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The ISO TC22/SWG2 - Brake Lining Committee established a task force to determine and analyze root causes for variability during dynamometer brake performance testing. SAE paper 2010-01-1697 “Brake Dynamometer Test Variability - Analysis of Root Causes” [1] presents the findings from the phases 1 and 2 of the “Test Variability Project.” The task force was created to address the issue of test variability and to establish possible ways to improve test-to-test and lab-to-lab correlation. This paper presents the findings from phase 3 of this effort-description of factors influencing test variability based on DOE study. This phase concentrated on both qualitative and quantitative description of the factors influencing friction coefficient measurements during dynamometer testing.

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.

Residual brake torque (RBT) is generated in disc brakes as a result of contact between brake disc and brake pads when the braking pressure is not applied. Among the negative implications of RBT are, notably, dispensable additional fuel consumption as well as increased pad (taper) wear. Several properties of the brake system have a direct influence on the level of residual torque [1]. A major effect is connected to the caliper properties determining the clearance gap. This is characterized by the default air gap between pads and disc and its distribution regarding vehicle inner and outer sides (piston and fist sides for floating type calipers). Initial air gap is mainly influenced by the sealing grove design (between piston and housing, where the sealing ring is positioned). The retraction of the piston due to the sealing ring, also called rollback, mainly depends on the load case (e.g. applied pressure and temperature).