Side-by-side on the test track, there’s no comparison. Disc brakes provide shorter stopping distances, better brake balance, easier routine maintenance-the sort of benefits that can have a glowing effect on the balance sheets at truck fleets and on the psyche of their drivers. But in North America, in the battle that matters most, disc brakes come up short. The S-cam drum brake is so well accepted and so inexpensive to spec that it seems nothing less than a sea-change of Biblical proportions will elevate disc brakes beyond its current status as a novelty on heavy trucks and trailers.

Well, there are signs that the tide is changing.

Faster-acting electronic controls on disc brakes promise to give drivers an even greater margin of stopping distance. More importantly, new technologies are opening up opportunities for electronic stability systems similar to those found only on luxury cars today. Demonstrations of air disc brakes and other advances that stem from electronically controlled braking systems are so convincing that we may see rates of acceptance here paralleling those in Europe.

The highlight of last fall’s meeting of the Technology and Maintenance Council was a day-long test-track demonstration of the latest braking and electronic stability-control technologies for heavy trucks. Staged at the Transportation Research Center in East Liberty, Ohio, the event showed trucking industry technical leaders the effectiveness of disc brakes and electronic braking control. It also introduced the concept of selective braking at different wheel positions for enhanced stability control, convincingly demonstrating enhanced long-combination safety and reduced rollover risk.

Ironically, the very regulations designed to promote truck safety in the United States and Canada have served to stifle advancements in braking technology. Specifically, this is the case with electronic braking systems, or EBS. With EBS, electronics replace the air lines, increasing the speed of response since the signal travels by wires-hence the term, “brake-by-wire.” These same electronics can process information about each individual wheel, so it can apply and balance the brakes precisely where and when needed. The electronics also allow for brake system diagnostics and overall integration of the braking into other vehicle systems.

However, to satisfy North American braking regulators, EBS still must function on top of a dual air system. This makes the brakes redundant three ways, and adds weight, complexity, and cost. Much could be saved if regulations were changed to permit a simpler system.

Duane Perrin of the U.S. National Highway Traffic Safety Administration points out that the safety agency understands the potential benefits of electronic controls for air brake systems, especially faster system reaction time and improved stopping performance. And overall, Perrin supports moves by manufacturers and truck users to adopt electronic braking controls. He’d like to close up the braking-performance gap between trucks and cars, while allowing for additional safety features, such as rollover protection.

However, he says the agency has concerns over compatibility issues with existing truck and trailer equipment and failsafe modes for the brake-by-wire systems. One issue has to do with the potential failure modes of the controller chip, Perrin explains. In a simple mechanical or pneumatic system, failure modes are predictable. With EBS, you can test for the system response with a loss of power, for instance, when conventional pneumatic systems back up the brake-by-wire system. But there is an almost infinite number of failure modes where the controller simply sends out the wrong instruction.

Perrin adds that there is no timetable to change to the U.S. FMVSS-121, the regulation that requires two pneumatic backups to the electronic system.

For the fleet to see value, EBS needs to do more than just apply the brakes, says consultant Dick Radlinski. “EBS is a platform,” he says. “With electronic controls as a basic [platform], we can add stability control and other features such as smart cruise or collision avoidance, diagnostics, and maintenance. When features are added [with their paybacks], there will be incentive to use the technology.”

To gain the full benefits of electronic braking, additional sensors must be provided on the vehicle, detecting brake temperatures, wear, and the vehicle’s lateral and rotational accelerations. This could lead to safety-related features like enhanced stability control, which promises to reduce accidents from rollovers and other conditions that drivers are not trained to handle.

If that pans out, Radlinski predicts that within 10 years most vehicles will have electronic braking.

Few of the 400 or so TMC members who climbed aboard the buses for the trip to the TRC track had seen EBS at work before. There, 14 different tractors and 13 trailers were driven by brake engineers from the major suppliers in a series of 40 different runs to show the value of enhanced braking and stability that comes from the adoption of two technologies: air disc brakes and electronic controls.

Some of these demonstration runs were out on the open handling area, others were on a low-friction skid-pad where split friction could be demonstrated.

The braking systems represented covered the map, from conventional ABS air-brake, S-cam braking systems to fully enabled, EBS-disc brake systems with stability controls. Trailers included tandem semis-one a flatbed with a load rack that lifted concrete blocks to a high centre of gravity to accentuate rollover tendencies. Fortunately, this outfit was equipped with outriggers to halt an actual rollover. Completing the lineup was a set of triples, also stabilized with outriggers.

An early test clearly showed the enhanced stopping potential of the new braking technologies. A bobtail Volvo VN conventional with air discs and EBS stopped extremely well alongside a Honda sedan from 75 mph. However, with its EBS disabled, the truck took an additional 30 feet more than the car to stop. Later, with all systems enabled, the bobtail VN stopped a full car length shorter than the Honda.

Other demonstrations showed the different capabilities and compatibilities of drums and discs on tractors and trailers, some with and some without ABS and EBS. All in all, these tests were mundane, with ABS functioning well in dry surface as well as split-friction-coefficient circumstances.

