TMC expands practice for comparing performance of aftermarket brake linings

The Technology & Maintenance Council (TMC) of the American Trucking Associations has expanded an industry practice designed to help equipment owners compare the performance of aftermarket brake linings.

The document, TMC RP 628B, Aftermarket Brake Lining Classification, provides information for judging the performance of aftermarket brake linings on air-actuated foundation brakes, including testing in accordance with the dynamometer test procedure in Federal Motor Vehicle Safety Standard (FMVSS) 121 and lining supplier qualification information. Such information will assist fleet operators in choosing aftermarket brake linings that will perform adequately on typical combination (tractor/trailer) vehicles and single trucks.

The revised RP will appear in TMC’s 2010-2011 Recommended Practices Manual, scheduled for release in April 2010.

While performance of original equipment (OE) brake linings is regulated by FMVSS 121, linings sold as replacements for these friction materials are not. Testing of small lining samples to SAE J661a, producing a two letter “friction identification code” (EE, FF, GH, etc), is not considered accurate in determining performance on a full size brake. As a result, brakes relined with certain aftermarket materials can have reduced braking output, cause a shift of work to brakes on other axles, and reduce overall stopping capability of the vehicle.

The Performance Review Institute (PRI), an affiliate of SAE International, has formed the Brake Lining Performance Review Committee to review results of FMVSS 121 dynamometer performance tests conducted by qualified laboratories, in accordance with the test conditions described in this RP. RP 628B’s appendix represents the most recent results available at the time of this manual’s publication. Results are also made available through TMC’s website and PRI’s website Lining Program. Readers should visit these websites for the most current list. Linings that have been successfully reviewed are permitted to display a new TMC-PRI RP 628 logo to so signify this status.

Brake torque output can be compared for linings that have been tested for FMVSS 121 dynamometer performance, using the test conditions of this RP.

Three torque values are listed for each lining at application pressures of 20, 40, and 80 psi. Most vehicle brake applications are typically non-panic stops at low pressures—usually 20 psi or less. Medium braking occurs around 40 psi, while heavy or panic stops can be at 80 psi or higher. Historically, the 40-psi value has been used to match brake torque of aftermarket linings. The 20- and 80-psi values are also now reported for additional fleet operator information.

Torque values of an aftermarket lining should approximately match that of the original equipment lining it is replacing. The vehicle manufacturer should be able to provide the original equipment lining formulation with the vehicle identification number. Brake lining output torque, by itself, should not be used to compare such things as lining wear. Due to variability in testing and lining composition, torques shown in the aftermarket lining classification list are approximations for the best information available for matching torque output—the key to making a vehicle stop.

An additional feature of RP 628B is the brake fade index. Fade is a characteristic of brakes in which braking torque is reduced as brake temperature increases. Fade can be a concern for vehicle operations in which high brake temperatures are experienced, such as mountainous operation or heavy brake usage in cities.

The RP 628B Brake Fade Index uses the brake power portion of the FMVSS 121 dynamometer test. In this test, a brake is required to complete 10 snubs from 50 mph in a timed sequence.

The RP 628B Brake Fade Index is the change in maximum braking force effectiveness from Snub 1 to Snub 10 during the Brake Power portion of the FMVSS 121 dynamometer tests submitted for the given lining and brake listed. A higher brake fade index number indicates a lining with more fade.

Additionally, RP 628B provides this information for each manufacturer and lining formula:
•Whether or not the manufacturer of the lining has certified the specific lining formula as “asbestos-free.”
•What quality certification is held by the manufacturing plant(s) that produce the specific lining formula.
•Whether or not the lining has been tested via either FMVSS 121 stopping distance vehicle test or FMVSS 121 parking vehicle test.

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