The National Highway Traffic Safety Administration revised its Federal Motor Vehicle Safety Standard (FMVSS) 121 to include new stopping distance standards that effectively shorten the majority of large tractor stopping distances by 30 percent.
In essence, the regulations have reduced the maximum allowable stopping distance to a minimum braking performance of 250 feet from 60 miles per hour, which is down from the previous standard of 355 feet.
Additionally, all heavy truck tractors must stop within 235 feet when loaded to their “lightly loaded vehicle weight.” Stopping distance requirements also were shortened for other load and system operating conditions.
FMVSS 121 is being implemented in phases. Three-axle tractors with a gross vehicle weight rating (gvwr) of 59,600 pounds or less must meet the reduced stopping distance requirements by Aug. 1. All other tractors must be in compliance by Aug. 1, 2013.
The regulation does not apply to retrofitting, as existing vehicles are not affected. Nor does it pertain to air-braked straight trucks, buses or trailers.
The truck manufacturer is responsible for making sure its vehicles are compliant with the new stopping distance regulation. The new rule will not significantly change brake system specifications or brake maintenance and service practices, but changes to foundation brake types and sizes are expected.
Today’s foundation brakes and air brake systems can meet the new requirements for the majority of the vehicles with some enhancements. However, for 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 longer than drum brakes. The drawback is they are heavier and more expensive than drum brakes.
Solutions available to meet the new requirements include higher performance steer axle drum brakes, which include larger diameter drum brakes, wider brake shoes and linings, air disc brakes or a hybrid combination of disc front and rear drum brake systems.
Manufacturers will offer options, so it will be up to the truck manufacturers to decide, by platform, how they will meet this regulation.
Progressive options are available to meet the new braking requirements:
• 15-by-5-inch or 15-by-6-inch drum brakes on the steer axle and 16.5-by-7-inch drum brakes on the drive axles.
• 16.5-by-5-inch or 16.5-by-6-inch drum brakes on the steer axle and 16.5-by-7-inch or 16.5-by-8.63-inch drum brakes on the drive axles.
• Air disc brakes on the steer axle and 16.5-by-7-inch or 16.5-by-8.63-inch drum brakes on the drive axles.
• Air disc brakes on the steer and drive axles.
Each progressive option provides more performance.
From a service standpoint, the new brake systems will not necessarily have components that can be interchangeable with the brakes on vehicles today.
It is anticipated that FMVSS 121 is not going to significantly affect the amount of drum brake maintenance that is needed.
Furthermore, the new upgraded steer axle brakes should now approach the performance capability of a rear brake. In some applications, the front brake can be doing more work and that work is distributed over more axles.
There’s an opportunity for longer brake life if everything is optimized across all axles of a combination vehicle.
With the larger drum and disc brakes maintenance costs should go down. Larger brakes will provide such advantages as increased lining volume to drive longer service intervals, lower operating temperatures, reduced fade and improved performance.
Air disc brakes are internally adjusted and factory-greased so maintenance requirements are reduced. Air disc brakes also have fewer parts to replace.
Tractor and trailer braking system balance will not be affected. The air systems will be balanced like they are today, with no need for pressure hold-off valves or changing chambers, etc., to get compatibility among tractor and trailer brakes systems. However, some algorithm changes on ABS systems might occur because of the performance difference.
The tractor will incur a little more of the braking workload, which will have some affect on the front axle and suspension components. The trailer will experience a proportionate reduction in workload.
Another thing to be aware of is that with trucks that can stop in shorter distances, more attention must be paid to properly securing cargo. BI