Discs or Drums? Brake System Specs Can Go Either Way

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Today’s Trucking / November 4, 2015

There are few ways of comparing air disc brakes to drum brakes that discs don’t come out on top. Still, fleet adoption rates in North America hover around 10 percent. That begs the question, are discs too good for their own good? Drum brakes get the job done, so is there any pressing need to look beyond our traditional and proven way of stopping trucks?

The short answer to that question would be no—in most cases. The thing with brakes is you never really know how valuable they are until you really, really need them. And brakes never fail in any way that’s less than spectacular. So, if no great need has ever arisen, you can’t be blamed for dismissing the arguably more costly and heavier air disc brake systems as an expensive luxury.

As of today, air disc brakes (ADB) carry a premium of about $3000, or about $500 per wheel end. Prices varies with the OE, the brand of ADB, size of the order, etc., but that figure is a good ball park. Fleets are right to ask why they should spend that much more on discs while drum brakes are still very much in the game.

“The advanced drum brake technology that emerged to meet the U.S. DOT’s most recent stopping distance requirements is more than up to the task,” says Jon Morrison, president of Wabco Americas, suppliers of ADB systems to Hendrickson, Daimler Trucks and others. “If you need additional stopping power or you operate in a severe application, like hilly terrain with heavy loads or in an intense start/stop cycle, discs do provide an extra margin of safety as well as lower maintenance costs over the life of the vehicle.”

Let’s break out some of the performance and operational attributes of each brake type. This might help you decide which is best for your operation.

Maintenance & Repair

Neither brake system is anything close to maintenance free, but both are getting close, at least for the vehicle’s first owner. The idea is to minimize the time, money and effort you spend keeping the brakes working. Discs do reportedly offer some advantage here, but as always, it depends on the application.

According to Meritor’s Director of Brakes for North America, Gopi Krishnan, a typical linehaul application with lots of miles and little stop/start exposure brake linings can be expected to last between 300,000 and 350,000 miles and up to 600,000 for the drums.

“The normal maintenance cycle for disc brakes is not quite double that of drum brakes in a typical application,” he says. “The pads in a disc brake system should go out to 500,000 or 600,000 miles. The rotors can be expected to last up to 1 million miles. As long as you don’t have to change the rotors, you will most likely see lower maintenance costs and therefore lower overall cost of ownership.”

Routine service of drum brakes, such as a reline, can take an hour or more per wheel-end, while a pad change on a disc system can take as little as 20 minutes, in some cases without removing the wheel. Rotor or drum changes can both be somewhat more complicated, but it’s likely that the first owner of the truck would never need to do that.

In fact, aside from a pad change, most disc brake systems in linehaul service would likely be traded in—following typical fleet trade cycles—without ever requiring any serious brake maintenance. The same could be said for the premium drum brake installations, but there are other parts to such systems that do require periodic inspection, such as lubricating brake adjusters, checking the clevis pins for free movement and inspecting the linings and drums for cracking, expansion of the linings, oil contamination, etc.

One fleet we spoke with said they had seen some water intrusion around the piston seals, which prompted them to increase the frequency of inspections and consider replacing the seals at certain intervals. So, apparently they are not totally trouble free.

Canada’s new Annual Inspection procedures (see Today’s Trucking, Oct. 2015) have specific requirements for both disc and drum brake inspections, and the drum brake inspection is more rigorous.

Routine inspections for drum brakes can be more cumbersome than discs. With drums, there are several moving parts to inspect and there are more areas for potential wear, including the cam bushings, the return springs, the brake actuators and of course the slack adjusters.

Meritor’s Director of Brake Engineering for North America, Joe Kay, says discs require much less in the way of inspection.

“With our disc brakes everything is pre-lubed and sealed essentially for life,” he says. “You’ll have to visually inspect the caliper for free movement, but not much more is required and there are fewer moving parts to wear out.”

So much for “typical” and “linehaul;” many applications are much tougher on brakes, and probably more frequent brake maintenance intervals. If relines and pad changes are required more frequently, the savings in labor, parts and even downtime over the life of the vehicle could be substantial with disc brakes.

Concerns over repairs would follow similar lines until you take into account the reason for the repair. Drum brakes, for as reliable as they are, are still regularly put out of service at rates approaching 20%. Most of the OOS events are adjustment related. With disc brakes, that problem basically disappears.

“Today there is not a good way to do a roadside inspection of a disc brake, although that could change in the future,” says Gary Ganaway, director of Original Equipment & Technical Sales at Bendix Commercial Vehicle Systems. “The disc brake adjustment mechanism is internal to the brake, and not as susceptible to manual adjustment. Provided the caliper is working as prescribed, there’s not much risk of a failed brake inspection.”

