It is widely expected that the first stage of the new stopping distance regulation for new heavy trucks issued by the National Highway Traffic Safety Administration (NHTSA)—to occur in mid-2011-- will be met by spec’ing either enhanced drum bake packages or air disc brakes (ADBs). But truck OEMs expect the second phase of the rule – to occur in mid-2013—may lead to wider use of ADBs as more severe-duty vehicles are impacted.

Once in effect, NHTSA’s new braking standards will mandate that most tractors when coupled to a trailer and traveling at 60 miles per hour will have to come to a complete stop in 250 ft vs. the old standard of 355 ft – or reduction of stopping distance of roughly 30%.

For a small number of very heavy severe-service tractors, the stopping distance requirement will be 310 ft under these same conditions. In addition, the rule will require that all heavy-truck tractors must stop within 235 ft when loaded to their “lightly loaded vehicle weight” (LLVW).

Regarding the initial phase-in of the rule, NHTSA will require new three-axle tractors with a gross vehicle weight rating (GVWR) of 59,600 lbs or less to meet the reduced stopping distance requirements by August 1, 2011.

Two-axle tractors and tractors with a GVWR above 59,600 lbs will have to meet the reduced stopping distance requirements by August 1, 2013.

NHTSA has noted that fleets may elect to comply with the rule earlier than those set dates.

Truck builders indicate they expect wider versions of the drum brakes currently used will provide more than enough stopping power to comply with the new rule.

“For the most part, drum brakes will handle the change, with associated systems being validated to determine if any additional changes will be needed,” Frank Bio, product manager-trucks for Volvo Trucks North America (VTNA), told FleetOwner.

“This includes suspensions, axles and brake components including linings,” he continued. “We expect a change in lining and the addition of brake shields as a method meet the test requirements. This will not eliminate the need for disc brakes in certain configurations, but for the first phase, we expect [drum] brakes to handle the requirements.”

But what brakes may be needed by vehicles affected by the second stage is a different matter. “For the 2011 stopping distance regulation ADBs are not required,” Jerry Warmkessel, marketing product manager-highway products for Mack Trucks, told FleetOwner. “Ffront ADB's, however, will be required” to meet the 2013 part of the rule. “Therefore, many astute customers are considering spec’ing front ADBs now so as to have several generations of brake parts on their shelves.”

“We’re confident that the new stopping distances can be met with minor modifications to our current drum and air brake systems,” Rich Shearing, director-product planning for Daimler Trucks North America, told FleetOwner. “But disc brakes can provide better stopping performance, and we will have a disc brake offering in the future, in particular for short wheel base vehicles. As stopping distances become more stringent, we anticipate an increased migration to disc brakes.”

ADBs are not a popular choice now for North American commercial trucks mainly because they are heavier and cost more to purchase than drums, Landon Sproull, chief engineer for Peterbilt Motors, told FleetOwner. “Even small swings of weight on the front axle are very critical,” he said.

The major advantages of ADBs is they do not fade after repeated applications and last much longer than drums. But those benefits are diluted somewhat by the North American operating environment, Ramin Younessi, Navistar’s group vp of product development and strategy, told FleetOwner. “Disc brakes are the standard in Europe because the trucks there stop so much more frequently over their life cycle,” he said. “In North America, you have long highway miles where stopping is infrequent.”

Yet as drum brakes necessarily get larger – and thus more expensive – to comply with NHTSA’s new stopping regulations, the lower cost and weight advantage they enjoy may be diminished, Younessi said.

“Disc brakes married to aluminum hubs could end up weighing a lot less compared to the larger drum brakes we’ll need to use – with the cost difference being much less than it is today,” he said. “That’s when the longer life of disc brakes would become a greater factor; that’s when we might start seeing some shifting.”