Braking systems on today’s heavy trucks are better than they’ve ever been, and maybe ever will be. Advances in drum brake designs and the increased penetration of air disc brakes into the North American market, were two of the...
Braking systems on today’s heavy trucks are better than they’ve ever been, and maybe ever will be. Advances in drum brake designs and the increased penetration of air disc brakes into the North American market, were two of the results stemming a US government mandate that required heavy trucks to stop in a 30% shorter distance.
Phase 1 of those rules, affecting most mainstream highway tractors, went into effect Aug. 1, 2011 and the remainder of trucks – including oddball configurations like tri-drives and twin steers – were brought up to speed Aug. 1 of this year.
While there was initially speculation that air disc brakes would be widely required to meet the new requirements, brake suppliers brought to market improved drum brakes that were up to the task, giving fleets a choice in how to meet the mandate. While the 30% reduction in stopping distance was a tall order for brake manufacturers, they exceeded the requirement, building in an extra 10% cushion beyond what the rules called for.
Unlike with diesel engine emissions standards, which have provided the industry a death by 1,000 cuts so to speak, with round after round of increasingly stringent emissions reductions, it’s not expected that further stopping distance requirements will be called for by government, even though the current rules will purportedly save 227 lives per year.
“We meet with NHTSA and the DoT fairly regularly, and all indications are that for the time being, their emphasis is not necessarily on increasing the performance of the technology, but really honing in on monitoring, to ensure the safety systems currently on vehicles are closely monitored and better maintained,” said Gary Ganaway, director of marketing and global customer solutions with Bendix Spicer Foundation Brake.
Besides, adds Chad Mitts, general manager, brakes and drivelines with Meritor, today’s braking systems are about as effective as they can be, without major modifications to the vehicles themselves.
“We’re at that point where the laws of physics with current truck designs won’t allow you to stop any sooner,” Mitts said. “There’s more torque there than the truck can manage. There’s nothing left to do from a brake perspective that wouldn’t require changes in tires, changes in control systems and things like that, which would be a pretty massive undertaking.”
While brake manufacturers were able to meet the new standard with no major issues, Ganaway said there has been some increased noise associated with the new-generation reduced stopping distance (RSD) drum brakes, which Bendix alerted customers to.
“We had a bit of a challenge and continue to see (issues with) brake noise,” Ganaway said. Drivers may have noticed some additional squeakiness emanating from their RSD brakes, but Ganaway said they can be a little more aggressive with the brakes, get them hotter to burn off the resins that are the source of the noise.
“The last data I looked at, we saw noise incidences on about half a percentage of our production,” he said. “Those are relatively low numbers, but we did see the need to council our fleets and to tell them to council their drivers.”
The disc vs drum debate
Besides stopping trucks sooner, the new mandate has also significantly narrowed the gap in performance between disc and drum brakes. Larger drum brakes with more effective friction materials have been introduced to the market. Those RSD drum brakes deliver stopping capabilities that can rival that of disc brakes and comfortably comply with the requirements.
“Certainly the performance gap (between disc and drum brakes) has narrowed tremendously,” Mitts acknowledged. “Disc brakes will provide a slight advantage during a full power stop, so when you’re running at a high speed and you hit the brakes, disc brakes will be a little better and the main reason is there is no in-stop fade. But the difference is very small and the difference is typically felt at the end of the stop. Drum brakes tend to deliver torque earlier on, so if there is a difference, it’s at the end of the stop when there’s not that much energy left in any event.”
Mitts said disc brakes really excel in situations where the brakes are running hot consistently, such as in refuse applications.
“That’s why you see early adoption in some of those markets,” he said.
However, Ganaway said while high-performance drum brakes are now better able to replicate the stopping power of disc brakes, there are other benefits to consider, including maintenance savings. A set of pads can be swapped out in a quarter of the time it takes to replace shoes on a drum brake, Ganaway pointed out. But the big seller, he noted, was the way the brakes feel to the driver when in use.
