In the automotive industry, the silence and ride comfort of the car body have been markedly improved by the development of the hybrid vehicles and fuel cell vehicles. However, the complicated frictional vibration is caused in brake system by the pursuit of a high brake performance. In high-speed areas of highways and roads with steep descents, the heat in car disc brakes sometimes causes a vibration problem. This phenomenon, called hot judder, is generated by hot spots on the surface of the disc. The generation mechanism of hot judder has not yet been clarified. In this study, hot judder is modeled as self-excited vibration due to the time delay caused by heat deformation of the disc, and the fundamental generation mechanism of hot judder is analyzed by a simple three degree-of-freedom system. In the analysis, the temperature of the disc is calculated by the equation of heat conduction. Then, the relations between disc temperature and the expansion due to heat and the shrinkage due to cooling is examined. Furthermore, the effects of disc surface wear are considered in the vibration analysis model because it has been known that the quantities of the surface wear is large at the position where the hot spot has occurred. It was found that (1) hot judder is an unstable vibration due to the time delay of the fluctuation of brake disc thickness caused by thermal deformation, (2) the number of hot spots is related to the natural frequency of the system, and (3) the wear increases in proportion to the quantities of expansion. Moreover, (4) when the vehicle speed changes, the number of hot spots also changes.
