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 121 and lining supplier qualification information. The recommendation is meant to help fleet operators choose adequate aftermarket brake linings for typical combination vehicles and single trucks.
The new information will appear in TMC's 2010-2011 Recommended Practices Manual, scheduled for release this month.
While performance of original equipment 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," 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 the 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.
Brake torque output can be compared for linings that have been tested for FMVSS 121 dynamometer performance, using the test conditions of this RP.
The revised best practice also includes a brake fade index. Fade is a characteristic of brakes in which braking torque is reduced as brake temperature increases, and it 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.
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