Reich, A., Sarda, A., and Semsch, M., "Drag Torque in Disk Brakes: Significance, Measurement and Challenges," SAE Int. J. Commer. Veh. 8(2):276-282, 2015, doi:10.4271/2015-01-2670.
Drag torque is generated in disk brakes as a result of contact (dynamic friction) between brake disk and brake pads when the braking system is not actuated. Among the negative implications of drag torque are, notably, dispensable additional fuel consumption as well as increased pad wear, which can also unfold as uneven along the pads' surfaces. The paper is based on extensive knowledge acquired through several measurement-based studies and contains a comprehensive assessment of the main topics related to the subject.Dedicated measurements illustrate the influence of different parameters on the level of drag torque and deliver a basis for a discussion about the conflict of objectives which may arise from the implementation of specific mitigation measures.Further emphasis is laid on the presentation of the state of the art concerning the main direct and indirect methods to evaluate drag torque characteristics of brake systems. The particular advantages, disadvantages and limitations of the individual methods are discussed. The experiment-based comparison leads to the identification of distinguishing benefits arising from dynamometer testing. Moreover, the reasons leading to the high complexity linked to measuring drag torque reliably and representatively are presented and advancements reached in this field are pointed out.Finally, key points for defining testing specifications, including important boundary conditions and evaluation methods are presented. Thus, it also represents a contribution for developing procedures which account for essential requisites to be integrated in the state-of-the-art of development of brake systems: Practicability, reliability, reproducibility and reference to real vehicle operational conditions.