The industry has been witnessing a trend with the increased adoption of specifying air disc brakes (ADBs). Specification of air disc brakes onto heavy duty commercial vehicles gained momentum in North America after the Federal Motor Vehicle Safety Standard (FMVSS) No. 121 for air braking systems enforced a reduction in heavy duty commercial vehicle stopping distances, with required compliance as early as 2011. The increased adoption rate of ADBs can be attributed to these changes, since ADBs aid in stopping a vehicle in a shorter distance than traditional drum brakes. The rate and extent of ADB use has been growing ever since.
Some fleets are specifying ADBs on all axles while others are specifying ADB/drum brake combinations. Whatever the case may be, fleets should be aware of the benefits ADBs can provide, service and maintenance considerations with these brakes, and the decisions to be made when specifying ADBs.
Differences in components and mechanics
ADBs and drum brakes both apply torque to slow the vehicle. They both take inputted air pressure applied within a brake actuator or brake chamber to translate a linear force to apply the brakes. The similarities, however, stop there.
Drum brakes take the linear force from the chamber to the slack adjuster to help maintain running clearance. The S-cam is turned to apply the force against the drum shoe friction pads, which are pushed against the inner surface of the rotating drum. This process creates the torque to help slow the vehicle.
ADBs take the linear force from the chamber to an adjustment mechanism to help maintain running clearance. A piston force against the backing plate of the friction pads applies pressure to both sides of the rotor, which is spinning between the pads. This clamping process produces the torque to help slow the vehicle.
These mechanical differences are the reason for performance differences. One of the performance aspects to consider is brake fade. Brake fade can occur when the heat, built from the application of friction, causes functional variations to the braking system. With drum brakes, as the drum surface heats up it expands away from the friction. This expansion increases running clearance and, in turn, increases stopping distance. With ADBs, as the rotor surface heats up it expands toward the friction. This expansion decreases running clearance and, in turn, decreases stopping distance.
“Operationally, the ADBs have the ability to greatly resist brake fade and provide a more consistent brake torque application during almost all braking situations,” says Tony Ryan, technical services and training manager for SAF-Holland. SAF-Holland is a global manufacturer and supplier of chassis-related systems and components for trailers, trucks, and buses, as well as a provider of wheel systems, brakes, axles, and more.
“[ADBs] reduce stopping distance, most notably at higher speeds,” says Keith McComsey, director of marketing and customer solutions, wheel end, Bendix Spicer Foundation Brake. Bendix Spicer Foundation Brake combines the wheel end foundation brake technologies of Bendix Commercial Vehicle Systems and Dana Commercial Vehicle Products. The joint venture is a single source for OEM brake system design, manufacturing, hardware, and support for foundation brake components and actuation systems.
“[ADBs] virtually eliminate brake fade … [brake fade] is a variable the driver sees; with a cold brake, the vehicle stops at one distance, and when the brake gets hot, that distance is extended. That variation to the driver is unpredictable. With ADBs, it is much more consistent and can be relied on by a driver,” McComsey says.
Another performance advantage of ADBs over drum brakes is the service life of friction material. ADB pad changes are less frequent due to their efficient on-road performance.
“With regard to tractor operation, the feedback I’m getting from customers, whether with ADBs on steer, or steer and drive [axles], is that they are getting 350,000-500,000 miles out of the brakes," says Greg Cooper, disc brake product manager, field product specialist, Stemco - Brake Products Group. "They are not seeing that kind of life out of their shoes. It is a significant difference.” Stemco is a manufacturer and supplier of commercial vehicle systems and components, including wheel end components, brake products, suspension products, and tire and mileage solutions.
“On the trailer side, it varies – depending on how much use [the brakes] are getting,” Cooper says. “If the tractor is equipped with drum brakes and the trailer is equipped with disc brakes, the trailer pads will wear out faster than normal. [ADBs] are a more responsive system, it works a little harder and tends to stop quicker; if it’s pulling the tractor, they may wear out quicker than anticipated.”
