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Stop Gap

FORT LAUDERDALE, Fla. - Steer axles will likely need to be equipped with larger and heavier brake designs once the U.S. National Highway Traffic Safety Administration (NHTSA) shortens allowable stopping distances for trucks, a number of brake expe...


FORT LAUDERDALE, Fla. – Steer axles will likely need to be equipped with larger and heavier brake designs once the U.S. National Highway Traffic Safety Administration (NHTSA) shortens allowable stopping distances for trucks, a number of brake experts agree.

It simply appears to be a matter of time. The regulator has already been speaking to an array of industry groups, and is widely expected to announce plans to slash allowable stopping distances by 30 per cent when it hands down a related Advanced Notice of Proposed Rule Making – a key step in developing U.S. regulations.

“This new rule is going to define stopping distances, and that’s going to define the brakes that we use,” said Dana Corp.’s Jim Clark, during a recent seminar held by the Technology and Maintenance Council of the American Trucking Associations.

The tighter requirements, which are expected to be officially unveiled in late May, will be based on an in-depth study of 1,000 fatal truck crashes.

“We’re going to look a little at (brake) maintenance as well, and that’s new for us,” added Jim Britell of NHTSA’s Office of Vehicle Safety. While tires, visibility and controls will also be addressed, brakes are the initiative’s primary focus.

About 98 per cent of drum brakes are S-cam designs, explained Paul Johnson, senior director of ArvinMeritor’s North American brake business. But the brakes come in different sizes. Sixty per cent of steer axles incorporate 15-inch cams, compared to 37 per cent that use 16.5-inch designs, he said. And that offers an important difference in stopping power.

Tractor-trailers with a Gross Vehicle Weight Rating (GVWR) of 52,000 lb. currently need to stop from speeds of 60 mph within a distance of 355 feet – a requirement easily met by either size of brakes. But problems emerge if the allowable distances are shortened to 248 feet, since trucks equipped with 15×4 brakes on steer axles will only come to a rest within 244 feet. A 16.5×5 brake on a steer axle will bring the truck to a stop within 206 feet.

“Manufacturers want a 10 per cent margin,” Clark said, suggesting that smaller designs may no longer be offered.

The shorter stopping distances come at a cost, even though they can be achieved with proven equipment. A 12 per cent increase in lining volume, for example, is accompanied by a 74-lb. increase in weight. (Johnson expects the use of different materials to shed some of that weight.)

But another approach could be the wider adoption of disc brake designs that are more popular in Europe.

In fact, a steer axle equipped with disc brakes will shed almost 10 feet from the minimum stopping distances enjoyed by one with a larger S-cam design, bringing equipment to a rest within 197 feet, said Clark.

“The disc is a better animal,” he said, referring to the shorter stopping distances. The output of torque is more consistent, and that means little to no pulling experienced at the wheel.

It’s also a bigger animal. A full-size disc brake will add 60 lb. to a steer axle.

Disc designs offer the added ability to maintain stopping power as brakes heat up, while drum brakes can face a problem known as brake fade, when drums expand beyond the reach of friction material.

But there are also suspension issues to consider, Clark added. The mounting of air chambers that feed disc brakes can interfere with steering arms and tie rod ends on some North American suspension designs. There are also concerns about the potential damage to valve stems that rotate past brake calipers.

Disc designs also have different maintenance needs. The pins on which the calipers slide, for example, have to be watched for signs of brake wear, whereas drum brakes are checked by inspecting the linings themselves.

The disc brake pads may simply drop into place when they need to be changed, but most disc designs require hubs to be removed before replacing rotors. (However, costlier multi-piece assemblies can be pulled out without removing hubs.)

There are other options for adjusting stopping distances, but they aren’t without their problems. By adjusting the length of the slack adjuster, you create adjustment problems and cause a number of performance issues, Johnson said. Larger brake chambers would require more space on wheel ends, and wider shoes that can extend brake life don’t add to the available torque.

The biggest impact will come from choosing a large-diameter design that can offer the added torque, he said.

Granted, it isn’t the final word on the issue, either. The May announcement is just the beginning of a process that will solicit industry comments.

“There’s going to be a lot of questions,” Britell admitted, calling on the industry to explain why it likes or hates any aspect of the proposed rules.

Mac Bradley, a Volvo engineer, suggested it’s also going to be important to investigate maintenance and inspection programs. Eighteen per cent of brakes were ruled out of service during the Commercial Vehicle Safety Alliance’s Operation Air Brake inspection blitz in 2003; 13 per cent were out of service because of adjustment problems.

“Are we focusing on the right issues?” he asked. “Can better maintenance and inspection programs be equally and more important?”

Canadian trucks are hardly immune from such rules, since Transport Canada tends to mirror standards drafted south of the border. (A noticeable exception was its refusal to require troublesome antilock braking systems in the 1970s – a move U.S. regulators eventually had to drop.)

The shorter stopping distances could also affect new load security rules, suggested Ron Szapacs, the moderator of the panel discussion.

“It will definitely affect cargo securement,” agreed a member of the American Trucking Associations’ engineering department.

Those rules just went into effect in January, and a seven-page memo released on Dec. 31 actually halved the related forces that the restraints have to withstand.

“With a 30 per cent decrease in stopping distances … it’s entirely likely FMCSA will revise that again,” he said. “It probably means slightly more stringent cargo securement regulations.”


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