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The Long Hello

Long goodbyes are one thing. But when it comes to being embraced by trucking, air disc brakes are a case of one very long hello. The most widely accepted technology in Europe for stopping heavy-duty trucks, air disc brakes have been a slow sell in North America for several reasons. For one thing, compared to the familiar S-cam drum brake, air disc brakes have cost much more to purchase. While few

Long goodbyes are one thing. But when it comes to being embraced by trucking, air disc brakes are a case of one very long hello.

The most widely accepted technology in Europe for stopping heavy-duty trucks, air disc brakes have been a slow sell in North America for several reasons.

For one thing, compared to the familiar S-cam drum brake, air disc brakes have cost much more to purchase. While few if anyone would argue that air disc brakes don't provide better overall braking performance than the old standby S-cam design, not many fleets have been willing to pay a premium for that performance.

To date, on this side of the Atlantic, air discs have mainly been sold to truck operators that are especially safety- or performance-conscious, such as haz-mat haulers and fuel tankers; they've also been adopted for fire engines and other emergency vehicles.

But according to leading brake suppliers, the air disc's days as an industry wallflower may soon be over, thanks to the benefits promised by newly engineered designs coming to market.

A proposal to shorten stopping distances being floated by the National Highway Traffic & Safety Administration (NHTSA) is also helping spark more interest in air-disc designs.

That proposal, which if it becomes a rule won't take effect any earlier than 2007, calls for reducing heavy-truck stopping distances by as much as 30%. Achieving that level of performance may require using redesigned — that is, larger — S-cam brakes or switching to air disc brakes.

However, according to all the major U.S. disc-brake suppliers, that proposal alone is not driving the new generation of heavy-duty air discs that are beginning to make their presence felt in the North American truck market. In fact, all three top disc suppliers plan on introducing new models over the next year or so.


They certainly don't foresee any wholesale movement to air discs on the part of fleets. Rather, they foresee new-generation air discs gaining a bigger foothold in the market by appealing to more non-specialized fleets, thanks to the performance benefits and lower lifecycle costs they will offer.

While the newest air disc brakes to be sold in North America are based on designs that originated in Europe, brake makers promise they will be much more Americanized than the products they now sell here.

The engineering specifics will vary from manufacturer to manufacturer, but in general terms that Americanization will involve tackling several issues that until now have held back the ready acceptance of the disc as a truly practical alternative to the drum.

The first of these issues — packaging — may not be obvious to most fleets. Because the design of most current air discs originated in Europe, it is more costly to engineer them into North American trucks. While that process may be invisible to truck buyers, it does add to the cost of buying trucks with discs.

Fleets naturally hate to spec any component that weighs more than they think it should. And discs have weighed more than comparable S-cam brakes. Suffice it to say, discs have gone on a diet, shedding some 20 lb. per wheel end in some cases.

But the two biggest issues stalling the adoption of discs by fleets have been cost — both initial and lifecycle — and plain old familiarity.

The latter will be harder to remedy. S-cams are what most fleet managers and maintenance personnel have cut their teeth on. They are dependable; they are a known quantity in a world of fast-paced equipment change. Displacing them at many fleets, with or without the impetus of federal regulations, will still require manufacturers to do a good job of educating buyers.


As for the former, suppliers will deploy a two-prong strategy. First, they intend to lower the operating and maintenance cost of discs by improving both lining and rotor life. Second, they promise the acquisition cost of discs will drop as purchasing volume increases. And they are betting that the promised performance of the newer discs — including the lower lifecycle cost — will increase sales.

Besides costing less to operate, next-generation discs will offer the same advantages that have led performance-oriented fleets to spec discs for years.

Chief among these are greater resistance to fade, especially under hard braking, and excellent directional stability, compared to S-cam brakes.

Disc brakes resist fade better because they can dissipate heat more quickly than drum brakes, even after repeated downhill braking.

Some engineers describe the resulting “feel” that air discs deliver to the truck driver as “car like.” That alone may make discs more popular with fleets struggling to attract and retain quality drivers.

It is also not too far-fetched to imagine that more fleets may eventually spec discs to seek lower insurance premiums or even to help protect themselves in liability lawsuits by showing they equip their trucks with state-of-the-art safety equipment. However, it should be noted that no brake manufacturer has claimed discs should be spec'd specifically for these reasons.


According to Tom Runels, senior project engineer, Dana Commercial Vehicle Systems will introduce its next-generation air disc later this year.

While Dana's current disc was initially developed from a European Haldex model, Runels says the new brake will be a Dana design using a Haldex adjuster mechanism.

“The new disc will be more specifically designed for North America,” Runels reports. “It will offer reduced weight and longer-life linings and rotors for better lifecycle costing. It will also be packaged for an easier fit in American trucks without sacrificing any of the performance advantages disc brakes offer.”

He points out that whether fleets are using currently available discs or the next versions to arrive, the “most impact” on braking performance can be seen by spec'ing them on steer axles. “Just putting the disc brake on the front will make an improvement in braking response. In other words, you get the most bang for the buck when you put them on the steer axle.”

Some engineers have argued that discs will be the most effective choice for most fleets once electronic braking systems (EBS) come into play. (See FO 8/00, “Orchestrated Stopping.”)

That's because, so the thinking goes, EBS will provide a cost-effective means of avoiding compatibility problems that may arise from the mating of disc-braked tractors to drum-braked trailers. Indeed, many of the fleets now using discs have them on tractor-trailer units that stay “married” most of the time.

