The Federal Motor Carrier Safety Administration (FMCSA) of the U.S. Department of Transportation (DOT) regulates the thickness of brake lining and pads in trucks pursuant to Title 49 CFR § 393.47 (Brake actuators, slack adjusters, linings/pads and drums/rotors.) These regulations are located in Subpart C of the parts and accessories necessary for safe operation requirements.
DOT is responsible for setting minimum standards for commercial motor vehicles, which are defined by law as vehicles used in interstate commerce to transport passengers or property that either (1) have a gross vehicle weight rating (GVWR) of at least 4,536 kg (10,001 lbs.); (2) are designed to transport at least seven paying passengers; (3) are designed to transport at least 14 non-paying passengers; or (4) are used to transport hazardous materials.
Steering Axle Brakes
Title 49 CFR § 393.47(d)(1) states that the thickness of the brake lining and pads of steering axle brakes on a truck, truck-tractor or bus "shall not be less than 4.8 mm (3/16 inch) at the shoe center for a shoe with a continuous strip of lining." The thickness requirements are at least 6.4 mm (1/4 inch) for two pads.
If air brakes are used, the minimum thickness is 3.2 mm (1/8 inch). If hydraulic disc, drum or electric brakes are used, the minimum thickness is 1.6 mm (1/16 inch).
Non-Steering Axle Brakes
Title 49 CFR § 393.47(d)(2) states that the thickness of the brake lining and pads of non-steering axle brakes on an air braked commercial motor vehicle shall not be less than 6.4 mm (1/4 inch).
If disc brakes are used, the minimum thickness is 3.2 mm (1/8 inch). If hydraulic or electric brakes are used, the minimum thickness is 1.6 mm (1/16 inch).