Brake Trends: Drums vs. Discs

Although 90% of heavy trucks still get drums, discs are gaining in popularity.

July 2014, - Feature

by Tom Berg, Senior Contributing Editor - Also by this author

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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.”   


  1. 1. Sunny [ April 07, 2015 @ 07:38PM ]

    want to order brake disc

  2. 2. Mekanic [ June 23, 2015 @ 10:03AM ]

    Disc brakes better? How! I've worked on trucks and buses for 20 years, the closest I got to the disc set-up was on buses. On our double decker fleet they were a nightmare! The rotors cracked all the time, sometimes they would crack about a third of the way into a set of pads rendering that set junk as you couldn't reuse them! On our MCI's (D4500) they would stop the bus marginally better then the others but worn down to nubs in less then 30,000 miles while the drums still had servicable life! And disc change outs were a pain in the ass especially on the drives where the caliper bracket was placed right up against the suspension brackets. All disc brake set-ups that I've found are designed by a europian company called Knorse Bremse, so this is just another way of shoving euro trash in our faces and telling us that it is somehow more superior then anything else. Just like MBE engines are better, yeah right!

  3. 3. Erik Smith [ November 25, 2015 @ 04:03PM ]

    I think for the owner operator who can work on his or her own rig, the disc's big payout is definitely going to help these people out, especially the long hauler's who don't get caught up in stop and go traffic all the time. The cool thing about having disc's on a semi, quite literally, is the fact that since air supplies are part of a tractors system as a norm, the blow off valves could be directed onto the rotor itself for the stop and go nightmares to help the cooling factor. This may help to alleviate the problem mentioned by the mechanic who has dealt with cracked rotors in a previous comment.

  4. 4. Michael Heisch [ November 26, 2015 @ 09:51AM ]

    Drum brakes is ancient technology, they overheat going downhill and stop the truck from the time you press the brake in different zip code. It's time like Europe to adopt this newer better and safer technology. And also it's time to say goodbye to rivets America and build good equipment like other places, I've been driving in North America for over 10 years and sadly the trucks here are inferior to European ones. The only advantage being the big sleeper

  5. 5. govind [ March 22, 2016 @ 07:00PM ]

    hiii frnds i want buying new cascadia truck give me advise plz which breaks good front axl disc or drum. some body told me disc s not good.which one i buying ?

  6. 6. Chris [ April 24, 2016 @ 08:20PM ]

    Is there anyone supplying an air drum to disc brake conversion kit?

  7. 7. Randy Kubick [ September 29, 2016 @ 10:39AM ]

    You guys are missing the whole problem - it's called air brakes - drum or disc! The problem is using air pressure (a spring) to apply brakes. This increases the lag time of the brake system by as much as 1/2 to 1 second which means a vehicle traveling 75 mph can go 50-100 feet before any braking occurs once the driver hits the brake pedal. Air brakes should be outlawed as it is a system that was invented back in 1869 by Westinghouse for trains - which have miles of track to come to stop and no sports cars pulling in front of them and slamming on the brakes. Hydraulic brakes are far superior in reaction time application, lighter, more powerful and self-adjusting. Hydraulics brakes have about a dozen less valves than air brakes, require no air compressor, don't freeze up and no maintenance fees. How much fuel is wasted running a 5-10 horsepower air compressor all the time? How much fuel is wasted lugging around air compressors, tanks, a 1000' of air lines, dozens of valves, air dryers, etc., etc. The reason we still have air brakes is because the manufacturers are too heavily invested in the manufacturing equipment to change. If air brakes were superior to hydraulic brakes every car in the world would have them. Another reason we have air brakes is because the manufacturers are reluctant to invest in the new types of devices necessary to couple-uncouple a tractor-trailer rig.

  8. 8. Steve [ October 21, 2016 @ 08:28AM ]

    Randy. Where did you come from, man? Talk about problems and points being missed. You are comparing hydraulics and air on cars (less than 5000 lbs) vs trucks (up to 80000 lbs) and trains (millions of lbs).

    Dude, you answered your own concern.

    What happens when the fluid in the brake lines boils due to heat generation after a few seconds of pedal application? Hydraulic systems are designed to work with liquid, not vapor. Drain your car's brake lines and try to stop. Try to drink vapor coming from boiling water... Does it quench your thirst? That is exactly how boiling brake fluid works. It changes state.

    If you are willing to stand in front of a truck or car stopping with hydraulics, more power to ya! But I will take air over my life any day.

    Do the world a favor, and don't become an automotive or brake engineer.

  9. 9. Xman [ November 07, 2016 @ 10:55PM ]

    Steve, you're convincing yourself only. Air is used because it is easier for trailer hook ups. Moisture in air freezes, you got no brakes. Give it a rest.

  10. 10. RuffriderAlec [ January 18, 2017 @ 03:18PM ]

    Xman noticed tou didnt answer the question. Hydraulics can only handle brakinv up to a certain class / weight. Once you exceed 32000 lbs hydraulics are utterly useless. There may be something else out there to useful in stoppinv a heavy truck but right now its not hydraulics. As Steve said after a few seconds hydraulic fluid is vaporized trying to stop and 80,000 lbs truck. The issue is how do I stop to refill the hydraulic fluid? Conundrum!

  11. 11. Brett pair [ February 28, 2017 @ 06:59PM ]

    They are available And work great those problems the guy mentions have been solved its the cost that comes with the manufacturing and every other phase of making the with everything else follow the money

  12. 12. Donovan [ May 03, 2017 @ 02:00PM ]

    There are far more problems to overcome with hydraulic than pneumatic brakes for heavy vehicles. First off, how are you planning to convert EVERY truck AND trailer in North America to hydraulic simultaneously since the systems won't work together? Not to mention trucks still need air, the suspension is air the transmission uses air, diff locks use air, so you're still going to have to spin that air compressor as well as carry the plumbing. Spinning a 5-10 HP air compressor on a 500+ HP engine pulling 80,000lbs is not hurting the fuel consumption nearly as much as you seem to think it is. Besides that any weight you do save you will gain back with the Hydraulic system since the amount of fluid that would be required would weigh significantly more than the air we currently use. Hydraulic would also be more prone to failure based on contamination which could easily occur when switching trailers. There's a reason trains use air, they are far more reliable in high demand applications because of their simplicity. Anyone that drives a truck for a living is also far better trained and far more experienced in sudden collision avoidance and can easily react fast enough to overcome the slight lag in an air system. That lag by the way is no where near one second as you put. If it is, you should probably get that thing to a shop on the double!

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