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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.

If this is true, air disc brakes may become necessary, especially if the standard requires multiple stops within a short period of time to more closely simulate actual driving demands for fade resistance.

Today's drum brakes stop the test rig in an average of 518 feet from 75 mph, 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.

Brakes fade, or lose stopping power, because of heat. Energy cannot be created or destroyed, but it can be converted to another form. When a truck brakes, its energy of motion is converted to heat through the friction material as it's forced against the brake drum or disc. The friction turns motion into heat, slowing the truck 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.

Brake fade

Future breaking systems will all be air disc. One benefit is these units resist brake fade. If heat builds up more rapidly than it can be drawn away, the friction material in the shoe or pad in a drum brake starts to cook. The reaction releases gasses that act as lubricants, reducing friction. That results in brake fade, often described as “the brakes going away.”

Disc brakes are better at dissipating heat and not releasing as much gas. They inherently resist fade more than drum brakes, especially with multiple applications such as descending long, steep, curved grades. Air disc brakes have been available in the United States for more than a decade, but have been used in limited applications. Today, four companies supply them: Bendix Spicer, WABCO, ArvinMeritor, and Haldex.

At TMC meetings, members who use air disc brakes on-highway report brake life of 500,000 to 700,000 miles with no maintenance issues. No members reported using them in vocational or in on- and off-highway applications.

Disc front brakes have other benefits. There is less side-to-side variation and more consistent brake torque. This improves steering stability during braking. This can provide an important safety margin, especially for mixers.

Should the rule be adopted, fleet managers must sacrifice something. Shorter stopping distances will cause greater weight transfer to the front, regardless of brake type. Trucks built for the new regulations will have heavier front springs and suspensions.

— Paul Abelson is senior technical editor for Road King and Land Line magazines, both serving the trucking industry. He is a former director of the Technology and Maintenance Council of the American Trucking Associations. E-mail him attruckwriter@anet.com.

Work Truck Show Coverage

Look for a special supplement in your March issue of THE CONCRETE PRODUCER for the National Truck Equipment Association's Work Truck Show, which takes place March 7–9, in Indianapolis.

Fleet managers will be very interested in this section, which will highlight product categories such as dump and service truck bodies, tires, and work truck tools. More than 400 truck equipment manufacturers will be at the show. We also will provide a glimpse of some of the show's special events.

For more information, visit www.ntea.com.