Hot spots phenomenon is a thermo-elastic instability (TEI) which could appear during high speed braking events. This could result in undesired judder vibrations and thermal damage of the disc brake. In order to assess the conditions in which hot spots could appear, experimental tests as well as numerical simulations have been conducted on two different disc brakes (Solid disc and vented disc). Results of these investigations are presented in this research work. On one side, experimental tests have been carried out on disc brakes in inertia dynamometer bench. A rich instrumentation has been used to measure in real time disc temperature, brake acceleration, braking torque variation, disc run-out and thickness variation during tests. Furthermore, disc surface and material have been analyzed after tests. These experiments clearly highlights the link between the frequency of brake vibration and the number of hot spots. Moreover, the observations done on disc surface after tests show how hot spots affect disc surface. In particular, disc thickness variation after tests correlate with the number of hot spots and the torque variation during braking. On the other side, a finite element model has been built to simulate hot spots. The numerical model is inspired from the scaling approach which assume braking pads to be axisymmetric [1]. Frictional heat dissipation is ensured by a special subroutine. From simulation, disc temperature and deformation are processed and then compared to experimental tests. For both tested brake discs, simulation results are in a good agreement with experimental tests. The predicted disc temperature and the number of hot spots are close to measurements. Based on the numerical model, a parametric study has been carried out to investigate the sensitivity of the model to braking parameters and to identify the most influent parameters. In particular, disc design, pad length and initial disc speed seem to be the most influent parameters. The study also shows how modifying these parameters could help reducing hot spots. In the future, it is intended to deepen the link between hot spots and judder vibrations by dynamic simulations. The idea is to integrate disc deformation in a multi-body model allowing to analyze braking torque variation and brake vibration dependency on disc hot spots. This will help reducing judder by optimizing brake housing.

Dr. Yassine Waddad, R&D Engineer, Hitachi Astemo; Ing. Enzo Ing. kassa, R&D Engineer, Hitachi Astemo; Dr.-Ing. Sylvain Thouviot, R&D Manager, Hitachi Astemo; Ing. Thierry Chancelier, R&D Manager, Hitachi Astemo