Experimental observations of hot judder

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. Subsequent holographic results confirm the positions with the appearance of thermal fringes “just off” the disc surface. The images also show a classical two-diameter mode order of vibration on the disc centre. The holographic recordings also indicate pressure waves being emitted from the trailing end of the pads with the mode shape of the pads confirming high amplitude bending; the existence of the pressure waves inferring the pads lose contact with the disc surface.

Other observations include hot spotting on the disc surface and a surface scan of the disc face indicates a depression at these positions. A metallurgical analysis of the blue spot is currently being carried out to determine if a material phase change has occurred within the disc body and this will be included in the paper.

The introduction of a pressure fluctuation absorber shows an improvement in the pressure amplitude and subsequent vibration signal and the possible development of this system is discussed