“What they came back with,” he said of fleets that have been implementing disc brakes into their fleets, “is the driver satisfaction in the day-to-day application. It’s one of those technologies – unlike collision mitigation and some other things – that you are going to experience every time you drive the vehicle. What fleets found was, the things we told them to expect (maintenance savings and cost of ownership reductions), they saw those things, but more importantly, it was the day-to-day experience with the brakes they had a hard time parting with.”
He said most fleets that try disc brakes, “absolutely” stick with them on future truck purchases.
Bendix has seen a 120% increase in demand for air disc brakes since the new stopping distance requirements were implemented. Though Mitts tosses some cold water on any idea that fleets are gravitating en-masse to disc brakes, noting a 100% increase or more of not very much, is still not very much.
“In percentage terms, it sounds dramatic,” he said. “There has been growth for sure, but it’s growing at a small pace; 90%-plus of Class 8 trucks leaving the factory have drum brakes on them.”
For those fleets that have tested the waters with air disc brakes, many have opted to try them initially on only the steer axle position. This has raised concerns in industry about the compatibility of disc and drum brakes, when mixed-and-matched on tractor-trailer combinations.
“Compatibility can be engineered in,” Mitts said. “There is a fundamental difference between disc and drum brakes in how they deliver their torque. There’s an early period in that drum brake (application) where it delivers a lot of torque quickly, then starts to level out, where a disc brake is pretty linear. So in the early part of a torque application, you get differences in how they behave.”
These differences in braking characteristics are most pronounced during light brake applications of 10-15%, which is the most frequent type of braking an on-highway truck will require. This can result in the drum brakes on certain wheel positions taking on more of the work, leading to premature wear.
“There are ways to get around that,” Mitts said. “Setting up an air system properly and lot of balancing-type things. But that’s a struggle to do, because a lot of different trucks can hook up to a trailer. It’s a pretty big challenge.”
But while Mitts admits compatibility issues could result in some inconsistent wear, he said it doesn’t jeopardize safety.
“Ultimately, if you had an emergency and you were laying on the brakes, they’re both going to come on and give you everything they have,” he said. “So it’s not so much an issue in a panic stop or anything like that. Safety is not compromised; it’s more the day-to-day use. Things have to be balanced properly so you don’t get any unintended wear, where you’re wearing the drums faster than the discs or vice-versa.”
Ganaway said compatibility between disc and drum brakes is less of an issue today, than it was with previous designs.
“The technology in disc brakes has come a long way,” he said. He admitted disc brakes introduced to the North American market in the mid-1980s suffered from “relatively high degrees of hysteresis,” which caused the disc brakes to experience a lag, or to hang up, when air pressure was released.
“That negated the attempts of the air system to properly balance the vehicle,” Ganaway explained. “They were much slower than drum brakes, so while the system was asking them not to be on, they were on for a little bit longer. Today’s designs have lower hysteresis. They’re far more accurate from that standpoint.”
In the past, Bendix wouldn’t allow disc brakes on the steer axle to be mated with drums in other positions.
“We do that relatively frequently now,” Ganaway said. “It’s really not a concern.”
He noted 40% of Bendix’s disc brake customers today run discs on the steer axle alone, “which is remarkable.”
The need for OE
Both Bendix and Meritor have expressed serious concerns about what happens when RSD brakes are relined with non-OE friction material. Technically, fleets and owner/operators aren’t breaking any rules by using a non-RSD aftermarket friction material during re-linings. But Bendix has done testing which shows that stopping distance can be seriously compromised when relining with non-OE friction materials.
Ganaway said testing has shown stopping distance can be extended by as much as five car-lengths when using a non-OE friction material.
“You have these new trucks and when they need to be serviced, you do a brake job and if you don’t put OE linings on there, then you’re definitely not going to maintain their stopping performance,” Mitts pointed out. “There are a lot of lining materials available in the aftermarket that aren’t regulated. You can put those on and not be non-compliant, because the regulations only dictate what’s used on new trucks. I think that’s probably one of the bigger issues right now that the industry is working through. All this work went into developing these advanced friction materials and improving the safety of the trucks, then the rules go out the window the first time you do a brake job.”
