Search Results

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.

A twin calliper brake system is investigated using the whole body visual technique of holographic interferometry. It is shown that the disc mode of vibration has a preferred position where a disc antinode is situated under one calliper and a disc node under the second calliper. The maximum angular space occupied by the pad antinode is, as predicted by the theoretical study of the disc/pad interface geometry, the angle subtended by the pad length. For a four-piston opposed calliper the minimum distance is slightly larger than the piston centers. There is evidence that the disc mode position, in relation to the two callipers, may be antinode/node, node/node or antinode/antinode. With these arrangements an accompanying revised theoretical study of the disc/pad interface geometry predicts two stable conditions are possible - if the callipers are positioned either at an angle between 125° to 130° or 165° to 175°.

High speed testing of a brake system using two different callipers and two different discs on a special test rig with a swinging calliper head mounting shows similar results with regard to hot judder. Holographic interferometry is used in an attempt to observe the disc mode shape during judder and although the classical fringe pattern was not obtained for the disc some useful and complementary information was forthcoming. Disc run-out measurements show that the disc takes up a permanent and increasing deformation with a two-diameter mode formation. This deformation is seen to give a brake pressure fluctuation that results in judder - the pressure being detected using a pressure transducer fitted at the calliper and the mechanical judder with an accelerometer mounted on the calliper body. The two signals allow the degree of phase shift to be estimated. A “strobing” effect, resulting from the combination of speed and video recording frequency, shows two hot-spots moving with the disc.

A test rig which replicates a one quarter vehicle of a rear wheel drive vehicle, including the suspension system, is used to investigate a low frequency noise. The cross beam is included along with the vehicle suspension spring which is loaded against a sprung loaded mechanism which represents the tyre stiffness exactly and supports the brake geometrically as the tyre would. Drive to the drum is from a DC motor through the wheel drive axle. Holographic Interferometry is used to observe the modes of vibration of the drum with mirrors strategically placed to observe additional features such as the backplate, spring pan and cross beam. Initial results show the mode of vibration of the backplate to be of a diametral mode order and to be moving in the direction of drum rotation. Additionally it is seen that the spring pan and cross beam exhibit high amplitudes of vibration.

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.

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.

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.

A test rig which replicates a one quarter vehicle of a rear wheel drive vehicle, including the suspension system, is used to investigate a low frequency noise. The cross beam is included along with the vehicle suspension spring which is loaded against a sprung loaded mechanism that represents the tyre stiffness exactly and supports the brake geometrically as the tyre would. Drive to the drum is from a DC motor through the wheel drive axle. Holographic Interferometry is used to observe the modes of vibration of the drum with mirrors strategically placed to observe additional features such as the backplate, spring pan and cross beam. Initial results show the mode of vibration of the backplate to be of a diametral mode order and to be moving in the direction of drum rotation. Additionally it is seen that the spring pan and cross beam exhibit high amplitudes of vibration.

An experimental investigation of passenger car disc brake noise has been made using the double pulsed laser holographic technique, developed to allow simultaneous recording of three orthographic visual images of a vibrating brake system. Mechanical measurements of parameters such as disc surface strain, torque, pad deflections, pressure and noise complement the visual images for both continuous and intermittent brake noise and enable phase relationships of the component parts to be determined. Visual images of the vibrational characteristics of a noisy brake show the disc to be in a bending mode with diametral modes. The pad is seen to vibrate in a variety of modes such as bending, torsion and often a combination of both and that pad end flutter is shown to make a significant contribution to brake system noise.

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.