There are good reasons why air disc brakes failed to make an impact in North America when they were first introduced in the late 1970s. Pricey to begin with, parts access left something to be desired, as did service expertise. The industry said no thanks, and just about every brake supplier left the market except one, Meritor Automotive, which never got rich off disc-brake sales in Canada and the U.S. but developed a following mostly among premium tanker fleets. Twenty years later, however, air disc brakes seem poised for a comeback.

The disc brakes available today are superior products compared to those the industry largely rejected. They’re a proven product in Europe, where fleets use air discs on almost all steer axles and 40% of drives.

And brake buyers have a wider choice of product. Both AlliedSignal Truck Brake Systems-a big player in the European brake market-and the partnership of Haldex Brake Systems with Dana’s Spicer Heavy Axle and Brake Division offer air discs here. And Meritor’s recent purchase of Lucas-Varity gives it one of Europe’s other premier disc-brake makers and expands its technical arsenal.

Another influence on acceptance perhaps is yet to come, according to Prakash Jain, director of technical support, stopping systems, at Meritor: a government mandate on shorter stopping distances.

WHY DISCS?

Indeed, shorter stopping distances is perhaps the biggest advantage of discs over drums and S-cams. Compared to S-cam brakes, AlliedSignal says its Bendix air disc can drag a 97,000-pound rig down from 90 km/h in a 49-foot shorter distance. That’s with discs just on the trailer, not the tractor. Put discs all around, add EBS (electronic braking system), and the gain is 75 feet-which cuts the braking-distance gap between cars and trucks nearly in half.

Jim Clark, chief brake engineer at Dana Corp., says the typical tractor with drum brakes on all corners will stop in 270 to 280 feet from 100 km/h. However, by increasing steer-axle brake torque (with disc brakes) to about the same level as drive-axle braking torque (still with drum brakes), you can bring that distance down to 200 feet or less. He’s achieved 209 feet in tests with that arrangement, and says a disc brake with greater torque could break the 200-foot barrier.

“For a loaded tractor-trailer, a 10% stopping-distance reduction is achievable with the use of disc brakes just on steer axles,” Clark estimates.

Other key advantages:

Fade resistance: There may be nothing more frightening to a driver than to feel his pressure on the brake pedal produce progressively less stopping power on a downgrade.

That’s what happens with repeated brake applications on ordinary S-cams. As brake temperature rises, the drum expands away from the lining. The brakes literally “fade” away.

When discs heat up, the rotor actually expands into the pads, not away from them. Furthermore, discs dissipate heat more effectively and just keep on working. And, a big comfort to drivers, pedal pressure remains constant.

Stability: There’s less likely to be a performance difference between individual disc brakes, and if the left side of the vehicle is braking as much as the right side, the result is a straighter stop. Bendix says an S-cam system can have side-to-side variances in brake torque of up to three times as much as a disc setup.

Servicing: On average, disc brakes allow faster friction replacement than S-cams, by three to five times. With Meritor’s DiscPlus, for example, the wheel is removed, the caliper swings out, and then the pad can be dropped in. It’s also permanently lubricated, as are some of the others. Replacing a rotor takes longer, and is thus more expensive, than switching a drum.

Fewer parts: An air disc is simpler than a drum brake to start with, but in many cases most of the disc assembly used on one axle is the same as that used on others, from the steer axle on back. That often includes caliper, rotor, pad, and slide-pin/retainer. The result is a reduced parts inventory.

WHY NOT DISCS?

Air disc brakes have a hurdle to leap over not unlike the one faced by anti-lock systems: a bad reputation rooted in lackluster product performance and support.

When discs were first introduced in the 1970s, the rotors simply didn’t last. New rotor families are being developed to overcome that problem, the key being appropriate size to fit the brake.

Another issue is friction life. No matter how you slice it, the disc brake’s pad is usually smaller (except compared to 16-by-four steer-axle linings) than an equivalent drum-brake’s lining and will thus wear faster. Also, if a disc-equipped tractor is mated to a drum-braked trailer, the drums will fade sooner under heavy braking, forcing the disc brakes to do more work. Fleets using disc brakes usually equip both tractor and trailer, which is the optimum arrangement. If you’re an all-drums fleet but considering discs, consult your brake supplier. You’ll likely start with your tractors, and on steer axles if you follow expert recommendations. That’s already a standard spec-deletable-on one tractor model.

PAYING THE PRICE

The higher initial purchase cost, made worse by shorter lining life, means that few fleets aside from those in some niche applications will choose discs without a legislative push. Those fleets should look at Europe, according to Anton Schneider, director of brake product marketing at AlliedSignal, where fleets are being driven to discs because of lower life-cycle costs. “The more air disc brakes a fleet manager puts on his vehicles, the more he simplifies his maintenance,” he explains. Mechanics have a lighter workload, freeing them for other work.

While Jain says only 1% of North American heavy trucks are equipped with air disc brakes, Schneider predicts that figure will be 30% by 2006, led by niche and vocational fleets. He acknowledges that “seeing [disc brakes] on linehaul vehicles is a relatively long-term vision.” Given the performance of disc brakes-and the pressure on governments to crack down on safety-it’s a vision that might one day be clarified by a legislated mandate.