Bendix also has been extremely vocal in raising this concern within the industry.
“When you think about the intent of the rule, the intent of the rule was to save lives,” Ganaway said. “Not just on those vehicles that are relatively new, but to follow the vehicle through its useful life.”
The regulations seem to fall short of that objective. Even so, Ganaway said he’s been encouraged by discussions with fleets, in which they’ve voiced a strong desire to remain compliant and do the right thing.
“As we talk to our fleets, they are alarmed that there is no requirement for serviced parts,” Ganaway said. “I give them an awful lot of credit. The fleets are much more safety-conscious than we give them credit for. What they have asked us for, is help and direction.”
Some fleets have requested that suppliers post a list of approved RSD friction materials. Others have gone so far as to suggest that suppliers design RSD brakes that won’t accept non-OE friction material. So, why don’t they do that?
“It’s one of the things we’re looking at,” Ganaway said. “One of the things we try to manage is to make sure we control the costs, and one of the things we’re concerned about is, if we begin to utilize unique parts, it would potentially drive up servicing costs of those components. So, for the time being, we’ve taken the path of being very vocal and very proactive in explaining the trade-offs of relining with something that is not like-for-like.”
Meanwhile, Ganaway said Bendix is also developing alternative friction materials that can keep the vehicle in compliance at a lower cost.
Yes, braking systems today are better than they’ve ever been. But the trade-off for that, noted Ganaway, is that they’re not as versatile as they once were. Think of it this way: If you buy a luxury sedan, you will have to fill it with premium gas and use high-quality replacement parts if you want to ensure the same level of performance and reliability over the life of the vehicle. The same can be said for today’s high-performance brakes.
“As the technology with brakes has evolved, they’re becoming a lot more sensitive to the inputs; things like bushings, lubrication and the right friction material,” Ganaway explained. “So, a cautionary note is, because we’ve really tightened up the tolerances and performance of the brake and what it does, we’ve made it much more sophisticated and we’ve lost a bit of freedom in terms of being able to throw whatever we want at it in terms of serviced parts.”
What drives brake experts nuts?
By James Menzies
Operation Air Brake took place in early September, a North America-wide enforcement initiative that targets braking systems. If the results are similar to previous years, then more than 10% of trucks inspected in Canada will have been placed out of service for brake-related defects.
Truck News asked brake experts Gary Ganaway of Bendix and Chad Mitts of Meritor, what they see in the field that drives them nuts. Both were quick to cite the common, but unnecessary, meddling with automatic slack adjusters as their main beef.
“I think the number one issue is education around automatic slack adjuster maintenance,” Mitts said. “We see this still to this day. People think every time that a truck has to be touched, that they have to adjust the slack adjuster. That is absolutely the worst thing you can do.”
“One of the things we are really surprised by, is the belief in a lot of circles that brakes need to be adjusted every trip, or every day or at every PM,” he said. “That is absolutely not the case. As a matter of fact, if a fleet or driver finds they need to adjust the brake, it’s probably an indication that their automatic slack adjuster is worn out, and that happens fairly regularly. Slack adjusters have a finite life expectancy – they do not last the life of the vehicle.”
Ganaway said slack adjusters typically last only four to five years before they wear out, depending on application.
Mitts added another frustration is when fleets adopt air disc brakes, thinking they’re a maintenance-free component.
“They often have this mindset that, ‘If I put disc brakes on there, I never have to look at this thing again,’ that it’s maintenance-free,” he said. “Generally, it performs pretty well, but it’s on the wheel-end of a truck, it’s in a horribly abusive environment. We’d never recommend that you don’t continually inspect disc brakes for damage to the air chamber and things of that nature.”
This phenomenon reminds Mitts of when unitized wheel-ends were introduced to the North American market. Many early adopters felt they were a maintenance-free item and that mistaken perception led to some misplaced dissatisfaction with the product.
“When I hear people put disc brakes on a trailer so they don’t have to worry about them, that is not the case and ultimately it could cause problems with adoption of the technology if people become disappointed,” Mitts said.