s h o p t a l k

Make Replacement Linings Match

TMC procedure qualifies friction levels so brakes remain balanced.

Jim Winsor
Executive Editor

Ideally, when buying replacement brake linings you want products whose performance will match that of the linings originally installed by the vehicle manufacturer. Any change in friction levels will make brakes more or less aggressive and may “unbalance” a good brake system.
     Since there are no DOT standards for replacement brake linings, it can be difficult to ensure that a replacement lining’s friction level is the same or equal to the original’s. Linings installed on new air-braked trucks, tractors and trailers must meet Federal Motor Vehicle Safety Standard 121, often referred to as the antilock brake standard. FMVSS-121 actually covers almost the entire air brake system, not just ABS.
     To take the mumbo jumbo out of buying replacement brake linings, The Maintenance Council of the American Trucking Assns. developed a Recommended Practice about four years ago, outlining a procedure for manufacturers to voluntarily qualify replacement linings.
     RP 628, titled “Aftermarket Brake Lining Classification,” provides information for judging linings for S-cam brakes that have been qualified in accordance with the brake dynamometer test procedure spelled out in FMVSS-121.
     Linings listed in the accompanying table have been tested to this procedure and reviewed by the SAE Brake Lining Performance Review Committee to verify they meet minimum FMVSS-121 requirements. Linings can stay on this list for up to three years, at which time they must be retested to make sure performance hasn’t changed.
     In addition, SAE conducts random off-the-shelf spot testing of linings purchased in the aftermarket. This data is compared against the original and if they don’t match up and/or fail the FMVSS-121 test, they’re removed from the list. Linings in this table are current as of August 1998. Four linings are listed for the first time, including one qualified for 22,000- to 23,000-pound axle rating.
     Since linings that pass have different friction levels, some are more aggressive than others. That’s why RP 628 lists the torque rating in inch-pounds at 40 psi brake pressure. This approximates fairly severe braking. Most routine braking/stops are under 25 psi.
     Linings are listed in ascending friction order, with 30-inch chambers first and 24-inch chambers last. Linings certified for 24-inch chambers (usually used on pre-1997 Macks and some tri-axle construction-type trailers) are more aggressive and friction levels are not interchangeable with linings for 30-inch chambers.
     The same friction range of linings are typically used on axles with 17,000- to 20,000-pound gross axle weight ratings (which include 34,000- to 40,000-pound tandems). More aggressive linings with different brand/model identification are used with the heavier 22,000- to 23,000-pound ratings (also 44,000- and 46,000-pound tandems).
     These are the two highest-volume brakes in use today. Only one manufacturer so far as qualified its linings for the 22,000- to 23,000-pound ratings. This does not mean other brands in the aftermarket don’t meet FMVSS-121. They just have not yet been submitted to SAE for qualification.
     SAE and TMC stress that qualification of a lining does not constitute SAE or TMC approval of any products. The listing only verifies that the brake lining material, as represented in the data presented, met the FMVSS-121 requirements when installed on a 161/2-by-7 S-cam brake and tested in the brake configuration specified in RP 628A. Linings are listed in ascending torque value as measured during the test at 40 psi. Order is not based on brake lining quality.

SIDEBAR
Replacement Brake Linings That Meet RP 628 Requirements