Disc brakes don’t offer much for roadside inspectors to write up. They aren’t get-out-of-jail-free cards, but adjustment violations are nearly unheard of.  -  Photo: Jim Park

Disc brakes don’t offer much for roadside inspectors to write up. They aren’t get-out-of-jail-free cards, but adjustment violations are nearly unheard of.

Photo: Jim Park

There aren’t many good reasons left not to spec air disc brakes, but drum brakes continue to out-sell discs. If you’re on the fence, here’s something to consider. Had the truck driven by Rogel Aguilera-Mederos been equipped with disc brakes, his 2019 crash near Lakewood, Colorado, probably would not have happened. That crash, where Aguilera-Mederos hit a traffic backup at the bottom of the grade at high speed after losing his brakes, killed four people and resulted in Aguilera-Mederos being initially sentenced to 110 years in prison before the governor stepped in.

Not many fleets prioritize performance over cost in a brake spec. Drum brakes work just fine. They are lighter, less expensive to purchase, and they’re well understood.

The weight and cost arguments have all but evaporated for most fleets, but there are instances where drum brakes still make sense. Like almost everything in trucking, there’s no right or wrong answer. It all depends on the application.

Fleet maintenance consultant Bruce Stockton of Stockton Solutions says the weight differential is now down to about 20 pounds per wheel-end on tractors. It’s still a little higher on trailers.

“When the stopping distance rules changed 10 years ago, drum brakes got bigger and heavier,” Stockton says. “That significantly reduced the weight delta between drums and discs. It’s now about 100 pounds on the tractor, and disc brakes continue to get lighter. If you’re really worried about weight, spec smaller fuel tanks.”

Along with the shrinking of the weight spread between discs and drums, cost has come down somewhat. There’s still a premium for discs even though they have become standard in every OEM’s data books. Stockton says the manufacturer’s suggested retail price on discs is about $600 — but nobody ever pays that much.

“It’s closer to $300,” he says, “And really only about $200 if you’re talking about just the tractor.”

But because many fleets don’t look at total cost of ownership, he says, they continue to buy drum brakes because of the initial purchase price.

“Disc brakes cost less over the typical service life of the vehicle,” he maintains. “Mind you, fleets that hang onto their equipment for a long time would see a different picture.”

Over a four- or five-year trade cycle, the brake maintenance spend will almost always be less with disc brakes. Even if the fleet does just one reline or pad change, the labor-cost savings alone makes discs worth the additional upfront cost. 

“Mechanics with air brake certifications are still required whether they are working on discs or drums,” Stockton points out. “It’s not like we are able to pay mechanics less per hour these days. So, the cost of labor alone, over the life of the truck, will usually make up the increased acquisition costs” for disc brakes.

Having to change out brake rotors during a trade cycle might change that calculus a little. That’s an expensive proposition. Fleets with extended trade cycles, or ones that operate in harsh, dusty environments, often report increased rotor wear, possibly leading to early replacement.

There would also be additional one-time costs associated with a transition from drums to discs, such as carrying additional parts inventory, technician training, etc. But once you have made space in the parts room and everyone is familiar with them, it’s business as usual.  

Differences That Make a Difference

If you can get past the cost and weight discussions, there are questions of performance and perception. While less tangible in an accounting sense, drivers tend to favor discs over drums, and who couldn’t use another perk to attract drivers? Then there’s the specter of plaintiff’s attorneys. They’ll have a field day with fleets that can be made to look like they prioritize profit over safety. Drum brakes make fleets easy targets, and that’s an argument ‘we’ is editorializing make it ‘fleets’ hardly ever win.

Chip Warterfield, fleet safety and equipment manager at Upstaging Inc., says his fleet eased its way into disc brakes, beginning with steer axles only several years ago, and more recently including discs on the drive and trailer axles.

Drum brakes are messy and more expensive to maintain than disc brakes, but among the pros are that they are lighter and cheaper to buy.  -  Photo: Jim Park

Drum brakes are messy and more expensive to maintain than disc brakes, but among the pros are that they are lighter and cheaper to buy.

Photo: Jim Park

“With the steer axle alone, we learned of the simplicity in maintenance and inspection,” he says. “With no faults whatsoever, we chose to step up to the drives as well. It was only natural that we followed with the trailers. Today, I would spec nothing else but air disc brakes.”

He says while drivers often comment on the smoother stops, they seldom say anything about the shorter stopping distances made possible by the faster application of the disc brakes. That, oddly enough, caused some consternation among drivers because they were getting more hard-brake event notifications.

“While reviewing dashcam videos of hard-braking events, I noticed the ADB-equipped trucks stopped quicker, triggering a higher frequency of events,” he says. “One of our best professional drivers had a great observation that I don’t often hear in the ADB discussion: ‘There’s no lag in the braking effort. When you push the pedal, and the caliper responds with its grip, deceleration follows,’ the driver told me. His comments made perfect sense when compared to average maintained drum brake equipment. There is the pedal effort, then the pushrods swing the s-cam, then the shoes [press] against the drums, and that’s if everything is in adjustment.”

Maintenance and Training

Almost nobody doubts that maintaining drum brakes requires some effort and cost. One of the selling features of discs is the simplicity of a pad change compared to replacing brake linings and possibly cam bushings and drums. Fleets with short trade cycles claim they can run a full trade cycle with only one pad change. If you normally have to do two relines with drum brakes over the life of the truck, the savings are even greater.

