OTR Trucks Roll with Reduced Stopping Distances

By Tom Berg, Truck Editor | September 15, 2011

Emerging from truck factories in the United States right now are road tractors that can stop in substantially shorter distances than those built until the end of July. That’s because new reduced stopping distance (RSD) requirements took effect on August 1.

The changes to Federal Motor Vehicle Safety Standard 121, the air-brake regulation, were announced several years ago but were anticipated years before. They require most highway tractors to stop in about 30 percent shorter distance than before during panic-type braking.

Affected are three-axle tractors grossing up to 59,600 pounds, which includes tractors that pull dump and other construction trailers. They must stop no more than 250 feet from when brakes are applied at 60 mph, compared to 355 feet under the previous rule. There are other requirements from slower speeds. Heavier three- and four-axle tractors, and lighter two-axle tractors, will be affected later.

The rules do not affect straight trucks, including heavy dumpers and mixers, because federal authorities believe they run at lower speeds and often have extra axles and brakes to help them stop safely. They also do not affect trailers because their axles and brakes help combination vehicles stop in about the same distance as tractors alone.

To ensure compliance, manufacturers build in a 10 percent safety factor. So affected tractors with stronger S-cam drum brakes can stop in about 225 feet from 60 mph, and those with air disc brakes can stop in 215 feet, say brake makers. Because the vast majority of stops are routine and gradual, tractor-trailer drivers won’t notice the stronger brakes on post-July 30 tractors unless they get into a panic situation, manufacturers say.

To achieve the higher performance, manufacturers are using larger front and rear S-cam drum brakes, or in some cases air disc brakes. Fasteners in the brakes are also slightly larger. Front axles and suspensions are slightly stronger to take greater forward weight transfer that comes in the more severe panic-type stops.

Traditional 15-inch-diameter by 5-inch-wide drum brakes on steer axles have been replaced with 16.5x5-inch or a 16.6x6-inch size, brake manufacturers say. Some drive-axle drum brakes are wider, with 16.5x7 and 16.5x8-5/8 sizes predominating.

Larger brakes have extra thermal capacity—the ability to absorb heat without fading on long downgrades. That and extra lining life are why some truckers already use 16.5-inch front brakes. Those who use engine or driveline retarders won’t need much of the increased thermal performance, but it’s there as a safety factor.

Compared to the pre-RSD 15-inch steer-axle brakes, the bigger drum brakes and their associated gear are heavier, by about 60 to 80 pounds per axle. They are somewhat more expensive, but the costs are built into the vehicles’ list prices so are hard to pin down.

Air disc brakes are not needed to meet the RSD requirements, but they offer other benefits. Side-to-side balance is better with discs, and fleet managers who have begun buying them say drivers like the more precise pedal feel. Knowing he has disc brakes can give a driver a more secure feeling, some think, but worry that this might lead to more aggressive driving. 

Discs are inherently self-adjusting while drums use separate slack-adjuster mechanisms that don’t always work and themselves require periodic attention. Disc pads are easier to change than drum linings, though some quick-change drum designs make replacing shoes almost as fast as disc pads. In some cases disc brakes will be equal in weight or lighter than drums, though drums can be ordered with lightweight mounting spiders and hubs.

Bendix Commercial Vehicle Systems and Meritor Inc., the principal brake manufacturers for power units, say they’ve made many advancements in drum brakes. That, along with reasonable prices and complete familiarity by fleet maintenance people, have kept them popular. S-cam drums will therefore continue to be used on most North American tractors and trucks.

Bendix and Meritor also offer air disc brakes, and they are optional from various truck builders.  Bendix Spicer discs are standard on the steer axles of most Peterbilt Class 8 models, including vocational trucks. In past years the brake manufacturers pushed hard to try to sell air disc brakes, but few fleets bought them because they were satisfied with the adequate performance and lower upfront cost of drums. Brake makers now say they’re happy to sell either kind to vehicle buyers.

Brake and truck makers say that preparing to meet the new braking requirements was a multiyear effort involving extensive lab, track and road testing, and use of newly formulated friction materials. The stronger foundation brakes had to be integrated into the vehicles’ braking systems, which meant much work with valves and other hardware.

Friction materials are more critical than ever to proper brake operation, and Bendix and Meritor engineers say that come relining time, vehicle owners should absolutely use original-equipment linings. This is not new, as brake makers for years have insisted that OE linings are needed to keep OE-level performance. And they discourage the practice of rebuilding shoes with new linings because tables might be warped. But it’s more true now than ever, the companies say.

In the real world, some truck operators will seek lower prices that many aftermarket products carry. Stemco, a major supplier of aftermarket linings, says its friction products are tested according to strict procedures to ensure proper performance. Anyone buying non-OE-brand brake products should look for proof of such testing.