The drama came when the high-centre-of-gravity trailer with EBS was coupled to an EBS-equipped and disc-braked Century Class tractor with a prototype electronic stability control system. In a fast lane-change manoeuvre with stability controls disabled, the trailer and load lifted not only its own inside wheels, but it took the tractor as well. Lifting the drive wheels, the combination had, for a considerable time, only 10 of its 18 wheels on the ground-the outrigger saved the combination from a full rollover. On the next run, at the same speed, the stability control selectively braked the wheels on the combination, and this time the lane-change was uneventful.

Throughout the demonstration, each level of technology brought an improvement in performance and stability. Discs stopped better than drums; discs with EBS were better than discs with ABS; and trucks with EBS could handle dynamics like sudden lane changes better than a driver ever could unaided.

Of all the displays, the most remarkable displays involved stability controls. Working in concert with the electronic brakes, the system uses computing power and sensors around the vehicle to alter the dynamics of a heavy vehicle in a turn. In essence, the stability control system selectively brakes the appropriate wheels to stabilize a vehicle that’s overstepped its limits.

The technology is already being outfitted on luxury passenger cars. Accelerometers measure linear and rotational acceleration and feed them to a computer programmed to correct an unstable condition by actuating the hydraulic brakes. The result is a significant margin of safety at high speeds in slippery and wet conditions.

Applying similar technology to big trucks is more difficult. The dynamics of an articulated vehicle are complex, and the hysteresis in an air brake system application renders the control ineffective. Add the speed of electronic braking and the fast release of disc brakes and many of those issues go away. Already, developmental systems from Bendix, Wabco, and Bosch have demonstrated the ability to keep a truck on a defined curve in slippery conditions when control is impossible for even an experienced truck driver. Furthermore, based on our limited experience with this system, the control is not obtrusive. In many-or even most-circumstances a driver won’t know that the truck has responded to the computer.

The effect of EBS combined with stability controls was so impressive, you have to see it to believe it. Indeed, the challenge suppliers face is to build market demand and promote regulatory acceptance in order to move these technologies off the track and onto the open road where they can do the most good.

SIDEBAR

UPHILL CLIMB The pros and cons of spec’ing discs

To be sure, discs brakes have performance and design advantages over your trusty S-cams. The combination of rotors (the discs) squeezed by piston/caliper-driven pads can provide shorter stopping distances than the shoe-and-drum type of brake. Discs resist “fade” – the loss of power you experience when heat causes drums to expand away from shoes. Discs don’t fade away from pads, and they keep working even when they get very hot. Discs also self-adjust and have fewer parts.

That’s why disc brakes dominate the automotive and light-truck markets, at least on the front axle. If you run newer medium-duty trucks, you probably have discs on them, too.

HYDRAULIC DISCS: Hydraulic disc brakes are now standard fare on many mid-range truck models, either as a front-disc/rear-drum combination or with discs at all four wheel positions. Because there are fewer choices among componentry in mid-range trucks, you probably can’t delete disc brakes in favor of hydraulic drums. And you’d be foolish to try, because discs do work better than drums in the lighter classes of trucks, right up to the threshold for class 8.

Hydraulic discs are certified on most chassis up to 33,000 gross vehicle weight. However, as you get into the heavy end of medium duty-at about 30,000 pounds – you may be among the buyers who tend to spec air brakes, and they’ll be S-cam-actuated drums, not discs. Air S-cam drum brakes are found on virtually all heavy class-7 and -8 trucks. Most operators think “air brakes” are synonymous with “heavy truck” and feel better about having them. They are not wrong.

Air drum brakes have much more lining area than the pads on discs, and in some cases those linings can better absorb the tremendous amounts of heat energy needed to stop a heavy truck. Also, parts for air drums are widely available on the aftermarket, which drives down prices. Discs stop the truck just as well, but their comparatively small lining area can cause their pads to wear out more quickly, and this raises maintenance costs (though suppliers say this is changing).

Then there’s the up-front cost of discs. Spec discs at all wheel positions and you’ll pay thousands of dollars more for them. They require compatible axle ends to mount, and the rarity of both disc brakes and appropriate axles compell truck builders to charge more to install them.

COMBINATION ISSUES: Discs work best if both tractor and trailer are disc-equipped, which they usually are in fleets that use them. But if all your tractors and trailers now have drums, you’ll introduce complications if you start mixing brake types among the vehicles. Special attention needs to be given to balancing the performance and wear characteristics of both brake types so they’ll work well together.

Sometimes discs work too well for their own good, say fleet managers who’ve used them. When mixed with drum brakes (as on a tractor with discs and a trailer with drums), the discs will do more braking work than the drums. This is partly because the disc’s built-in adjustment mechanism keeps the pads “right up there” while the drum brakes have gone out of adjustment (even with automatic slack adjusters), and partly because of the disc’s superior performance.

The disc pad and rotor will not “fade” from heat like the shoe and drum, and will continue to exert force even after the pad lining has worn out and metal-to-metal contact ensues. It will then quite literally work itself to death.

MOVING FORWARD? Of course, these pros and cons are intended for the here and now. In the future, combined with EBS, disc brakes appear to present too many advantages for the market to simply ignore.

But while the changeover may seem certain, it won’t be swift, given the prevalence of drums, the comfort level with S-cams, and the cost and compatibility issues surrounding discs and drums.