Adding heavy-duty air disc brakes like the Meritor ES225, shown above, will ensure new trucks meet the National Highway Traffic Safety Administration's new shortened stopping distance requirements. Photo: MeritorWABCO

The National Highway Traffic Safety Administration (NHTSA) issued its long-awaited revisions to Federal Motor Vehicle Safety Standard FMVSS 121 in the July 27, 2009 issue of Federal Register, calling for a 30% reduction in stopping distances starting Jan. 1, 2011. This rule will affect all applicable 2011 models.

While written around heavy-duty tractors, the executive summary indicates that stopping distances apply to all air-braked vehicles and to hydraulic-braked vehicles greater than 10,000 pounds gross vehicle weight rating. This includes dump trucks, snow plows, and other heavy vehicles that use braking systems similar to tractor-trailers.

Current regulations call for a tractor pulling an unbraked test trailer to stop from 60 mph in 355 feet. Virtually all of today's tractors beat that requirement by 15% or more, currently stopping within 300 feet. But the new stopping distance will be 250 feet.

Today's brakes — 15x4-inch drums on steer axles and 16½x7-inch drums on drive axles — cannot achieve the new target. The best stopping distances are between 255 and 300 feet.

To achieve the 30% reduction in stopping distance, industry tests conducted by brake system suppliers and fleets indicate that, at the very least, front brakes will need to be larger. Also, air disc brakes (ADBs) on the steer axle coupled with larger drive axle drum brakes can consistently stop in less than 250 feet. Oversize front drum brakes, at 16½x5½ inches, coupled with normal 16½x7-inch-wide drums on drive axles can also meet the requirement, but with little margin for error. More effective in consistently stopping test trucks from 60 mph are wider 16½x8 5/8-inch drums on drive wheels used with the larger front drum brakes.

Today's brake configurations stop test trucks in 300 feet. Replacing steer-axle brakes with ADBs lowers that distance significantly. ADBs on all axles will stop those trucks in just 234 feet. For comparison, cars stop from 60 mph in less than 130 feet.

Brakes work by converting kinetic energy to heat. Linings or pads made of high-friction materials rub brake drums or discs. The drum or disc conducts the heat away from the lining or pad, to be dissipated to the air flowing over the brake. If heat builds up more rapidly than it can be drawn away, the friction material in the shoe or pad cooks and releases gasses that lubricate the contact area. That reduces friction and creates what we feel as brake fade, or “the brakes going away.”

Disc brakes are better able to dissipate heat and to release any gasses; they are inherently more resistant to fade than drum brakes. Air disc brakes have been available in the United States since the 1980s, but have been used in limited applications. Today, three companies supply them — Bendix, MeritorWABCO and Haldex — through all truck manufacturers.


The augmented brakes will add to the cost of new trucks. NHTSA estimates that enhanced drum brakes would add $85 for each steer axle and $65/drive axle for a three-axle truck. The agency's estimate for disc brakes is $500/axle regardless of position.

Truck manufacturer Freightliner informed NHSTA that the incremental cost for enhanced drum brakes would total $222, while ADBs would add $963 to a two-axle rig and $1,627 for three axles. According to WABCO, disc brakes are about equal in weight to high-performance drum brakes.