More Changes Seen in Shoes, Friction Materials as Phase 2 of Stopping-Distance Rule Approaches

By Stephen Bennett, Special to Transport Topics

This story appears in the July 2 print edition of Transport Topics.

While shorter stopping distances mandated last year by the National Highway Traffic Safety Administration prompted the introduction of new brake configurations in Class 8 tractors, a pending set of stopping standards for a subset of tractors is now prompting even more changes, according to truck equipment manufacturers.

The second phase of the stopping distance regulation takes effect on Aug. 1, 2013, and will apply to the balance of on-highway tractors — the 10% to 20% of them with two axles, and tractors with three or more axles that have gross vehicle weight ratings of more than 59,600 pounds.

The required stopping distances for those power units vary according to weight rating and number of axles. For a 3-axle tractor with a weight rating between 59,600 and 70,000 pounds, the distance will be 250 feet; for a 3-axle tractor greater than 70,000 pounds, the new stopping distance will be 310 feet.

To deal with Phase 1 of Federal Motor Vehicle Safety Standard 121, as the regulation is known, brake manufacturers said they made drum brakes bigger and used improved friction materials to provide the added stopping power needed to meet shorter stopping distances. Air-disc brakes are being offered as an option in some cases, they said.

For example, Peterbilt Motors Co., Denton, Texas, last year made air-disc brakes standard on all its Class 8 tractors (4-4-11, p. 3).

“The truck’s application and GVWR largely determine the solution, whether it’s brake size, brake material or installation of air-disc brakes,” said Frank Bio, product manager for Volvo Trucks North America, Greensboro, N.C.

To meet the upcoming stopping requirements, manufacturers said, they are taking similar measures a step or two further.

Most tractors in the United States are made with drum brakes, and the NHTSA regulation does not require that the reduced stopping distance be achieved via one type of brake or another, manufacturers noted.

A drum brake, according to the American Heritage Dictionary, is a brake “in which the friction is caused by a set of pads that press against the inner surface of a rotating drum.” A disc brake is defined as a brake in which the friction is caused by a set of pads that press against a rotating disk.

Phase 1 of Safety Standard 121 required a 30% reduction in stopping distance for new 3-axle tractors with a GVWR up to 59,600 pounds produced after Aug. 1, 2011. The new stopping distance for those vehicles is 250 feet from a speed of 60 mph.

Manufacturers have estimated that 80% to 90% or more of new on-highway tractors rolling off assembly lines are subject to Phase 1 of the regulation.

“I think there was a lot of misconception leading up to Phase 1 that truck manufacturers were going to have to go to air-disc brakes,” said Matthew Creech, brake business manager for Meritor Inc., Troy, Mich. In stopping-distance tests, Creech said, the manufacturer’s drum brakes stopped tractors within about 10 feet of the distance achieved by the air-disc brake.

“You essentially have to generate more brake torque to achieve [reduced] stopping distance,” said Tom Hewer, vice president of development engineering for MHT Brake & Wheel Ends Group, Cullman, Ala., which manufactures brake drums, rotors and hubs. More torque requires either switching to air-disc brakes or changing aspects of drum brakes to enhance their performance, Hewer said.

Hewer had worked in the same capacity for Webb Wheel Products before its parent company created MHT Brake & Wheel Ends Group.

“Disc brakes work,” said Hewer. “They work very, very well.” But they are not in widespread use in the United States, in part because they come at a cost premium, he said.

To enhance drum brakes, manufacturers said, they created new material for brake linings that is better able to withstand the high temperatures generated during braking.

“One of the enemies of effective braking is heat,” Hewer said. At about 600 degrees, resins in lining material “degrade in their friction effectiveness,” and the drum ex-pands, resulting in what is known as “fade” — which requires more pressure to apply braking power, Hewer said. To offset the fade, manufacturers said, they increased the size of drum brakes to provide a lar-

ger surface over which heat can dissipate.

As a result of these changes, lining life is showing improvement, said Gary Ganaway, director of marketing and global customer solutions at Bendix Spicer Foundation Brake LLC, Elyria, Ohio.

“The wear rate of the friction material goes down dramatically,” he said.

The chairman of the Heavy Duty Brake Manufacturers Council, Jim Szudy, who is a Bendix employee, said that brake manufacturers are packaging slack adjusters and actuators to accommodate the new brake sizes.

“When you have a bigger brake and a bigger actuator, particularly on the front axle, you have to consider the articulation of the suspension and the steering angles,” Szudy said.

