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Of discs & drums

If you think today's bigger and beefier drum brakes now offer the type of stopping power and performance once reserved for air disc brakes (ADBs), you're right but you're also wrong. While the stopping power of both brake types is now very similar (along with their initial price points, it should be noted), their stopping characteristics, along with ease of maintenance and lifecycle costs, remain,

If you think today's bigger and beefier drum brakes now offer the type of stopping power and performance once reserved for air disc brakes (ADBs), you're right — but you're also wrong. While the stopping power of both brake types is now very similar (along with their initial price points, it should be noted), their stopping characteristics, along with ease of maintenance and lifecycle costs, remain, in many cases, worlds apart.

What's driven the two types of brakes closer together in terms of both stopping power and price is the result of government intervention, specifically new stopping-distance rules promulgated by the National Highway Traffic Safety Administration (NHTSA), which started going into effect in August of this year.

Those new rules require a typical three-axle “tandem” tractor with a gross vehicle weight rating (GVWR) of 59,600 lbs. or less coupled to a trailer and traveling at 60 mph to come to a complete stop in 250 ft., compared to the former 355 ft. stopping standard — reducing tractor-trailer stopping distance roughly 30%.

For heavy, severe-service tractors with GVWs above 70,000 lbs., the stopping-distance requirement is reduced to 310 ft. at the same speed, with the caveat that all “heavy truck tractors” must stop within 235 ft. when loaded to what is termed their “lightly loaded vehicle weight.”

Two-axle (4×2) tractors and tractors with a GVWR above 59,600 lbs. (6×4) must meet the 250 ft. at 60 mph rule by Aug. 1, 2013. Heavy, severe-service tractors sporting GVWs above 70,000 lbs. are required to meet the shorter stopping distance of 310 ft.

Truck brake engineers emphasize that the majority of tractors — in this case those with tandem rear axles, which account for 60 to 70% of the highway rig population — can comply with the new stopping distance using nothing more exotic than wider “enhanced S-cam” 16.5-in. drum brakes. The larger size and design refinements of these brakes dissipate heat more effectively and, as a result, stop trucks within a significantly shorter distance than their predecessors.

“Four years ago, there was a big gap in performance between drum and ADBs,” says Joe Kay, chief engineer-brake systems for Meritor Wabco. “Now, with the regulatory changes driving design improvements, we've significantly narrowed that gap to where there's now very similar performance between the two.”

Yet those design changes to drum brakes don't come cheap; as a result, the price differential between them and their ADB brethren has narrowed considerably, helping fuel renewed interest in ADB systems among fleets.

“We have seen a dramatic increase in ADB demand; this year alone it's up 140% over last year, and we've installed a new assembly line at our Bowling Green, KY, assembly plant to meet this growing demand,” notes Gary Ganaway director for drum brakes, marketing, and global development at Bendix Spicer Foundation Brake LLC.

“With the reduced stopping-distance rules, the base [drum] brake system became larger and more expensive while, in turn, ADBs became less expensive,” he explains. “ADBs also reduce the required amount of periodic maintenance, as pad and rotor life is typically improved over their drum brake counterparts. In addition, they are sealed and lubed for life, negating the need for any periodic lubrication or adjustment.”


Another issue driving the adoption of ADBs, especially for tractor steer axles, deals with the need to increase stopping power while confronting space limitations.

“The greatest opportunity to improve overall braking is by increasing the torque of the steer axle brakes, which have historically been smaller and less capable than their drive and trailer axle counterparts,” Ganaway says. “Drive and trailer axle brakes are typically 16.5 in. in diameter and 7 or 85/8 in. wide. By contrast, front brakes [typically] have been 15 in. in diameter and 4 in. wide.”

“The real focus has been getting more stopping power ‘output’ from the steer axle,” adds Meritor's Kay. “You don't need them on the drive axles so much because you can't really increase brake torque back there anymore. You're already pretty close to ‘lockup’ in terms of the stopping power on the drivers, so there's no benefit to trying to boost the brake capability on them any further.”

Bendix's Ganaway believes that the tendency to use disc brakes on just a truck's steer axle is largely a matter of economics, as well, a perspective shared by Ken Kelley, president of Webb Severe Duty.

“The North American market penetration for ADBs will heavily depend on the overall cost benefit of disc brakes over drum brakes,” Kelley points out. “Today, air disc brakes still carry a cost premium over drum brake systems. If a fleet is given two choices that work, they normally gravitate to the option with the best overall cost per mile.”

That view is also echoed by Jeff Geist, director of product & business development for brake system maker Motor Wheel. “Certain fleets are presently evaluating ADBs for overall cost and performance benefit,” he says. “Since the present up-front costs are greater and ADB systems have yet to demonstrate long-term reliability with low maintenance costs, we expect the conversion to be slow.”

As discs and drums become closer together in terms of performance and price, other issues may start to come into play, noted Ramin Younessi, Navistar group vice president for product development & strategy.

“Disc brakes are the standard in Europe because the trucks there stop so much more frequently over their lifecycle,” he explained to Fleet Owner in an interview last year. “Yet as drum brakes necessarily get larger — and thus more expensive — to comply with NHTSA's new stopping regulations, the lower cost and weight advantage they enjoy may be diminished.”


Younessi added that ADBs “married to aluminum hubs could end up weighing a lot less compared to the larger drum brakes we'll need to use, with the cost difference being much less than it is today. That's when the longer life of ADBs would become a greater factor; that's when we might start seeing some shifting [in the market].”

Bendix's Ganaway posits that three characteristics of ADBs may be the critical fulcrum around which such a shift away from drums would begin to gain significant momentum:

  • Faster pad changes

    With the wheels off, it takes about 15 min. to reline an air disc brake versus the 60 min. for the typical drum brake.

  • Sealed for life

    Disc brakes are sealed and thus do not require any periodic lubrication.

  • Longer life

    Bendix noted that its pre-stopping distance experience showed a significant life expectancy advantage for ADBs versus drums. However, current advances in friction technology and the larger size of today's drum brakes are significantly lessening this gap.

“Field inspections of ADBs present some level of challenge, too,” Ganaway adds. “Unlike drum brakes, the movement of the brake during actuation is less noticeable. Also, due to its sealed design, it is not possible to visually check or measure the actuation stroke. That's why several industry groups are working on a uniform inspection method.”

Webb's Kelley explains that there are a variety of brake combinations available today that achieve the reduced stopping distance and results will vary from fleet to fleet depending on vehicle specs, duty cycle, geography, and trade cycles. “The North American commercial vehicle market, however, is a tight-knit community and there's no doubt that word will get out over time as to ‘what works best,’” he says.

In the end, the brake system that best fits a particular fleet's overall stopping power, cost, and maintenance needs will win the day.

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