With the heightened awareness of vehicle defects under the U.S. DOT’s Compliance, Safety and Accountability (CSA) program, minimizing the chances of a failed inspection has greater value than it once did.

That exposure is directly proportional to how aggressively a fleet inspects and maintains its brakes. Fleets with good attention to detail may never have a brake problem at a scale.

Getting technicians up to speed on ADB is a bit of a hurdle, but it’s not impossible to overcome.

“As with any new product, it’s important to conduct training and make sure there’s a thorough understanding of the maintenance practices, but disc brake maintenance is generally easier than that of a drum brake” says Ganaway. “Key maintenance items for disc brakes include inspection during regular PM intervals—looking for damage to boots, ensuring the caliper slides freely, and checking pad/rotor wear.”

Performance

Back in 2005, when the U.S. National Highway Traffic Safety Administration (NHTSA) issued a notice of proposed rulemaking to shorten the stopping distance of heavy trucks by between 20 and 30 percent, many people believed it would open the door to air disc brakes. It helped, but at the same time brake manufacturers came up with an advanced drum brake, engineered to develop the increased torque necessary for shorter stops using wider and longer brake lining blocks along with larger chambers. The result was a drum brake that met the stopping distance requirements without forcing the industry over to disc brakes.

We’re at a point now where brakes are about as aggressive as they can be without compromising traction. Simply put, when brake torque is greater than road friction coefficient, wheel lock-up will occur.

Hendrickson has produced an interesting White Paper called “Understanding Your Brakes: Considerations When Specifying Air Disc Brakes.” The first few pages discuss the dynamics of stopping, as a preface to a discussion about spec’ing ADB in potentially unsuitable applications.

The essence of the discussion is no matter how “good’ you can make a brake, there are practical limits to how much you can ask it to do. If the brake causes the wheel to lock up and break traction you have a potential jackknife situation on your hands. With an Anti-Lock Brake intervention, these situations are nearly eliminated, but you’re asking another system to compensate for the overly aggressive brake.

It’s to the point now where a disc-brake-equipped 5-axle tractor-trailer combination at 80,000 pounds can stop in about the same distance as a car. Try such a full application stop with an empty or lightly loaded truck, or on wet pavement, and you’ll have lots of ABS activity, which will actually increase stopping distance.

Where disc brakes have real potential to outperform drums is in severe service—heavy loads, big hills—or in frequent stop/start applications such as P&D or regional applications.

The refuse industry is probably the best-suited sector in the industry from a performance and ROI perspective. They are making hard stops hundreds of times a day with little opportunity for the brakes to cool. Lightly loaded truckload carriers would probably have the toughest case to make from a performance standpoint, although trailers that may not be seen for months at a time, being out there in some drop yard or another, might benefit from a brake that needs very little hands on maintenance.

“Applications where brakes are used frequently can see an earlier ROI with disc brakes due to longer service intervals and shorter maintenance times,” says Ganaway. “And most fleets will also experience a reduction in brakes out of adjustment.”

These days, when drivers are harder to find the customers, it’s harder to ignore driver preferences, and few would argue against disc brakes. The pedal feel is superior to drum brakes and there’s always plenty of stopping power under foot. That boosts driver confidence considerably, especially with newer drivers.

“If I’m a driver and I’m getting out-of-service violations related to brakes, if I could drive a truck that wasn’t going to affect my score I’m certainly going to choose that truck,” says Morrison. “Driver retention has become critical and I see disc brakes as a very positive recruiting tool.”

Brake Balance

One question that often arises around disc brakes is that of balance. Do you equip tractors but not trailers? Trailers but not tractors? Which do you do first?

When one vehicle in the combination has discs while the other does not, there can be balance concerns. The disc-equipped vehicle could do more of the stopping, that is, incur more brake wear, then the other. In a panic stop, ABS will level the playing field to where the more aggressive brakes get more ABS events to prevent wheel lock up.

“Performance characteristics of discs and drums are bit different,” says Kay. “Due to their self-energizing capabilities, where the leading shoe grabs the drum first, drum brakes can feel more aggressive at low speeds. Disc brakes tend to apply in a more linear fashion, so they tend not to “grab.” Discs are also more effective at higher speeds and feel better through the pedal. The bottom line is, fleets should consider the balance implications when considering a switch to discs, especially if some larger portion of the trailer fleets is older and more prone to brake adjustment issues.”

While each brake has its advantages, disc brakes have some advantages over drums that are definitely worth considering. On the other hand, disc brakes may be more than you really need, and therefore not worth the price premium. As we said from the beginning, the disc brake call is anything but a slam-dunk.