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The future of braking
OEMs eye air disc brake technology to meet shortened stopping distance standards

By Paul Abelson
Senior Technical Editor

The room at the Technology and Maintenance Council’s Annual Meeting was packed in March 2004.

Everyone wanted to hear NHTSA’s announcement about what the next generation of brakes would be required to do.

The news was that a Notice of Proposed Rulemaking was not ready, but could be expected that summer.

It is now almost 16 months later, and the industry is still waiting for the proposed rulemaking from NHTSA. The feds expect the proposed rulemaking to be ready in August.

Expectations throughout the industry are that the major impact on heavy-duty trucks will be to shorten required stopping distances. Current regulations call for a 56,470-pound GCW test rig – a tractor pulling an unbraked test trailer – going 60 mph to stop in 355 feet.

Making their own rule

Truck OEMs are eager to learn what the changes to FMVSS 121 will be. A highly placed industry source told Land Line that the Truck Manufacturers Association brought up the idea of the industry proposing its own rule to NHTSA, to stimulate them into action. The TMA proposal, discussed at a recent SAE Heavy Duty Brake Committee meeting, will call for a 25 percent reduction in stopping distance to 266 feet. Applying Clark’s 10 percent margin, the target could be 239 feet.

Virtually all of today’s tractors exceed the requirement by 15 percent or more. At the Society of Automotive Engineers and the Technology and Maintenance Council, respective members think the required stopping distance will be reduced 30 percent, to 248 feet.

Tests indicate that the 248-foot target cannot be achieved consistently using today’s brakes – 15-inch by 4-inch drums on steer axles, 16 1¼2-inch by 7-inch drums on drive axles. With that configuration, stopping distances range from 255 to 292 feet.

Industry tests, conducted by brake system suppliers and fleets, as reported at TMC meetings, indicate that at least front brakes will need to be larger to achieve the expected 30 percent reduction in stopping distances.

Air disc brakes on the steer axle coupled with standard drive axle drum brakes consistently stop in less than 248 feet. Other configurations can achieve the expected standard, too. Using 16 1¼2-inch by 5 1¼2-inch drums in front is effective with normal 7-inch wide drums on drive axles. Even more effective in consistently stopping test trucks from 60 mph are combinations of 8 5¼8-inch wide drums behind 5 1¼2-inch wide front brakes.

In other words, the anticipated 248-foot stopping distance requirement from 60 mph can be met with drum brakes.

But Jim Clark of Bendix Spicer Foundation Brake LLC, a joint venture between Bendix Commercial Vehicle Systems and Dana Corp., believes that to consistently meet a 248-foot requirement, a 10 percent safety margin is needed, making his target 220 feet.

According to NHTSA’s Jim Britell, research is ongoing. He declined to be specific, but an industry insider active in TMC and SAE thinks that NHTSA is considering stopping distance requirements from 75 mph.

If true, air disc brakes may be required, especially if the standard requires multiple stops within a short period of time to more closely simulate actual driving demands and the need for fade resistance. Today’s drum brakes stop the test rig in an average of 518 feet, with the shortest distance measured at 450 feet. Front air disc brakes shorten that to an average of 390 feet. Air disc brakes all around drop that to 206 feet, with 196 feet being the shortest.

Brakes fade, or lose stopping power, because of heat. Energy cannot be created or destroyed, but it can be converted to another form. Trucks – and cars and airplanes – stop when their kinetic energy, or energy of motion, is converted to heat. Friction material is forced against the brake drum or disc. The friction creates heat, slowing the vehicle in the process. The drum or disc conducts the heat away from the pad or shoe 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, starts to cook, releasing gasses that act as lubricants, reducing friction. That is felt as brake fade, often described as “the brakes going away.”

the technology  is already in use

Thirty years ago, NHTSA mandated ABS before the technology was proven. Systems were rushed into production. Unproven systems, full of bugs, caused crashes. It took a lawsuit to overturn NHTSA’s ruling. For years, truck drivers distrusted ABS. Meanwhile, research and development continues, unhindered by the pressures of federal regulation. Several progressive fleets, members of TMC, worked with OEMs, suppliers and NHTSA to de-bug the systems and prove them in the real world. When tractor and trailer ABS were again mandated a decade ago, the systems already had millions of miles of testing. They were available. Regardless what new requirements NHTSA proposes, they will be based in existing, proven technologies that are already in use.

Disc brakes are better able to dissipate heat and to not release any gasses. They are inherently more resistant to fade than drum brakes, especially with multiple applications such as when descending long, steep, curved grades. Air disc brakes have been available in the U.S. for more than a decade, but have been used in limited applications. Today, four companies have them: Bendix Spicer, WABCO, Arvin Meritor and Haldex.

Ron Szapacs specs trucks and equipment for Air Products. He said the private fleet has been using air disc brakes for more than a decade. In 1998, the company stopped using them on drive axles when they switched to 22 1¼2-inch wheels. At the time, air disc brakes could be used only with 24 1¼2-inch wheels. Today, all manufacturers make ADBs for 22 1¼2-inch wheels.

Air Products continued to spec air disc brakes on their steer axles. They get more than 500,000 miles with no brake maintenance at all needed. There have been no rotor problems and pads still have a great deal of life left.

Marty Fletcher has had similar experiences at US Xpress Enterprises. The company has been part of a NHTSA-sponsored five-year test of electronically controlled braking systems.

Fletcher reports no serious brake-related problems with the 50 trucks equipped with air disc brakes. He projects pads will last between 750,000 and 1 million miles.

Electronically controlled braking systems, used widely in Europe, do away with pneumatic control systems, taking air from tanks near each brake chamber. FMVSS 121 currently mandates dual pneumatic systems. ECBS replaces air with a faster acting “brake by wire” system.

The advantage of ECBS is that signals to brakes are virtually instantaneous with maybe only about a two-tenths of a second lag from the time the driver steps on the brake’s air treadle valve until the time the brakes start to apply. Because redundant air control systems are still mandated by FMVSS 121, there is little likelihood of ECBS gaining acceptance until the standard is revised. Perhaps the upcoming proposed rulemaking from NHTSA will include ECBS.

Paul Abelson may be reached at