One of the primary advantages driving the more widespread adoption of air disc brakes are the reduced maintenance requirements and associated costs with disc brakes when compared to drums. But there should be no expectation that disc brakes are maintenance-free.
“The level of maintenance is less on air disc brakes than on drum brakes, but they still require some attention,” warns Keith McComsey, director of marketing and customer solutions, wheel-end, with Bendix Spicer Foundation Brake. He says there’s a perception among some users that disc brakes are practically maintenance-free, which is a fallacy.
“There’s no such thing as maintenance-free,” agrees Jim Kennedy, vice-president of maintenance for 275-truck tanker fleet McKenzie Tank Lines. To say McKenzie Tank Lines is an early adopter of air disc brakes is an understatement. The company deployed them in their first go-round in the 1970s, before the technology was ready for prime time.
It went back to disc brakes in 2011 and now makes it a standard spec’ on new tractors and trailers. Kennedy said the maintenance savings are impressive, even though they do require regular inspections.
“One of the first test units we had was a vehicle that ran nitrous acid. It was a team truck that runs 23,000 miles a month, loaded both ways, and I know that the disc brakes on that unit were right at 290,000 miles before we had to do a pad change. If you run that back to the standard, where you’re loaded only 50-60% of the time, you can extrapolate that to greater than 500,000 miles, maybe even 600,000,” Kennedy said. “For maintenance, I can sit there and watch guys slip in new pads in no time. I’ve never had any rotors fail through the first change.”
Still, McKenzie Tank Lines is vigilant about inspecting and maintaining disc brakes, even though pad changes are infrequently required.
“We’re inspecting them during every preventive maintenance (PM), every 60 days. We have certain commodities that are on 30-day inspections,” Kennedy explained. “You want to make sure the slide has free movement. If it’s sticking, you’re going to accelerate wear on the rotor and pad. We check the gap clearance between the rotor and the shoe. Nothing in this world is maintenance-free.”
“They are not maintenance-free but it’s more of an inspection-type procedure,” added Roger Jansen, product manager, SAF-Holland. “During PM cycles we like them to go in and check the pads for wear, check them for even wear and check the slide pins for any slack. Put a crowbar between the carrier and the caliper and check for excessive play. Check the slide pins and brake pads for wear and the rotor for the proper thickness. We recommend doing this every three months.”
Bendix suggests making a visual inspection of the air disc brake part of any routine PM cycle.
“There are a lot of fleets that think that because you don’t have to grease them and measure brake stroke, that there is no maintenance to them,” said Kevin Pfost, coordinator, technical services with Bendix Spicer Foundation Brake. “There is nothing that’s truly maintenance-free. But with disc brakes, the nice thing is, when you do PM on a tractor or whenever you are underneath the tractor, you just need to visually inspect the brake to make sure that parts are still fastened properly, that nothing came loose, and look at the pads to make sure the inboard and outboard pads are wearing evenly. Look at the rotors to make sure they’re wearing evenly.”
Wear indicators on the rotor will inform a mechanic when it’s time for a pad change. Even when pads do need replacing, it takes up to 50% less time than a drum brake job typically requires, McComsey said. Pfost also suggests checking the caliper to ensure it is sliding along the guide pins.
“If a guide pin gets contaminated, it won’t slide,” he warned. “Then you will find one pad is wearing more than the other. That’s an indicator you need to do a wheel-off inspection.”
This is especially important in applications that include off-highway usage. Oilfield fleets, for example, can run into problems with disc brakes if mud and debris isn’t washed away daily.
“The mud and everything gets caught in the wheel, it impacts the cooling fins on the rotors and it hardens,” McComsey explained. “When they get in at night, they really need to pressure wash the wheels out to get the mud out of them, otherwise it hardens and cakes in there.”
Disc brakes, though they have some axle weight restrictions, are increasingly being spec’d on a wider variety of applications, including logging and heavy-haul. In addition to reduced maintenance, they offer better stopping performance as well.
“You’re still getting 15-20 feet less stopping distance than what you see on RSD (reduced stopping distance) drums,” said McComsey. “When you couple that with the maintenance advantages, quicker pad changes, longer service life intervals, fleets are seeing that added value.”
Also, maintenance managers have the opportunity to simplify their parts inventory.
“With drums, you need two different sizes of brakes and two different frictions for steer and drive. With disc brakes, one set of pads does steer, drive and trailer. That’s a bonus because it’s less inventory sitting on your shelf,” said Pfost.
Disc brake users also have a lower risk of running afoul of enforcement agencies, since they don’t need to be adjusted.
“Usually when trucks get taken out of service on the road under DOT inspections, it’s because they’re out of adjustment,” Pfost said. “With disc brakes, you don’t have that.”
They’re also less susceptible to rustjacking, in which corrosion forms where the friction block is riveted to the shoe. The shift towards disc brakes continues, even with better performing RSD drum brakes now on the market. This is largely driven by the standardization of discs by certain OEMs, McComsey said. Due to their better braking performance and reduced maintenance requirements, the wider-spread use of disc brakes is a positive – as long as end-users don’t mistakenly think it’s a set it and forget it product.