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The efforts of the ISO “Test Variability Task Force” have been aimed at improving the understanding and at reducing brake dynamometer test variability during performance testing. In addition, dynamometer test results have been compared and correlated to vehicle testing. Even though there is already a vast amount of anecdotal evidence confirming the fact that different procedures generate different friction coefficients on the same brake corner, the availability of supporting data to the industry has been elusive up to this point. To overcome this issue, this paper focuses on assessing friction levels, friction coefficient sensitivity, and repeatability under ECE, GB, ISO, JASO, and SAE laboratory friction evaluation tests.

Inertia dynamometers are commonly used to determine the friction coefficient of brake assemblies. Dynamometers are a well-established platform, allow testing under controlled conditions, exhibit a good correlation to many situations encountered in real driving, and are comparatively economical and less time-consuming than full vehicle test. On the other side of the spectrum is the use of scaled tribometer. These test systems make possible a test without the entire brake corner. This separation allows the investigation of the frictional-contact only (frictional boundary layer) speedily and independently of a given brake system or vehicle configuration. As the two test systems (inertia dynamometers and tribometers) may have different users with possibly different tasks, the question remains regarding how comparable the two systems are. These issues provide incentives to better define the fields of investigations, correlation, and applicability for the two systems.

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.

The SAE Brake Linings Committee launched an evaluation of the current SAE standard J840 “Test Procedures for Brake Shoe and Lining Bonds,” specifically the bond plane shear test. The aim of the study was to identify what settings in the shear test lead to the most repeatable results for a variety of linings. An initial four factor two-level design of experiments (DOE) was conducted, with 15 pads sheared for each condition. The DOE looked at pad material type (NAO and semi-metallic), backing plate attachment type (integrally molded and mechanical retention system), ram offset position (1.0 mm and 4.0 mm) and normal pressure (0 and 0.5 N/mm2). The test measured shear force and percentage material retention for each pad tested. The objective of the DOE was to guide revision of the SAE J840 shear test procedure. For example, the application of a normal force raises the nominal value of the shear force by 30-50%, and reduces the variability of the test results.

This paper presents three main topics which proved useful during the systematic resolution and testing program to confirm the ability of the proposed friction material to conform to the performance requirements indicated on the TP-121D [1] dynamometer test. Initially, the paper presents some commonalities and differences between the vehicle FMVSS 121[2], the dynamometer TP-121D and the SAE J2115-06 [3] test protocols. The second part of the paper elaborates on the implementation of the methodology established on the ASTM E1169-07 [4]. This standard relies on Design of Experiments (DOE) methods to assess the robustness of a given test method when testing on the extreme values allowed for key test conditions. The DOE used a three-factor, two-level, fractional factorial design to investigate the influence of (a) cooling air speed, (b) brake power as the combination of test inertia and deceleration settings, and (c) brake adjustment method.

There is vast literature and peer-reviewed methods to estimate losses1 during aerosol sampling. However, there are no current published models for transport losses during laboratory measurement for brake emissions. This paper presents an open source (Microsoft® Excel) Macro using three different models (Particle Loss Calculator - PLC - from the Max Planck Institute [1]; AeroCalc [2] from the United States Center for Disease Control and Prevention; and SAE AIR6504™:2017-10 [3] for calculation of non-volatile particulate matter penetration). The fourth model (LINK) provides the average value from the three initial models. The LINK PALS2 Microsoft® Excel Macro (or ‘Macro’ for short version) also includes calculations for isokinetics not included on any of the three initial models.

Modern project management including brake testing includes the exchange of reliable results from different sources and different locations. The ISO TC22/SWG2-Brake Lining Committee established a task force led by Ford Motor Co. to determine and analyze root causes for variability during dynamometer brake performance testing. The overall goal was to provide guidelines on how to reduce variability and how to improve correlation between dynamometer and vehicle test results. This collaborative accuracy study used the ISO 26867 Friction behavior assessment for automotive brake systems. Future efforts of the ISO task force will address NVH and vehicle-level tests. This paper corresponds to the first two phases of the project regarding performance brake dynamometer testing and presents results, findings and conclusions regarding repeatability (within-lab) and reproducibility (between-labs) from different laboratories and different brake dynamometers.