to be investigated over a full cycle of excitation and so create an animation of the mode of vibration. It is seen that the in-plane displacement is complex and that its amplitude may be about twice that of the out-of-plane displacement.

Investigation of drum brake noise from a brake mounted on a half vehicle test rig

The paper considers a drum brake mounted on a 1/2 vehicle test rig including suspension, cross beam and transmission differential. It is a continuation of earlier work and so reviews the characteristics of a drum brake when generating noise on a 1/4 vehicle test rig and compares them to those found on the 1/2 vehicle rig. Frequencies of 960, 850, 1400 and 4600 Hz are examined in some detail using the technique of holographic interferometry. It is seen that the modes of vibration of the component parts vary notably over the frequency range considered. This observation allows the significance of each part to be evaluated for each frequency range. With the accumulated information it was possible to predict other possible unstable frequencies and although these were not observed within this series of tests the predicted instability frequencies have been observed on earlier work