ADBs can also reduce the potential for Compliance, Safety, Accountability (CSA) violations. When it comes to CSA violations mandated by the Federal Motor Carrier Safety Administration, a large portion are due to brakes that are out of adjustment. A common cause leading to brakes being out of proper adjustment is rust jacking, or the buildup of rust between the brake shoe and lining. ADBs are less susceptible to rust jacking due to their ability to keep contaminants out from underneath friction material.
Changes in maintenance
Of the differences between drum brakes and ADBs, the most notable are those related to maintenance and servicing.
“Fleets should expect simplified maintenance schedules,” says Savvas Constantinides, senior commercial product manager, Haldex. “Air disc brakes have different maintenance procedures, but [the procedures] are simpler and [are] required less often [than drum brakes].” Haldex is a developer and supplier of brake and suspension solutions in the global commercial vehicle industry, as well as a developer of technologies for electric and autonomous vehicles.
On the ADB side, there are longer pad replacement and longer rotor replacement intervals when compared to the drum shoe and the drum itself for drum brake systems. Not only is the friction material replacement interval longer with ADBs, but the maintenance required to change the friction material is also much quicker and simpler. Manufacturers have simplified the replacement process so that a technician need only swap a new pad in place of the old pad within a mounted bracket. It is always recommended to carry out a thorough inspection when replacing brake pads. A drum and shoe system is more labor-intensive, as technicians must work on a bigger, bulkier, and heavier system than ADBs.
“Service times [with ADBs] compared to drum brakes are significantly less," Ryan says. "With the vehicle jacked up and the wheels off, a pad change and a few prudent inspection checks will take only a few minutes per wheel end.”
“The ADBs friction will last longer; and if there is a chance that you have to replace the pads before you trade the vehicle out, the replacement and labor time to do that is much less – around one quarter of the time you would spend on replacing drum brake friction,” says Bendix Spicer’s McComsey. “It’s that kind of difference that is a real key benefit from a service perspective for the fleet for reducing their overall maintenance and labor costs.”
Along with longer pad life, the rotor in the ADB system has a longer service life than its comparative counterpart, the drum. With drum brake systems, it is not uncommon for the drum to be replaced at the time of friction material change. With ADB systems, McComsey says that the rotor should last two to three pad changes, further saving fleets in costs associated with parts, labor, and downtime.
Another significant difference in brake maintenance is that the servicing of slack adjusters, S-cam bushing wear checks or shaft greasing, free stroke checks, and other maintenance procedures are all eliminated when servicing ADBs rather than drum brakes.
“While the perception may be that air disc brakes are maintenance-free, the reality is that, like any other braking system, a regularly planned maintenance schedule is integral to maximizing performance, enhancing safety, and achieving optimal life,” says Jeff Wittlinger, business unit director of wheels and braking systems, Hendrickson Trailer Commercial Vehicle Systems. Hendrickson is a global manufacturer and supplier of medium and heavy duty mechanical, elastomeric, and air suspensions, axle and brake systems, and other components for the commercial transportation industry.
Another change to routine maintenance will be the focus on new and different areas of inspection on the vehicle. For example, technicians who have spent a majority of their careers working on S-cam drum and shoe systems will need to understand the critical service and maintenance points on an ADB system. There are fewer parts to inspect, maintain, and stock with ADBs when compared to drum brakes. This should translate to less necessary inventory and less downtime associated with preventive maintenance (PM) schedules.
Establishing a regular PM schedule that incorporates ADB inspections will be crucial in the fleet’s ability to capitalize on the adoption of ADBs and ensure that the braking system's operation is optimized. Inspections on integral ADB components, such as brake pads and calipers, will impact the system’s service life and performance.