While the brake and tractor manufacturers collaborate closely, it is the tractor makers who must prove, in testing, that their vehicles can stop fully loaded rigs in the required distance using tractor brakes only. Besides the specifics of the test, there are real-world operational challenges to consider, tractor manufacturers said.

“It is relatively easy to design a brake system that meets the new distance requirements,” said Jerry Warmkessel, highway marketing manager for Mack Trucks Inc., Greensboro, N.C. “However, it is challenging to do so while providing compatibility with the trailer so that there is little noise, vibration, or pulling under the variety of operating conditions our customers experience on a daily basis.”

Volvo’s Bio said, “Developing a new braking system isn’t just about the truck’s reaction. It entails a more holistic focus on the entire tractor and trailer combination to retain ride, drive and maintenance characteristics.”

Some fleet managers said they have continued to spec drum brakes, while others said they planned to spec disc brakes before long, possibly as soon as their next order of tractors.

“We did look very seriously at air-disc brakes,” said Ben Curtis, fleet manager for Boyle Transportation, Billerica, Mass. The carrier, with 80 tractors and 230 dry van trailers, is likely to spec disc brakes on its next purchase, Curtis said. He cited “cost, weight and maintenance costs” as the reason the carrier took a pass on disc brakes for now.

“Also, we’re a smaller fleet and we decided that we would let the larger fleets get more time on disc brakes, work the problems out,” he said.

Miller Transporters, Jackson, Miss., runs a fleet of 150 tractors and approximately 1,200 trailers, with drum brakes spec’d fore and aft. The carrier also contracts with some 350 owner-operators.

Nelton Qualls, maintenance di-rector, said the carrier could have spec’d disc brakes on the steer axle or on all axles on its tractors, but “we elected to wait another year or so on that to see how it’s going.”

Qualls said the cost for disc brakes on all axles of a tractor “is way up there — five grand per truck.”

LoneStar Transportation, Gaines-ville, Texas, runs a fleet of about 300 tractors and 2,000 flatbed trailers, augmented by a roster of about 300 owner-operators.

“We move all kinds of specialized equipment,” said Garland Brown, vice president of maintenance. “All types of heavy stuff,” which includes wind-tower, oil-field and construction equipment.

Phase 2 of the stopping regulation will apply to LoneStar, Brown said: “We’re 80,000 pounds and up.”

LoneStar continues to spec drum brakes and does not expect to adopt disc brakes in the near future.

“We’re still waiting for the infrastructure,” Brown said — meaning widespread availability of disc brake service and parts in the field. “We don’t feel like it’s out there.”

But despite the reluctance of the smaller carriers, the use of disc brakes is growing.

Ganaway said Bendix had more than doubled its production of disc brakes for two consecutive years. He said that indicated growing acceptance of disc brakes.

Meeting the requirements of Phase 2 might involve making disc brakes standard in selected cases, some tractor manufacturers said.

“We’re likely going to require front air-disc brakes” on tractors that must comply with Phase 2 of the stopping regulation, Navistar’s chief engineer, Bernie LaBastide, said. Front air-disc brakes will greatly reduce brake fade, he said.

Tractors subject to Phase 2 will also be upfitted with a 6-channel automatic braking system that LaBastide said would enhance efficiency by enabling the sensing and controlling of all axles on the tractor.

LaBastide said the only Navistar models that will not be equipped with disc brakes are the 4-by-2s — tractors that have four wheels with drive power on the rear axle’s two wheels. Instead, they will feature bigger drum brakes on the steer axle, and increased braking power.

For Kenworth — which along with Peterbilt is a division of Paccar: “Most of the design changes have to do with brake size, chamber size, slack length, and lining performance,” said Kevin Baney, chief engineer.

Thomas Macmenemy, brakes manager for Daimler Trucks North America — which is headquartered in Portland, Ore., and owner of the Freightliner and Western Star brands — said that because heavy tractors require higher brake torques, the original equipment manufacturer’s solution in many instances will be drum brakes at higher torque output levels; in other cases, there will be a greater dependence on air-disc brakes.

The OEM also will be releasing a new high-friction brake lining for drum brakes, he said.

“In many cases, our customers prefer drum brakes due to the lower cost and familiar maintenance,” Macmenemy said. “At the same time, disc brakes are becoming more widely accepted, and many customers also find that they are worth the added cost.”


Follow Us


Newsletter Signup

Subscribe to Transport Topics

Hot Topics