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This paper discusses the analysis and measurement of the dynamic centre of pressure of a brake pad during a normal braking event. The technique is unique in its design and implementation. The process is progressive whereby the interface static measurements are first taken and then dynamic testing is carried out under braking. Two different measurement systems are considered during the analysis with one used to measure the center of pressure. Both the in-board and out-board pads are measured for wear but the piston pad was selected for pressure measurements. Validation of the spragging process is undertaken on both test rigs and vehicle trials. Pad wear measurements complement the collective information. The results show the position of the centre of pressure to vary considerably during a braking event, both radially and axially along the pad.

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.

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.