“One factor that can negatively impact maintenance cost and system performance is allowing brake pads to wear past minimum thickness,” Wittlinger says. “When this happens, the pad’s metal backing plate can wear against the rotor. The disc brake mechanism will continue applying force to the worn pad, which, in turn, continues to grind against the rotor. If left unchecked, the rotor will likely require premature replacement and repair costs can skyrocket.”
When it is time to change the ADB friction materials, most suppliers recommend utilizing OEM or like-for-like parts. These are components that were designed to work together and meet OE specifications when the vehicle was manufactured. McComsey advised Bendix Spicer tested a number of aftermarket friction materials, and confirmed some severely reduced performance and increased stopping distance.
For those fleets researching the use of aftermarket friction materials, be sure to verify compatibility and work with a trusted supplier.
Stemco has developed a comprehensive parts interchange that takes original pad equipment numbers and translates them into Stemco numbers. The system then uses the Friction Material Standard Institute numbers to culminate an interchange that determines what types of vehicles a pad fits, pad to caliper compatibility, and more. The interchange is available at stemco.com through the site’s parts search section.
With such drastically different architecture, mechanical operation, functionality, parts, and size, aftermarket specification to ADBs from drum brakes is a major undertaking. Although not unheard of, it is rare, as there is much to take into consideration when making the change from drum brakes. It would not only be more expensive to specify ADBs as a retrofit option but would also require significant downtime and labor. Most fleets will opt to specify ADBs on the vehicle at the time of production.
Fleets can specify ADBs through the OE on any and all axles of heavy duty commercial vehicles.
“The more axles that have ADBs on them, the more improved the performance is – the shorter the stopping distance [and] the longer the brake friction [material] and rotor will last – because the work is spread across all the axles more evenly and efficiently than when you have mixes of drum and ADBs,” says Bendix Spicer’s McComsey.
Not all fleets, however, are specifying all (both tractor and trailer) axles with ADBs.
“Market acceptance is currently being driven by ADBs being used on the tractor side, first on the steer and then on [the] drive axles. This is also in part due to the reduced stopping distance laws introduced for tractors,” says SAF-Holland’s Ryan.
The FMVSS mandated air brake systems to improve stopping distance performance on trucks with a 30 percent reduction in stopping distance, per FMVSS Title 49, part 571, Standard No. 121. The standard requires most vehicles to stop in no more than 250 feet when loaded to their gross vehicle weight rating, and that all heavy truck tractors stop within 235 feet when loaded to their lightly loaded vehicle weight, when tested at a speed of 60 miles per hour.
“The ideal situation would be to have ADBs across the whole tractor and trailer combination, or at least on the trailer [with drum on the tractor] so that it pulls the combination up straight and is nicer for the driver,” Ryan says.
It would seem that the best braking performance would be to specify ADBs across the board. However, Hendrickson’s Wittlinger poses thoughtful considerations.
“While there are many positives with disc brakes, we do think the business case for disc [brakes] on a trailer may prove to be different than a tractor. Factors like trailer trade cycle length, differences in brake pad wear between tractor and trailer, and overall stopping distance contribution from the trailer affect return on investment.”
“[Fleets] are specifying ADBs in many cases because of stopping performance, and how well ADBs respond with ABS; some systems have roll stability, traction control, ABS, etcetera," Says Stemco's Cooper. "The ADBs seem to adapt better to that technology and work more efficiently.”
A common configuration in today’s fleets is the specification of ADBs on the steer axle of the tractor, with drum brakes on the drive axles. The trailers will be fitted in uniform with either drum or ADBs.
Adoption across heavy duty markets
“ADBs can help fleets reduce their total cost of ownership and improve efficiencies and safety. This applies to fleets of all vocations. All fleets can benefit from ADBs,” says Haldex’s Constantinides.
Air disc brakes present many benefits to fleets. With reductions in stopping distance, easier and faster maintenance procedures, fewer replacements with longer pad service life and rotor service life, and flexibility in terms of their specification, it is clear why fleets have continued to adopt air disc braking systems.