Disc brakes offer many benefits, including improved performance and ease of service.
One friction material supplier says there are risk involved in spec’ing disc brakes only at certain positions, as they don’t properly ‘work share’ with drums and can take on more load than they were designed to withstand.
It was an introduction to Canadian trucking that Mark Lee, Truck News columnist and a longtime professional driver imported from the United Kingdom, will never forget.
“I was behind the wheel of a big truck before the Timmies I bought at the airport had gone cold,” Lee recalls. “I pulled out of the yard and when I got to the first stop sign I was shocked to find that the middle pedal didn’t work properly. I pushed it down, it just didn’t appear to be connected to the brakes, so I pushed it some more and it slowed down a little. By the time I reached the stop sign I had to do a panic stop.”
Turns out, Lee was simply getting acquainted with drum brakes. Air disc brakes, which are pretty much standard in Europe, provide more responsive braking and other benefits, but they come at a higher price that Canadian fleets have traditionally been reluctant to pay. As a result, drivers here have simply come to accept the comparably spongy feel of drum brakes.
“If I had used the same pedal pressure on a disc braked truck, I would’ve been chewing on the steering wheel way before the stop sign,” Lee claims.
While disc brakes are popular in other parts of the world, they’re just now catching on in North America, where the almighty dollar and poor past experiences have caused most fleets to persistently hang onto the familiar drum brake.
Drum brakes have served the trucking industry adequately for many years, but now under pressure to reduce the stopping distances of heavy trucks, vehicle manufacturers are beginning to more aggressively promote disc brakes, which cost more at the outset but provide more effective braking as well as maintenance savings over the life of the equipment.
Last year, Peterbilt became the first truck manufacturer to make disc brakes standard on the front axle of all its Class 8 truck models. Now, Truck News has learned, about 60% of disc brake-equipped Peterbilt trucks are rolling off the assembly line with discs on only the steer axles while the other 40% are fitted with disc brakes at every position, a testament to how quickly customers have warmed up to the technology.
It wasn’t always that way. In the mid-1980s, disc brakes were thrust into the market with disastrous consequences.
“This is not the same disc brake we saw in the past,” Steve Bell, engineering manager, air disc brakes with Bendix, claims. Early generation disc brakes featured undersized rotors and didn’t have the ability to absorb the energy generated by heavy vehicles. Overheating resulted in cracked rotors and a lot of unhappy customers.
“We understand that phenomenon much better now,” Bell says of high temperatures and other challenges which tainted an entire industry’s perception of air disc brakes.
Despite their early shortcomings, redesigned air disc brakes are making a comeback, largely because of new regulations mandating shorter stopping distances for heavy trucks. Disc brakes aren’t needed to comply with the 2011 National Highway Traffic Safety Administration (NHTSA) stopping distance requirements, which called for a 30% improvement in heavy truck braking capabilities. Larger, more effective drum brakes can meet the new requirement, but as drum brakes become bigger and more expensive, the value proposition for disc brakes grows stronger. That, and the willingness of truck manufacturers to promote – and in some cases, standardize them – has resulted in a sharp increase in demand for the products.
Gary Ganaway, director of the foundation brake group with Bendix, said from 2005 to 2011, the company produced 250,000 air disc brakes. This year alone it will build 155,000 units.
“Interest in air disc brakes has grown immensely and we think that’s largely attributable to the fact fleets are paying more attention to their brakes,” Ganaway says.
One of the biggest deterrents to disc brake adoption has been purchase price, which remains significantly higher than the traditional drum brake alternative. Conventional wisdom suggests higher production volumes should soon translate to lower purchase prices, but that has not yet materialized.
“We haven’t really seen that so far,” Ganaway admits. “We’ve maybe seen some opportunity-taking on the part of the vehicle manufacturers.”
Even at today’s higher prices, however, a strong case can be made for the investment, even in the most cost-sensitive fleet operations. Fleets interested in disc brakes have been encouraged to spec’ several of their new trucks with disc brakes on either the steer axle only, or better yet at all positions, and then run them alongside traditionally-spec’d tractors for direct comparisons.
Over time, the repair and maintenance savings on the disc brake-equipped tractors will usually deliver a return, Ganaway says.
“Our rule of thumb is in linehaul applications, a fleet can expect to double their lining life with disc brakes and the expectation is the rotor would last two pad cycles, typically,” he says. “So, if a fleet is getting 400,000 miles they should expect to get about 800,000 miles. But that number is not absolute; every driver is different, every route is different and every load is different, so there’s a lot of variance in there.”
It also takes less time to change the pads on a disc brake; about 15 minutes compared to an hour on a traditional drum brake. The maintenance savings are influenced by many variables, but the performance advantages of disc brakes are difficult to dispute in any application. Disc brakes simply work better than drums; they are more resistant to fading and they offer more responsive braking, as Lee can attest.
