Emerging from truck factories in the U.S. since Aug. 1 are road tractors that can stop in substantially shorter distances than those built earlier. It's due to a change in government stopping distance requirements
New braking rules require shorter panic stops, but the resulting stronger brakes will have otehr benefits like increased fade resistance on long downgrades.
New braking rules require shorter panic stops, but the resulting stronger brakes will have otehr benefits like increased fade resistance on long downgrades.
that were in the works for several years.

The revisions to Federal Motor Vehicle Safety Standard 121, the air-brake regulation, require most highway tractors to stop in about 30% less distance during panic-type braking. Truck builders have therefore been installing bigger drum brakes, and sometimes air disc brakes. As the new types of brake parts wear out, customers will need like parts as replacements, and distributors have to be ready to sell them.

This will come gradually, say suppliers we talked with, but there will be a learning curve for distributors as well as an opportunity to rise above competitors who are slow to adapt. A few types of brake parts might eventually go out of use, raising issues over core values.

Tractors only for now

Vehicles affected are three-axle tractors grossing up to 59,600 pounds, which constitute a large majority of those on the road. 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.

To ensure compliance, manufacturers build in a 10% 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.

As with the previous rules, trailers are not affected because although they carry a truck combination's payload, their extra axles and brakes help a rig stop within the required distances. In fact, formal track testing of components involves only the tractor's brakes, not those of the short test trailer it tows during the runs. The trailer is loaded to simulate the weight the tractor alone carries, and the trailer's brakes are disconnected for the testing.

Straight trucks are also not covered by the new "121" rules, and the National Highway Traffic Safety Administration, which wrote the regs, has announced no plans to tighten their braking performance requirements. NHTSA officials believe heavy straight trucks travel at slower average speeds, and heavier, multi-axle trucks have extra brakes that allow them to stop safely.

Because the vast majority of stops are routine and gradual, tractor-trailer drivers won't notice the stronger brakes unless they get into panic situations, manufacturers say.

Getting there

To get the reduced stopping distances, or RSDs, manufacturers are using larger front and rear S-cam drum brakes, and 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 16.5- by 5-inch or a 16.6- by 6-inch size, brake manufacturers say. Some drive-axle drum brakes are wider, with 16.5 by 7 and 16.5 by 8.625 sizes predominating. And friction materials have been reformulated for higher performance.

Larger brakes combined with enhanced friction materials 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. And they're somewhat more expensive, but the costs are built into the vehicles' list prices, so are hard to pin down.

Discs have benefits

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 others worry that this might lead to more aggressive driving.

Discs are inherently self-adjusting. 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. Other builders also have begun using discs, and all builders offer them. 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 buyers.

Discs unfamiliar

But the swing to discs is something new, and many distributors do not handle any parts for them. As discs become more numerous on road tractors, some customers will begin seeking aftermarket replacements. That means pads at first, followed by rotors and calipers as they wear out. Distributor employees will have to learn about them and determine what to stock.

Air disc brakes themselves are not new, with current products on the market at least for several years and some going back to the '90s. So there is information on them from suppliers. Aside from Meritor and Bendix, which supply ADBs to truck OEMs, aftermarket suppliers post information on the Internet. Marathon, for instance, says it plans to post an illustrated catalog of disc brake parts in February.

Long effort

Brake and truck makers say that preparing to meet the new braking requirements was a multi-year 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 the reality is that many truck users prefer aftermarket brake products, partly because they cost less than OE parts, and that's not likely to change. Distributors therefore should start learning about the new products and be ready to sell them. There are likely to be several kinds of customers: Those who want to preserve new-truck braking performance and are willing to spend reasonable money to get it; those who want good alternatives; and those who want cheap substitutes.

The cheapskates are shunned by many distributors who feel a responsibility to contribute to highway safety, and who don't want