Decoding the disc vs. drum brake debate

Though a decade ago it was far from the case, nowadays, the likelihood for a technician to work on a newer truck with air disc brakes (ADBs) is about the same as one with drum brakes.

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“Almost half of the builds for new equipment are for disc brakes today,” noted Mark Holley, director of marketing and customer solutions for wheel ends at Bendix Commercial Vehicle Systems. “With new trailers, it’s about 20%.”

Both types can provide some value to fleets, and when evaluating whether a drum or disc brake is the better overall fit, fleets should consider the benefits relative to the application they will be placed in. The total cost of ownership, including the maintenance and service, also factors into a brake’s longer-term ROI.

To help you calculate which is more suitable for your duty cycles and assets, we talked to leading brake experts about how service is done on each and what to look out for.

Service time is money

ADBs and drums have a few things in common, of course. Primarily, they both arrest vehicle movement, which takes a toll each time it happens.

“Friction and rotors will wear, as will friction and drums,” said Kevin Pfost, wheel end product specialist at Bendix.

But to discern the amount of wear, technicians must go about this in different ways.

“Visually and with a tool, a technician can measure friction and drum thickness,” Pfost said. “With a disc brake, we have a tool to measure rotor thickness. A technician can also measure the percentage of friction. So comparatively, disc brakes and drum brakes are the same in terms of visual inspections.”

The difference presents itself when the time comes to actually service the brakes.

Changing ADB pads takes roughly half the time of drum brakes, according to Joe Kay, director of engineering for Cummins-Meritor, a global power technology company whose products include disc and s-cam drum brakes.

Drum brakes not only require additional time, but oftentimes more effort.

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“With disc brakes, you don’t have to pull off a heavy drum in order to change the friction,” Pfost explained. “With disc brakes, you’re only pulling wheels off.”

Furthermore, with a drum brake system, the technician is nowhere near done after removing the drum. There are brake shoes to deal with, which Pfost said can be a challenge in their own right. After that, the cam head should be cleaned up and inspected for wear or damage.

Air disc brakes (ADB) are now standard for many new trucks built after 2018 and can be ordered on either the steer or drive axles, or both. ADB can also be ordered for most trailer types. From a safety standpoint, ADB can help reduce the risk of rear-end crashes. According to Fleet Equipment Magazine, “The main strong point for disc brakes (over drum brakes) is reduced stopping distance. With disc brakes, that distance can be anywhere from 17% to 33% shorter than drum brakes, depending on speed.”

Other advantages of using ADB include:

• No brake fade caused by heating up the brakes due to excessive usage.

• Generally less maintenance, although not maintenance-free.

• No slack-adjuster-related “out of adjustment” violations from roadside inspections.

• Longer life before replacement.

• Use of a caliper/pad mechanism, compared to the lining/drum/S-cam mechanism found on drum brakes.

  Consider the type of operation you run before choosing the equipment to best fit your needs.

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