Customer expectations for increasingly efficient vehicles together with stronger CO2 emission legislations put a high pressure on the development of next generation brake systems in terms of a reduction in residual torque. This becomes even more evident by the fact that on the vehicle level, 1Nm drag torque at each wheel can cause up to 2g CO2 per km, e.g. in the European NEFZ cycle.
In order to reduce the residual torque, strong efforts are currently undertaken into the direction of optimized caliper concepts and components, e.g. by fine-tuning piston and sealing ring geometries or by optimizing how a brake pad is held and moved inside the caliper. This paper addresses the topic of residual torque from the material point of view and discusses, in which way the friction process can contribute to a drag torque reduction in situations where unwanted contact between pad and disk is unavoidable.
Clearly, the energy conservation is maximized with a brake pad whose friction coefficient is minimized at very low contact stresses, which resemble the contact situation in typical off-brake scenarios with no full separation of pad and disk. In this work, tribological measurements with current and future (also copper-free) brake pad materials are shown that cover the very low normal force region equivalent to brake pressures below 3bar. The results are discussed for different sliding speeds (urban, interurban and highway scenario) and allow drawing first conclusions, e.g. on principal dynamic characteristics and differences between different material concepts. Along with the data, the high requirements on the measurement system are outlined, together with a suggested measurement procedure to quickly and reproducibly fingerprint a brake pad for its performance in terms of low μ at low pressure.
Kai Bode, Tobias Schramm, Nils Perzborn, Stephan Raczek, Jan Münchhoff, Georg-Peter Ostermeyer - Audi AG, TU Braunschweig, Institute of Dynamics and Vibrations