Image
Image
Brake shoes for old (15X4-inch) front brakes (right), compared to newer, larger (16.5X5-inch), more aggressive front brakes, which will be needed to meet proposed standards.

The room at the January 2004 Technology and Maintenance Council's (TMC) annual meeting was packed. Everyone wanted to hear the National Highway Traffic Safety Administration's (NHTSA) announcement about what the requirements for the next generation of truck brakes would be. But all we learned was a Notice of Proposed Rulemaking was not yet ready, but could be expected that summer.

Three years later, the industry is still waiting for NHTSA's proposed rulemaking to change Federal Motor Vehicle Safety Standard 121. Federal officials expect the proposed rulemaking “to be ready soon.”

I don't think there will be any earth-shaking news. The industry expects the new ruling's major impact on heavy-duty trucks will be to shorten required stopping distances by either 20% or 30% for tractors because the overwhelming majority of fatal crashes involving trucks are with tractor-trailers.

Current regulations call for a 56,470-pound GCW test rig—a tractor pulling an unbaked test trailer—to stop in 355 feet from 60 mph. At both the Society of Automotive Engineers (SAE) and the TMC, supplier-members think the required stopping distance will be reduced by 30%, to 248 feet for tractors.

Requirements for single-unit (straight) trucks, including dumps and mixers, should be unchanged. Test single-unit trucks now must stop from 60 mph in 310 feet. However, dumps towing dozers and equipment may be subject to the tractor requirements.

Virtually all of today's trucks can exceed current requirements by 15% or more. In the real world today, most tractor-trailers stop from 60 mph in a range between 275 and 300 feet. Currently, most single unit trucks actually stop in 200 to 210 feet.

Fleet managers can anticipate much discussion on this new ruling. Recent tests demonstrated that the 248-foot target cannot be achieved using today's standard brakes—15-inch by 4-inch drums on steer axles, 16 ½-inch by 7-inch drums on other axles. With this configuration, stopping distances range from 255 to 292 feet.

Some believe that the NHTSA wants truck brakes to conform to those of cars, which stop from 60 mph within 125 to 150 feet. Eventually, the agency will turn its attention to straight trucks. The agency wants all vehicles' performance characteristics to be as similar as possible.

Larger front brakes

Suppliers say they need an extra 10% safety margin to assure consistency after multiple stops. That makes the target 223 feet for tractors and 195 feet for single-unit trucks. Supplier testing reported at TMC meetings indicates that to meet a 248-foot requirement, changes must be made to most brake systems. Larger front brakes will be needed to achieve the expected 30% reduction in stopping distances. Even so, there is little margin for fade unless larger drums are used all around.

One industry insider active in both the TMC and the SAE thinks that the NHTSA is considering establishing stopping distance requirements for 75 mph.