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This paper discusses the measurement of the dynamic centre of pressure (CoP) of a brake pad during a normal braking event using a modified 12-piston opposed calliper. The modifications allow the centre of pressure to be controlled both radially and along the length of the pad, inducing a leading or trailing centre of pressure as desired. The technique is unique in its design and implementation. Both the centre of pressures of the inboard and out-board pads are recorded simultaneously with varying pressures and speeds. The results, which include pressure and force maps, show the position of the centre of pressure to vary considerably during a braking event, both radially and axially along the pad. The CoP offset is related to the calliper mounting geometry which is subsequently compared to the effective “spragging angle” and the generation of brake noise.

The out-of-plane vibration characteristics of a noisy brake are generally better understood than in-plane characteristics. The fundamental reason for this is that in-plane vibration was not considered a significant effect until recently when technology has allowed the in-plane vibration characteristics to be determined with some degree of confidence. Detailed investigations of the side views of out-of-plane holographic images indicated that the in-plane displacement could be quite significant and possibly larger than the out-of-plane displacement. It was because the fringe pattern could not be attributed solely to out-of-plane displacement that a study of in-plane vibration was initiated. The paper discusses the measurement of both out-of-plane and in-plane vibration of a twin calliper disc brake during noise generation.

Previous work on generating animations from real disc brake systems generating noise (squeal) has been consolidated and developed. Using the method of double pulsed laser interferometry a series of holograms (typically ten per half cycle) can be recorded from the brake during a cycle of excitation. From these holograms a considerable amount of data can be obtained about the vibration of the disc and pad surfaces. Standard methods from image processing and algorithms developed to investigate hologram fringe lines can be used to generate three-dimensional representations of the surfaces. Furthermore although part of the disc surface and even more of the pad surface are obscured by the calliper, etc., it has been possible to form a reliable numerical reconstruction of the whole disc and pad surfaces partly by using standard mathematical approximation techniques and partly by intelligent extrapolation of the available data.

Laser holographic methods have been successfully used to produce animations showing the out-of-plane vibrations that take place in both the disc and pad of a real brake system generating noise (squeal). A series of holograms made at different times in the cycle of vibration were used to give the data on which the animations were based. Further it has been shown that mathematical approximations made to the fringe patterns obtained in the holograms by this method give phase and amplitude information about the wave motion involved. It establishes the existence of travelling waves on the brake disc with a speed given by the angular frequency of the noise divided by the mode order. This approach has now been extended to examine a time-related series of holograms of the disc rim of a brake system. From the data obtained from these holograms it has been possible to develop animations that show the in-plane vibration in the disc of a brake system producing noise.

The paper considers a drum brake mounted on a ½ vehicle test rig including suspension, cross beam and transmission differential. It is a continuation of earlier work (1) and so reviews the characteristics of a drum brake when generating noise on a ¼ vehicle test rig and compares them to those found on the ½ vehicle rig. Frequencies of 960, 850, 1400 and 4600Hz 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 test the predicted instability frequencies have been observed on earlier work.

The paper overviews the modes of vibration of the principal component parts of a brake and their contribution to system instability during noise generation. It is shown that both in-plane and out-of-plane vibration are present and that both can be related to the vibration of the pad. It is further shown that the pad and its region often provide a solution or “fix” towards noise prevention and it is this area that forms the focus of this investigation. The collective evidence, proposals and associated theory are applied to real brake case studies when it is demonstrated that disc/pad interface “spragging” may be the source of brake noise. Measurements of the position of the dynamic centre of pressure (CoP) support the theoretical predictions that a leading CoP induces brake noise. Design proposals are suggested that may be applied early in the design phase as a means to reduce the propensity of a brake to generate noise.

Asymmetry is applied to a heavy-duty commercial twin calliper 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.

Brake noise investigations using the whole body visual technique of double pulsed holographic interferometry have been extended so that a series of interferograms may be recorded over a cycle of excitation providing information about the amplitude, direction, phase relationship and the mode of vibration of the principal component parts of the brake. This work investigates the possibility of automatically interrogating the holographic images and creating an animated 3 dimensional image of a brake generating noise.