While air disc and advanced drum brakes are nearly equal in performance, discs don't fade. That's a big advantage on long grades.

While air disc and advanced drum brakes are nearly equal in performance, discs don't fade. That's a big advantage on long grades.

Despite their acknowledged technical superiority and performance advantages, penetration of air disc brakes on Class 7 and 8 trucks in North America is a paltry 10%. The ratio of drum brakes to disc brakes in European Union nations is practically the opposite. All on-road commercial vehicles in the EU are now fitted with ADBs. Some off-road trucks, such as mining and construction still use drum brakes, along with some military vehicles there. Drum brakes comprise about 18% of total EU brake demand.

Why, then, have North Americans not embraced air disc brakes? We're different from Europe, that's all, says Chad Mitts, Meritor's general manager for North American Brakes.

"Europe is a different market with different technology drivers," he says. "North Americans operate different trucks on different roads with different business models. It's not a given that we would embrace the same technology for the same reasons they do."

Air disc brakes relieve less-skilled drivers of worrying about brake fade on long hills.

Air disc brakes relieve less-skilled drivers of worrying about brake fade on long hills.

Speaking at a trade press gathering at Meritor's European Brake headquarters in Cwmbran, Wales recently, Mitts cited several reasons why North American fleets lag behind the Europeans in ADB uptake:

  • In Europe, the OEs decide what the truck is going to look like. In the US, the customer still has a lot more input to the spec. North American truck makers build the truck the customers wants; in Europe, fleets get what the manufactures build.
  • Different service infrastructure: Most European trucks are serviced at dealers with factory trained technicians. They can manage the service infrastructure better. Here, trucks are serviced by fleet technician, jobbers, dealers, etc., which makes adoption of new technology more difficult. You have to deal with new parts and new procedures, and there are technician training issues to overcome.
  • Our longer conventional trucks are dynamically different from the short wheelbase EU cab-over-engine trucks. A typical North American conventional has a completely different distribution of brake work in a hard stop. The benefits of ADB are still there with North American conventional tractors, but they are not as pronounced as on a shorter wheelbase COE models.
  • North American fleets are very cost-sensitive, and as long as low-cost options exist, they usually get the nod. Mitts points out that truck costs have escalated by 30% over the course of the three generations of EPA emissions reduction mandates. "We're still reeling from that, so discretionary items like ADB are just one more added cost," he says.

That said, there are some niche markets here where discs have grown substantially, specifically the refuse and transit bus markets. "Almost all new vehicles in those markets are sold now with air disc brakes," says Mitts. "Those are severe service applications for brakes where operators used to relining brakes in a year or less are seeing several years of service before major maintenance is required on air disc brakes."

For on-highway trucks, as far back as 2009, many assumed that new stopping distance requirements brought in with the 2011 changes to Federal Motor Vehicle Safety Standards No. 121 would be the tipping point for ADB here. Brake system manufacturers, instead, came up with advanced drum brake systems with larger friction surfaces that easily met the standard. That took the wind out of the ADB sails, but it was what fleets wanted.   

Drum vs. Disc: What Drives the Decision?

At the risk of over-simplifying it, drum brakes are good enough for most applications and they are getting better, Mitts says -- though not in as many words.

He explains that current advanced drum brakes meet the new stopping distance requirements, so there's little need for improvement from a regulatory compliance perspective. He notes that today's advanced drum brakes are capable of going 500,000 miles or more before requiring a reline, which means that fleets with typical trade cycles and usage profiles may never have to do major brake maintenance for the first life of the truck.   

"Zero-overhaul was our goal and we have achieved that in a lot of cases," Mitts says. "As long as there is about 30 percent left on the linings, there's no need to reline at trade in. The case for drum brakes has changed a lot in terms of what the first owner can get out of them."

Before the stopping distance requirements changed in August 2011, a typical North American drum brake package was 15x4-inch front axle brakes and 16.5x7-inch drive and trailer axle brakes. With the 121 change in 2011, they went to 16.5x5 on the front with advanced friction materials and wider linings on the other axles.

"You have better-wearing lining material and more of it, so they tend to run a bit cooler which also helps with wear," says Mitts. "The case for drum brakes has changed a lot in terms of what the first owner can expect from them."

When it comes to parts and service availability, drum brakes can be serviced anywhere.

Drum brakes however remain our Achilles' heel when it comes to CSA. Brake problems are typically the top 5 out-of-service or citation items, Mitts says. Even with automatic brake adjusters, roughly 20% of trucks are sidelined each year during the Commercial Vehicle Safety Alliance's Road Check and Operation Air Brake events. Similar numbers are reported at numerous inspection blitzes that take place throughout the year.

Part of that is product based, Mitts admits, but a big part of it is education on how to keep the brakes functioning.

Electronic stroke sensors detect pad thickness and alert fleets to maintenance concerns.