That said, history tells us that trucks operating in dusty or gritty environments such as gravel pits could see high rates of rotor wear. Probably no worse than drum wear under similar conditions, but rotors are much more expensive to replace. For that reason, many fleets that tried disc brakes have gone back to drums.

Corrosion seems to be playing a similar role in some instances. It’s not doing any more harm to the rotors than you’d expect, but it’s causing calipers to stick, and it’s eating ABS tone or exciter rings off the hubs.

“There’s so much calcium chloride on the roads now, it’s getting into the calipers and causing corrosion,” says Darry Stuart, CEO at DWS Fleet Management Services. “The pads aren’t lasting like they should, and the tabs on the ABS ring are corroding. That gives you an ABS malfunction and the ABS doesn’t work. This affects drum brakes too, but a standard hub is less expensive to replace than a rotor hub and the calipers, etc. And it’s all pretty labor-intensive.

“There’s nothing wrong with disc brake in this case; it’s just the expense of replacing all the corroded parts,” Stuart says.

Corroded ABS tone rings are a problem for some fleets. It’s no better with drum brakes, but new hubs are less expensive than new rotors.  -  Photo: DWS Fleet Maintenance

Corroded ABS tone rings are a problem for some fleets. It’s no better with drum brakes, but new hubs are less expensive than new rotors.

Photo: DWS Fleet Maintenance

It’s more of a problem for fleets operating in the Northeast and areas of the country where lots of calcium chloride is used. Stuart says he knows of fleets that are going back to drum brakes because of the expense.

And a final word on training. It’s fairly obvious that technicians would need to be brought up to speed on inspection and maintenance procedures. But drivers should get an update, too. The daily inspection requirements are different for disc brakes than for drum brakes. The upside is they are no longer required to do mark-and-measure brake adjustment checks (not that they are typically done anyway). The downside is drivers may get a bit dirty when inspecting caliper travel when the caliper is mounted high on the wheel-end.

“You wouldn’t think driver training on ADB’s would be difficult, but you have to get them used to inspecting a different system,” says Warterfield. “ADBs do call for an extra measure of effort when inspecting the vertical caliper versus the horizontal nature of the drum brake. And they aren’t going to figure it out for themselves.” 

Disc Brakes Outperform Drum Brakes Where It Matters

It’s understood and accepted that modern tractor drum brakes (those produced after the stopping distance rules changed in 2011) perform as well as disc brakes in stopping distance tests. If they didn’t, they wouldn’t be installed on trucks.

But that’s not the whole story.

The stopping distance rule change didn’t apply to trailers — yet trailer axles contribute roughly 40% of the truck’s total braking effort. That figure would probably be somewhat less when you consider the likelihood that some trailer brakes are operating beyond their adjustment limit, or have been refitted at some point with aftermarket friction material that might not be the same as the OEM spec.

Additionally, drum brakes can suffer some performance degradation over time as linkages and bushings wear and linings thin out. Automatic brake adjusters are designed to compensate for some degree of wear, but they can only do so much.

By design, with the exception of the performance and thickness of the friction material (brake pads in disc brakes), none of the mechanical conditions common in drum brakes can afflict disc brakes. It would be rare to hear a complaint about “brake failure” from a driver of a truck equipped with disc brakes.

In the heavily publicized case of Rogel Aguilera-Mederos, the driver convicted in a 2019 Colorado crash and initially sentenced to 110 years, said during his court testimony that his brakes had “failed.”

More likely, bad driving practices, and possibly worn or mis-adjusted brakes were behind his inability to maintain a reasonable speed during his descent of that long hill in I-70 leading into Lakewood, Colorado.    

It’s understood that if truck weight is doubled, stopping power must be doubled. But if speed is doubled, stopping power must be increased four times. If both weight and speed are doubled, stopping power must be increased eight times.

In practical terms, that means a fully loaded truck descending a hill at 60 mph will require eight times the stopping power of a lightly loaded truck traveling at 30 mph. Put another way, the brakes on the faster, heavier truck will be doing eight times as much work as the brakes on the slower, lighter truck. The brakes must absorb or dissipate eight times as much heat.

The brake drum and the lining material are designed to dissipate heat, but too much heat that cannot dissipate quickly enough will eventually overwhelm the brakes, rendering them less and less effective, and eventually almost useless in extreme cases. As brake drums expand due to the extreme temperatures, brake stroke increases, lessening the force applied by the actuator via the push rod.

Exactly the opposite happens when disc brakes get hot.

The rotor expands in the direction of the brake pads, reducing the length of the stroke and increasing the force applied to the rotor by the actuator. Disc brakes glowing bright cherry red actually have more stopping power than cold brakes.

That’s probably not a big concern for fleets operating in the Midwest. But when poorly trained drivers venture into mountainous terrain with poorly maintained equipment, brakes become front-page news.   

“Even though manufacturers can meet or exceed stopping distance standards with drum brakes, not adopting the latest technology of disc brakes leaves a fleet open for criticism by our friends in the litigious realm,” observes fleet maintenance consultant Bruce Stockton of Stockton Solutions.

This article first appeared in the March 2022 issue of Heavy Duty Trucking. 

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