Abstract
ABSTRACT The stopping capability of disc brakes is an important part of vehicle active safety system, which is predominately affected by the rate at which heat is dissipated due to forced convection and the thermal capacity of the rotor. It is widely known that concentrated high temperatures are responsible for most problems in vehicle braking systems. Any improvements to the thermal mass or cooling characteristics of a braking system will reduce the risk of temperature associated problems and provide safer transport.
In order to improve the safety and to ensure that brake discs will operate within the normal operational range, which is specified by standards, homologation tests were developed to examine the newly developed brake discs operation in extreme braking conditions. Each car manufacturer has developed its own homologation procedures to best correspond to the predicted operation of a specific vehicle.
The current form of a mid-sized passenger car brake disc failed to pass homologation tests due to excessive temperature suggested by specifications at sequential braking test and excessive deformation at deformation – deflection test. Therefore, numerical analyses of different disc brake geometries and their affect to the heat dissipation and deformation characteristics were performed. The thermal and deformation behaviour was first analysed on an existing design using thermo-mechanic finite element analysis (FEA). Non-linear material physical data tested in laboratory combined with wall heat transfer coefficient obtained from CFD airflow analysis were used as a boundary condition. The airflow properties at all operating temperatures and speeds were examined and data were then organized in such a way that could be transferred into FEA analysis. The “swan neck” section and the ventilation channels were especially carefully examined.
Two different brake disc models were proposed, one with increased thermal capacity in the critical areas, best for short single stops and one with improved cooling properties, best suited for multiple sequential followed stops. FEM simulation of the portions of brake dynamometer homologation tests was done on the proposals using the same procedure as on the original brake disc.
The prototypes with proposed modifications were then manufactured and tested on the state of the art brake dynamometer. It turned out that numerical simulation and dynamometer experiment test have closely comparable results. The proposal with the increased cooling properties and modified “swan neck” area was finally chosen because its temperature and deformation are within the proposed boundaries of the homologation test.
KEYWORDS – ventilated brake disc, cooling efficiency, computational fluid dynamics (CFD) and finite element analysis (FEA), dynamometer tests, thermal deformation