Air disc brakes for heavy trucks offer performance advantages to drum brakes

Despite the "technical performance superiority" of air disc brakes (ADBs) for heavy trucks compared to drum brakes, adoption has been slow in North America (NA).

Whereas in Europe, at the end of the 1990s, all commercial vehicle manufacturers (truck, bus and trailer) introduced ADBs as standard, Dietrich Zaps, General Manager, Brakes, CVS Europe, Meritor, told Fleet Maintenance while on an exclusive European press tour to some of its brake facilities. ADBs are now accepted as the primary foundation brake in Europe.

Meritor is a leading global supplier of drivetrain, mobility, braking and aftermarket solutions for commercial vehicle and industrial markets (www.meritor.com).

Drum brakes are still used on off-road vehicles (mining, construction, military, etc.) and on vehicles for export to other continents - 18 percent of total European Union (EU) brake demand, he added.

Chief choice reasons

Among the reasons for introduction of ADBs in EU, says Dietmar Knoop, Head of Research & Development, Meritor Heavy Vehicle Braking Systems, United Kingdom, are:

With ADBs, brake fade is virtually eliminated, proven from Alpine testing.
Inherent high-efficiency (greater than 95 percent) and low hysteresis ensure a negligible pull (different brake performance left and right) to deliver controlled vehicle steering and braking stability.
This same high-efficiency and stability enable the highest-quality of control functions for electronic control systems like ABS, electronic braking systems (EBS) and electronic stability systems. (ADBs were introduced in parallel with EBS in the EU during 1996.)
ADBs support intelligent functions, such as continuous wear sensors, brake pad wear monitor and, in the future, electronic clearance control.
New ADBs designs reduced stopping distance up to 30 percent at the time of introduction in the EU, compared with then-current drum brakes.
ADBs enable simpler, quicker pad change vs. drum shoes and have an integrated automatic wear adjuster function.

Knoop noted that Europe is the "center of excellence for air disc brake technology."

ADB use in North America

Meritor air disc brake systems have been in use in North America since the 1970s with the Dura Master ADB designed for transit and waste industries. In 1999 with the purchase of Lucas Verity, Meritor was able to add over a dozen new braking systems to the market.

In 2010, Peterbilt Motors became the first North American heavy truck manufacturer to offer air disc brakes as standard equipment on a heavy truck.

Yet, more than 90 percent of trucks in the United States still spec s-cam drum brakes. However, ADBs are widely used on refuse trucks and transit vehicles.

It should be noted that drum brakes are larger and have higher-torque following August 2011 FMVSS changes that mandated reduced stopping distances for heavy trucks.

Factors affecting growth

There are several reasons contributing to the slow adoption of ADBs in North America explained Chad Mitts, General Manager, North American Brakes, Meritor. Firstly, in Europe, the truck OEMs decide the vehicle specs, whereas NA is predominately a customer spec market.

Next, there is a different service infrastructure. Trucks are serviced at OEM dealers in Europe by factory-trained and equipped technicians, he continued, so technology changes can be more easily managed and facilitated. In NA, vehicles are serviced at a wide variety of service locations, so conversion to new parts and training is more complex.

Another factor is that trucks in North America are dynamically different so the impact of ADBs is less. Plus, the upfront costs for ADBs is more than drum brakes.

Further, Mitts observed that, in general, technology application lags in North America compared to Europe.

Trends and drivers

Some of the brakes trends in the North American market, according to Mitts, are:

CSA is putting more emphasis on brakes for service and reliability.
Weight is becoming a factor as larger drum brakes, emissions equipment and pending GHG regulations all shift the focus.
Air disc brakes are widely available - a factor that influences choice.
An array of safety innovations, such as collision mitigation and lane departure warning, compete for ADB dollars
Data sophistication is growing with real-time telematics-based systems.

Looking ahead for the next five years, Meritor believes ADBs will continue moderate growth, but drum brakes will remain the market standard, said Mitts. ADBs and drum brakes are both important and product development investment will continue for both by Meritor with weight, service and reliability being the focus areas to meet the challenges facing its customers.

Major differentiators

Among the major differentiators of ADB vs. drum brakes are feel and safety, Mitts said. As for feel, the ADB's linear output and stability drive the preference.

With regard to safety, the difference is multi-faceted, he said. With better brake feel there is less driver fatigue. Because there is less brake fade, less skill is required, making for safer drivers. In addition, stopping distance is slightly better.

Mitts acknowledged that ADBs cost more than drum brakes but this has to be factored against future truck residual value, maintenance and service savings (pad changes are up to 75 percent faster than drum shoe changes and no periodic lubrication is required) and uptime improvement (adjuster mechanism and pistons are environmentally sealed for life and there are no current "out-of-adjustment" conditions).

Greater adoption of ADBs in North America will continue to progress as vehicle owners and operators become more educated on their benefits and advantages compared to drum brakes. "The right choice is unique for each business," as they say.