Five years ago, when federal authorities announced that stopping-distance rules for heavy tractors would get stricter, there was talk that this would finally cause a conversion from drum brakes to air disc brakes in the U.S. Even well before that, some supplier representatives claimed that discs were so superior that a switch to them was inevitable, just as has happened in Europe and other overseas markets.

Yet here we are, a year after the final rules change became effective and more than three decades since they first appeared in North America, and disc brakes still comprise a small minority of air brakes being bought by truck operators.

Put another way, 90% of heavy truck buyers today still choose tried-and-true drum brakes, according to manufacturers we talked with. These are not good ol’ drum brakes, but improved designs that stop better and last longer than past products. They have to perform better to meet Federal Motor Vehicle Safety Standard 121’s shorter limits, and to do so many of them are larger. That, in turn, allows their linings to stay in service for more miles, supplier reps say.

”Air disc brakes cost more, so there isn’t a more cost-effective way to stop a truck than a drum brake,” says Joe Kay, director of engineering for brake systems at Meritor, which offers both types. He means on tractors, which the “121” rules affected, and for which drum brakes were specifically improved. But discs can stop those same tractors quicker, which is why some fleets are adopting them. And discs hold up better in severe-service applications, like trash collection trucks, many of whose operators have embraced them.

“There’s an added expense with disc brakes and operators need time to get a proper payback,” says Gary Ganaway, director of marketing and global customer solutions at Bendix Spicer Foundation Brake, which also offers both drums and discs. Cost for air disc brakes varies by truck builder: At Kenworth and Peterbilt they’re standard on the steer axle. For all the other builders the cost of upgrading is $800 to $1,000 per axle.

In some cases there can be a payback through maintenance. With discs, “refuse trucks doubled their brake life,” Ganaway says. With drums, “they get hot and wear rates increase exponentially.”

It’s easier and quicker to change a set of pads on disc brakes than shoes on drums, Kay and Ganaway both note. Pads take as little as 15 minutes to change out compared to almost an hour for a set of shoes. That’s where the maintenance payback can come into play.

But the advantage is less if original linings on a drum-brake system can be made to last a long time, which is possible with larger brakes. To meet 121 stopping requirements, steer axle brakes have gone from 15x4 inches to 16½x5 inches, while drive-axle brakes stayed with 16½x7s, Ganaway explains. Daimler Trucks chose to make 16½x8-5/8-inch drum brakes standard on drive axles for Freightliners and Western Stars.

However, there is a movement to air disc brakes and it’s accelerating. “We announced the sale of our half a millionth air disc brake at the Louisville truck show, which was only two years after we announced the first 250,000th” following introduction of current models in 2005, says Bendix Spicer’s Ganaway. “Fleets are paying more attention to foundation brakes and began experimenting with air disc brakes. They are becoming more and more comfortable and are coming back for more.”

“Air disc brakes are progressively increasing,” says Steve Hampson, director of sales and marketing at Wabco, which sells discs. “There are two levels of fleets: those looking for brake performance — like tankers and heavy steel haulers —  and those who want a longer lifetime. With discs you don’t have to touch the brakes as often.” Adoption percentages vary: On trailers it’s 8 to 10%, on trucks and tractors it’s 12 to 15%. The usual configuration for tractors is discs front and drums rear.

Brake adjustment is another reason for disc brake growth. Out-of-adjustment brakes are often an issue in roadside inspections, and citations now affect a fleet’s Compliance, Safety and Accountability ratings, Hampson adds. “Disc brakes are encapsulated and have fewer moving parts, so out-of-stroke is not an issue. Some fleets are looking at disc brakes so they don’t have the concern over adjustment.”

Disc brakes are inherently self-adjusting and have no “stroke” to measure, so are difficult for inspectors to gauge unless they have dynamometers to measure performance. Some states do.

Aside from refuse fleets, firms adopting discs are those sensitive to safety, like haulers of petroleum, chemicals and industrial gases. Freight haulers who are squeezed more by costs are more likely to stick with drums. “Customers have to choose among disc brakes and electronic aids like lane-departure warnings, electronic stability control and so on, and they have only so many dollars to spend on safety,” says Meritor’s Kay. “There are also automated transmissions, for driver ease and fuel economy.”

One who’s staying with drums is Kirk Altrichter, vice president, maintenance for Crete Carrier Corp. “Cost and weight” argue against discs, he says. He wasn’t clear on price, but puts the weight penalty at 100 or more pounds per axle.

“We’re struggling constantly to take weight off,” he says. “The cost for that is $1.50 per pound — that’s a figure we use. So there’s not enough justification to make discs worth it. Drums meet the stopping distance but the jury’s still out as far as how they’re lasting. I don’t think there’s extra life.”

Air disc brakes might some day account for 25% of sales, says Ganaway. More bullish is Randy Petresh, vice president, technical services, at Haldex Commercial Vehicle Systems, which sells a variety of components for brakes and air systems. He thinks disc brake usage might go to 70% in 10 years. Even so, “There’s never going to be a complete conversion. There’s always going to be segments that will stick to drums no matter what. And cost —  drums have been in production for 30, 40 years and tooling has been amortized, so drum-brake parts are dirt cheap.”

Parts pricing is one reason air drum brakes are popular among buyers of medium-duty trucks. “In Class 6 and 7, greater than half the buyers buy air brakes,” says Meritor’s Kay. “Some Class 5 buyers will spec air if they have other air systems on the truck, though the lower end of Class 5 use hydraulic discs. It depends on gross axle weight rating; there’s a point where hydraulic brakes just can’t do the job anymore, about 32,000 pounds.” Beefy drum brakes absorb heat and work better in many cases.

“There are also parking concerns,” Kay continues. “Disc brake systems are more complex, and you don’t get the same power, so OEMs go with some kind of driveline brake.”