For that reason, owner/operators are actually outpacing fleets in adopting disc brakes, Ganaway claims.
“The owner/operator is actually driving the equipment and he or she gets to enjoy the benefits of that added feeling of safety – the better feel – and typically owner/operators keep their equipment longer than fleets do, so we find it’s a pretty attractive option to them,” he explains. “They’re willing to spend a little more because they’re going to enjoy the benefit of the utility of the technology.”
Transitioning to disc brakes, however, isn’t without its risks, at least according to TMD Friction, which produces OE and aftermarket linings for disc and drum brakes. Shabbir Hakim, vehicle program manager, commercial vehicles with TMD, warns that compatibility issues can arise when mixing and matching disc and drum brakes, a common practice as many fleets begin their transition by spec’ing disc brakes on the steer axle of a tractor and drums on the drives. That’s to say nothing of the trailers, the vast majority of which are equipped with drum brakes.
According to Hakim, problems can arise when drum brakes fade, which tends to occur when subjected to high heat such as when descending a grade. As drum brakes fade, the disc brakes on the steer axle could be forced to take on too much of the load and could crack or fail as a result.
“The compatibility issue really comes in when you use disc brakes in combination with drum brakes,” Hakim warns. “When you use disc brakes, you have to be very careful that they don’t do all the work and that the drum brake also puts in its fair share of work. Otherwise, what’s going to happen is the disc brakes will get worn out and they’ll get a bad reputation in the market and people will say disc brakes don’t work, when in fact they’re working too much. The point is, you have to use the right type of friction material for the disc brake that is compatible with the properties of the drum brake.”
Typically, an air disc brake uses a metallic lining where friction is sustained, allowing the bra
ke to produce torque to temperatures of up to 1,500 F. Drum brakes, on the other hand, utilize a friction material bound together by high-temperature resins, allowing the brake to produce torque only to temperatures of about 600 F before fading occurs.
The disc brake’s resistance to fading is one of its strongest attributes, but this same characteristic can result in it taking on torque loads it wasn’t designed to withstand.
These concerns, if valid, are heightened in Canada with the prevalence of mountainous terrain and the industry’s widespread use of multi-trailer combinations.
The issue has been recognized by the Technology & Maintenance Council, which in response developed Recommended Practice (RP) 628, establishing guidelines for aftermarket friction material for disc brakes. TMD is thus far the only supplier to develop a friction material that meets the RP 628 specification, with its Textar T3070 product.
“We have looked at it using a systems approach, as opposed to a one-brake approach,” Hakim explains. “We took into consideration the drum lining, the materials used in drum brakes and we formulated a disc brake material that works with the drum brake. When the drum brake starts losing effectiveness, we don’t want the disc brakes to continue working very powerfully. This system is such that they both work together.”
In essence, TMD has developed an air disc brake friction material designed to fade in synch with the drum brakes, which seems counterintuitive. Not everyone is a believer in the concept. Count Bendix’s Bell among the skeptics.
“I don’t understand why one would want to pay additional money for disc brakes and then want it to fade to perform like a drum brake,” he says. “It’s unfathomable to me.”
“I completely agree with Steve,” Ganaway adds. “I have always been less than enthusiastic about RP 628. Bendix’s recommendation has been for fleets to reline with the material they began with, that’s always the safest bet.”
Ganaway says fleet managers and technicians should resist the urge to overthink the science that goes into developing friction materials.
“(RP 628) really is asking for the fleets to take on the role of brake system engineers and that always comes with some peril,” Ganaway claims.
Still, Bendix officials admit there are some applications where more than the bare minimum number of positions will need to be fitted with disc brakes. For instance, some heavy-haul applications with multi-axle trailers, Super-Bs and long combination vehicles will require disc brakes on more than just the steer axle. With this in mind, fleet managers would be well advised to discuss their application with their dealer to ensure they won’t be placing too much strain on a set of disc brakes.
“In our approval process, we have it set up such that…it’s gross combination weight-dependent,” Bell explains. “We do take that into account and limit the approval based on the number of disc braked axles in a (configuration).”
The TMD folks, on the other hand, insist that ignoring the risk of incompatibility issues between disc and drum brakes could lead to another bad experience with air disc brakes, and threaten their widespread adoption.
“TMD knows that the existence of a large numbers of air disc brakes will likely lead to compatibility issues between disc and drum brakes,” Jim Clark, director of engineering with TMD said during a recent presentation on the subject. “Very simply, incompatibility issues seen in Europe when disc brakes were introduced in large numbers will be repeated and likely will be worse here in North America.”
While warnings of compatibility issues may give some fleets cold feet, drivers who’ve driven trucks with both types of braking systems will have their preferences, and many will decidedly favour discs. Just ask Lee: “If I could make one transplant (from the European experience), it would be the braking system,” he says. “Those drums would be junked in favour of a set of discs all around.”
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