Electronic stroke sensors detect pad thickness and alert fleets to maintenance concerns.

It's another story for another day, but brake adjustment issues are easily detected in routine inspections, if they are done properly. Also, the industry really needs to move away from the practice of simply readjusting brakes with excessive push-rod travel. As any brake manufacturer will tell you, if an automatic brake adjuster is out of adjustment, it's probably a symptom of some problem with foundation brake, not a problem with the slack adjuster.  

Mitts says drum brakes will remain a large part of the market for the foreseeable future and so Meritor will continue to invest in drums as well. The focus will be on maintenance and reliability and ease of inspection, he says.

"There's a lot of life left in the concept, particularly when you look at weight and reliability," notes Motts. "The basic structural elements haven't changed much in a couple of decades, but we're now focused on service and reliability improvements as well as weight improvements. It's important to recognize that drum brake developments are not standing still."

Drum brake development plans at Meritor include overhauling the basic design, and introducing new structural components and materials. Mitts says there are a lot of ideas on the table, but there's a limit to what fleets will pay for improvements, and at some point the cost delta between disc and drums will close, which could prove to be the tipping point that accelerates discs in the market once and for all.  

North American Technology Drivers

Fleets want -- or don't want -- ADB for different reasons. Everything else being equal, Mitts calls them a discretionary spec. Fleets focused on safety have an array of other safety products to spend money on, such as lane departure warnings, collision mitigation systems, etc. It's harder to build a safety case for ADB because in the vast majority of brake applications, disc and drum brakes perform equally well. Even in a panic stop, drums and discs run up against pavement adhesion limits with the tires. Again, discs provide little advantage over drums.

Other technologies are available for fleets wanting to invest in safety, making air disc brakes a discretionary decision.

Other technologies are available for fleets wanting to invest in safety, making air disc brakes a discretionary decision.

In situations where brake fade due to prolonged hard application is a concern, discs have a decided advantage -- the hotter they get the better they work.

In the in-service environment, mixing drums and discs on tractors and trailers can create some wear issues. Because of the difference between drums and discs in the way torque rises during the brake application, in a combo with discs on the tractor and drums on the trailer, the trailer brakes will be doing most of the work during normal stops, and thus wearing out sooner.

Mitts says weight really isn't a factor in the spec'ing choice although weight-sensitive fleets could make such a case. He says the difference between the two on an axle might be 40 pounds either way. "If you care about weight, you spec the disc brake package the right way or you spec the drum brake package the right way," he says. "You have to look at total wheel-end weight, which includes wheels, hubs, etc."

Air disc brakes are not yet the dominant choice for trailer brakes, but market share is increasing.

Air disc brakes are not yet the dominant choice for trailer brakes, but market share is increasing.

Finally, the ease and cost of maintenance are big drivers of spec'ing decisions these days. There are fewer moving parts on disc brakes, and those are housed in the caliper and for the most part protected from the elements. Inspections are easier because there are few external parts to wear, and pad changes are much faster than a brake reline. But as we noted earlier, advanced drum brakes are easily capable of going the first life of the vehicle without replacement, which reduces the lifecycle cost of those brakes. A brake rotor replacement can be costly, as can a wheels-off brake overhaul.

"Much of the decision depends on the duty cycle, how long you plan to keep the truck, how heavy you run and where you run," says Mitts. "Disc brakes are world class, but how's the availability of parts? Drum brake parts are available everywhere and any tech can service them. There's no real black or white answer to this question. It's very fleet and application specific."


As purveyors of both drum and disc brakes, Meritor is going to get your business either way you decide, but they do recognize there are differentiators in the two products.   

Number 1 is feel. "Drivers can feel the difference in the brake pedal, they feel the deceleration as well as the performance and fit and finish of disc brakes," Mitts notes. "Better feel equals less fatigued and safer drivers."

Number 2 is safety. "Safety is a multi-faceted animal," he says. "As I have pointed out, there are other ways to spend money on safety, but brake fade is not a factor with disc brakes. Therefore, less skilled drivers won't have to worry about recognizing and managing fade."

Number 3 is ROI. "Even though costs are moving down in drum and discs, discs still have higher acquisition costs," Mitts acknowledges. "But balanced against a couple of other factors such as operating costs, maintenance costs and less tangible safety costs, there's a case to be made for some fleets. The wild card still is residual value. Will you recoup some of that difference at resale? The market isn't there yet, so we can't yet make that case."

The bottom line, Mitts says, is the right choice is unique to each business. "It's not a black and white situation. Spend some time with your OE reps to see what factors will come into play when choosing the spec."

As for the future of air disc brakes on our side of the pond, Joe ElBehairy, Meritor's vice president engineering and quality sees growth remaining slow. "Outside of some catalyst; either legislative or an OE saying they want to push the technology, left to natural tendencies, we're going to see relatively slow adoption of air disc brakes in North America."

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