But Runels offers a different perspective. “In our research, we haven't experienced significant compatibility problems from mixing the two brake types,” he relates. “We may have had to modify an air system slightly. Still, compatibility should not be regarded as a huge issue. However, we would not suggest ignoring it as a spec'ing factor, as every fleet's equipment is set up a little differently.”

Runels also contends that no matter what NHTSA may do about stopping distances, air discs will continue to offer superior fade and pull characteristics over S-cams.

For the time being, he expects that fleets most interested in performance will continue to gravitate to discs on their own. “If NHTSA does get involved with stopping distances again,” Runels adds, “that may change the dynamic enough to give the adoption of discs more of a push.”


Next year ArvinMeritor plans to “supercede” its current ADB 1560 air-disc brake, which has been in the U.S. market since 1981, with an expansion of its DiscPlus series.

According to Gary Ganaway, director-North American air brake business, DiscPlus will then consist of two models. The DX195, which debuted in 1997, is for use on steer axles and low-profile drive axles and is designed to fit 19.5-in. wheels. To be introduced later this year is the larger DX22, sized for use with 22.5-in. wheels.

Ganaway says both DiscPlus models offer key improvements over the ADB 1560. “The DiscPlus design provides a more flexible packaging system that allows mounting actuators in four different positions,” he explains. “We also have four brackets available for moving the air chamber to different positions.

“Once in fleet service,” he continues, “DiscPlus requires no periodic lubrication. That compares favorably to the ADB 1560, which had to be lubed at every PM or 30,000 miles.”

Ganaway says another key design change is the “larger swept area” of the DiscPlus. “The area of the rotor that comes in contact with the brake pad is larger, so heat dissipation is improved.” He says this makes the brake “far less prone to rapid lining or rotor wear.”

Although Ganaway contends unit weight is not a major hurdle to disc acceptance by fleets, the DiscPlus trips the scales lightly. “Depending on a vehicle's setup,” he explains, “the weight of a DiscPlus can vary from being on par with an S-cam to offering a fairly significant reduction. To avoid confusing buyers, we'd rather simply emphasize there is no weight penalty incurred with spec'ing DiscPlus brakes.”

What Ganaway says has been the “basic issue”' thwarting wider acceptance of air discs is cost of ownership: “the acquisition price plus the cost to operate and maintain them.”

While he says the cost of ownership for DiscPlus will be “significantly better” than for ADB 1560, he points out that low sales volume initially will keep the purchase price higher than that of S-cams.

“But what must be kept in mind,” Ganaway says, “is that when you look at a disc brake you are seeing a premium product. Because the S-cam has been around so long, brake makers have been able to take cost out of the product. Until we have the same economy of scale with discs, they will cost more to buy than drums.”

While Ganaway doesn't expect any air disc to suddenly turn the brake market on its ear, he does predict greater adoption of new-generation discs by owner-operators.

“Because owner-operators are purchasers as well as users,” he explains, “they are often extremely safety-conscious. And because they tend to keep their vehicles longer, they are more likely to look more closely at the value proposition offered by disc brakes.

“As demand for company drivers increases, we are also seeing fleets converting some of their trucks to discs as part of a strategy to reward and retain top drivers,” Ganaway adds.

At the Mid-America Trucking Show this spring, Bendix Commercial Vehicle Systems announced it was nearing completion of fleet testing of the ADB 225, which it calls its first air disc brake designed specifically for North American applications. The brake is slated to enter production starting next year. It was developed through Bendix's joint venture with Germany's Knorr-Bremse.

According to Ernst Eichel, director of special products marketing, preliminary performance data shows the ADB 225 will reduce stopping distances by as much as 39% over today's regs, thus putting it well ahead of the 30% reduction proposed by NHTSA for '07.

Designed as its name implies for 22.5-in. wheels, the new disc boasts a “mono block caliper design” that distinguishes it from the European two-piece caliper model currently sold here. The caliper design is Bendix's answer to the packaging problem that all disc brake makers are addressing.

As for how the new brake compares to Bendix's current SB7 air disc, Eichel points out the older brake is a “global design.” He says it was engineered “very robustly” for European conditions and OE requirements over there. “It made the most inroads here with safety-conscious fleet managers for whom weight and initial cost were not much at issue,” Eichel points out.

“But,” he continues, “we did hear the message that weight and especially cost had to be lower for wider acceptance in this market. After all, the S-cam is already sold here at essentially a commodity price.”

What Bendix did, according to Eichel, was keep the key internal features of its existing disc brake's design and worked to trim the weight and bring costs down.

Eichel reports that the ADB 225's mono-block caliper, smaller carrier and splined-rotor technology make the disc up to 10% lighter than the lightest drum brake available. Other key features of the ADB 225 include integrated pad- and rotor-wear sensing diagnostics and internal automatic adjustment mechanism.

“We are doing fleet testing now under various operating conditions,” Eichel relates, “and so far we are seeing pad life that is 30-50% better than that of S-cams. Our goal is to achieve 500,000-mile pad life in linehaul applications.

“Overall,” he continues, “lifecycle costs for the ADB 225 will be lower than those of S-cams. And we now feel we have a product that will attract enough users to give us real economies of scale in this marketplace, which will ultimately bring down the premium charged. The price of discs is prohibitive now only because the volume isn't there yet.”

Eichel says Bendix is confident enough about the growing acceptance of air discs here that it expects to justify the cost of manufacturing them in North America by '03.

All the new air discs on the way may not spell “goodbye” to the tried-and-true S-cam. But it's a sure thing trucking will give them a firmer “hello” than their predecessors have received.

Dish on discs

For more information on air disc brakes, contact these and other suppliers.

ArvinMeritor 334

Bendix 335

Dana 336

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