The Meritor ES22525 heavy-duty air disc brake.

Disc brakes are better able to dissipate heat and to release any gasses. They are inherently more resistant to fade than drum brakes. ADBs have been available in the U.S. since the 1980s, but have been used in limited applications. Today, three companies supply them, Bendix, MeritorWABCO and Haldex, through all truck manufacturers.

The augmented brakes will add to the cost of new trucks. NHTSA estimates that enhanced drum brakes would add $85 for each steer axle and $65 per drive axle for a three-axle truck. The agency's estimate for disc brakes is $500 per axle regardless of position.

Freightliner indicated that the incremental cost for enhanced drum brakes would be $222, while disc brakes all around would add $963 to a two-axle rig and $1627 for three axles, according to comments made to NHTSA. Disc brakes are about equal in weight to high performance drum brakes, according to WABCO.

NHTSA did consider stopping distance requirements from 75 mph. If they had been adopted, ADBs would have been needed, especially if the standard had required multiple stops within a short period of time, conditions that more closely simulate actual driving demands and the need for fade resistance. Consumer safety groups favored multi-stop requirements.

High-speed braking

Since ready-mix trucks normally do not operate at such high speeds, there is no need for more expensive disc brakes to meet requirements. But if higher standards are set in the future, concrete producers may have their options taken away. High-volume often dictates manufacturing specifications

One of the greatest benefits expected form these shorter stopping distances will be reducing impact velocity in rear-end, truck-into-car crashes. In some cases, “according to the final rule, reduced stopping distances will stop a crash from occurring.”

During the comment period before NHTSA issued the final rule, several organizations mentioned unintended consequences of shortening stopping distances. Brake maintenance intervals may be shortened, and wear to system components could increase due to increased torque levels.

Cargo and load restraint systems, drum supports, and drive mechanisms may also need strengthening as g-loads increase. The counter argument is that except for emergency braking, most deceleration will be at current rates.

Paul Abelson is a former director of the Technology and Maintenance Council of the American Trucking Associations and is currently on the Board of Truck Writers of North America. E-mail