Enhanced Braking

New federal braking regulations for heavy trucks

Last July, the National Highway Traffic Safety Association (NHTSA) issued new stopping distance standards that effectively shorten the majority of large tractor stopping distances by 30 percent. Federal Motor Vehicle Safety Standard (FMVSS) 121 affects all new air-braked vehicles including trucks, buses, tractors and trailers.

In essence, the regulations have shortened the maximum allowable stopping distance to a minimum braking performance of 250 feet from 60 mph, down from the previous standard of 355 feet. For severe-service tractors, the new stopping distance requirement is 310 feet, also down from the previous 355-foot distance. That is for all loaded vehicle conditions.

In addition, all heavy truck tractors must stop within 235 feet when loaded to their "lightly loaded vehicle weight" (LLVW). The stopping distance requirements were also shortened for other load and system operating conditions in the FMVSS 121 regulation.

NHTSA considers a severe-service vehicle as a three-axle tractor with a gross vehicle weight rating (gvwr) greater than 70,000 pounds or a four or more axle vehicle with an 85,000-pound or greater gvwr.

FMVSS121 will be implemented in phases, says D. Mike Pennington, senior director, global marketing, communications and media relations for ArvinMeritor. Three-axle tractors with a gvwr of 59,600 pounds or less must meet the reduced stopping distance requirements by August 1, 2011. All other tractors must be in compliance by August 1, 2013.

ArvinMeritor is a premier global supplier of a broad range of integrated systems, modules and components to the motor vehicle industry, and serves commercial truck, trailer and specialty OEMs and certain aftermarkets.

The regulation does not apply to retrofitting, as existing vehicles are not affected, or to roll stability or collision mitigation systems, notes Paul M. Johnston, senior director, compression and braking, for Meritor WABCO. Nor does the regulation pertain to air-braked trucks, buses or trailers or to hydraulic-braked vehicles.

Meritor WABCO is the leading supplier of braking systems and controls and active safety systems for commercial vehicles in North America.

Highway crash and fatality data for heavy vehicles show that the majority of crashes involve tractor trailer configurations. The goal of NHTSA and its activity with revising the FMVSS 121 braking regulations was to get commercial vehicles to stop as fast (short) as a car - 195 feet, Johnston says, and the industry has made "significant improvements" towards achieving this.

While single trucks, buses and trailers are not included in this rule, he says future research by NHTSA may impact future stopping distance requirements for other air-braked and hydraulic braked vehicles.


"The truck builder is responsible for making sure its vehicles are compliant with the new stopping distance regulation," says Pennington. "The new rule will not significantly change brake system specifications or brake maintenance and service practices, but it is expected that there will be changes to foundation brake types, as well as sizes."

No change in trailer foundation brakes is necessary.

Today's foundation brakes and air brake systems can meet the new requirements for the majority of the vehicles with some enhancements. However, for the heavier gvwr tractors and two-axle tractors, the consensus is that air disc brakes will grow in use.

The major advantage of air disc brakes is that they don't fade after repeated applications and last much longer than drum brakes. The drawback: they are slightly heavier and more expensive than drum brakes.

In Europe there has been a significant movement away from drum brakes to air disc brakes because the vehicles there are of a different configuration and stop much more frequently over their life cycle than do heavy trucks in North America, Pennington points out.

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