With Phase Two of the Federal Motor Vehicle Safety Standard (FMVSS) 121 reduced-stopping-distance regulation going into effect in August, three brake executives gave an air disc brake overview during the National Tank Truck Carriers' 2012 Tank Truck Show & Maintenance Seminar October 22-24 in Louisville, Kentucky.

Matt Creech, North American business manager for Meritor Inc, said drum brakes are still the market leader and offer stopping-distance controlled-test performance relatively close to air disc brakes, but in real-world conditions, the two brake systems are not equal.

NTTC Creech “Drum brakes may be subject to fade,” he said. “That means reduction in braking power as they heat up. The drum expands away and the shoes have to chase the drum, so that affects braking performance.

“Directional braking stability can vary. Instability may cause the vehicle to ‘pull’ to one side when braking. Drum brakes have the lowest acquisition costs but higher costs to operate because of more frequent maintenance.”

Air disc brakes provide the shortest stopping distance in a variety of real-world situations and are less subject to fade, he said.

“Directional braking stability is very stable,” he said. “Vehicles are less likely to ‘pull’ to one side when braking. Improved driver feel mimics passenger cars. There's a higher cost but multiple total cost of ownership (TCO) benefits.”

Rich Litecky, fleet and original equipment account manager for Bendix Commercial Vehicle Systems LLC, said today's disc brakes: eliminate brake out of adjustment; have an internally sealed automatic adjuster, increased service intervals, integral molded friction pads, a simple pad exchange that reduces service time by over 50% versus drum brakes, and a large useable pad material volume; are available from all major axles and suspension suppliers; feature a choice of aluminum or steel hubs, and spindle types; and provide increased driver comfort that enhances driver retention.

Creech said a lot of customers are buying air disc brakes for performance, noting that there's a big difference between drum and disc brakes in handling vehicle speed, with the pedal input required not being consistent at various speeds with drums. “Drum brake performance is speed-sensitive,” he said.

NTTC LiteckyLitecky added that air disc brake advantages include:

  • Shorter stopping distances — 15% shorter than FMVSS requirements (250 feet).

  • Stable braking with virtually no fade; excellent downhill performance.

  • CSA scores: no slack adjusters means no out-of-adjustment concerns.

  • Higher driver comfort. Braking feels more like that of a passenger car.

  • Less downtime. Significantly longer service intervals

  • Serviceability. Less than half the time to perform lining change.

He said the Internal Adjuster Mechanism is completely sealed.

“There's no service required — it's lubricated for life,” he said. “It adjusts brake-pad clearance to compensate for pad wear. It senses each time the brake is applied. Automatic adjustment determines whether adjustment is required to maintain running clearance using designed-in tolerances. No intervention is required between pad changes. Manual adjustment must only be made at a pad change.”

Creech said maintenance shop inspection involves periodically inspecting the brakes:

  • Check caliper, torque plate, pads, and rotor for signs of wear and tear.

  • Check for any loose or missing screws.

  • Check for appropriate amount of clearance between the pad and rotor; .024" to .042".

  • Measure the pad and rotor to ensure both meet manufacturers' operating guidelines; replace when necessary.

He said periodic inspection is necessary using the following schedule: fleet chassis lubrication; chassis manufacturer schedule; at least four times during lining life; or at tire replacement.

He said the brake's adjuster mechanism is internal so limited visual inspection is required:

  • Inspect the lining-wear indicator to determine if replacement or further shop inspection is required.

  • Inspect the rotor for cracks, deep groves, blue marks, and heat checking.

  • Inspect the air chamber to verify that the mounting bolts and air lines are securely fastened and not damaged.

“According to fleet users, air disc brakes have the opportunity to reduce DOT (Department of Transportation) violations and expedite DOT brake inspections,” he said. “We're hearing that inspectors will see air disc brakes, maybe look for hoses rubbing and generally move on. There's nothing mechanical to inspect. We don't want to say they're error-free, but they generally speed up the process and have a propensity to reduce violations.”

Rick Dinkel, applications engineer for Wabco North America, said packing should be verified.

NTTC Dinkel “Air disc brakes wear inboard, so the caliper moves inboard,” he said. “We've seen issues in the field where they're knocking steel covers off, back into the brake, which causes brake failure. So you have to make sure all stuff is cleared.

“When replacing pads check that they are installed correctly, not backwards. The back of pads are metal. When pads are put in backwards, rotors will break apart.” 

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