Changes to the trucking industry's braking systems can happen in a relatively short period of time. A little more than a decade ago, everybody was still wondering whether a trailer's anti-lock brakes ...
BETTER STOPPING: Changes in stopping distance requirements for heavy trucks could mean bigger drum brakes, the switch to disc brakes, or a combination of solutions. Pictured is MeritorWabco's dual-piston disc brake (left) and single-piston option (right). The single-piston offering reduces weight and cost.
Changes to the trucking industry’s braking systems can happen in a relatively short period of time. A little more than a decade ago, everybody was still wondering whether a trailer’s anti-lock brakes would require a second power cord between tractors and trailers. Some equipment configurations have even emerged with air disc brakes and stability control systems.
There are more changes on the horizon.
Despite delays in its final ruling, the US National Highway Traffic Safety Administration is on the verge of mandating a large cut in allowable stopping distances. Under current rules, an empty tractor-trailer that travels at 60 mph must be able to stop within 355 feet (108 metres). Regulators are pondering whether that distance should be slashed by 20, 25 or 30%.
Most industry groups have recommended a 25% cut, which could be met with changes that include larger brake linings and drums, and air disc brakes on selected vehicles, says Dan Philpott of MeritorWabco.
The shorter stopping distances will still present some challenges, however.
“Tire, suspension and chassis structures will be affected by putting more brake torque across the wheel,” Philpott explains. If the brakes on a steer axle become more aggressive, there could be an impact on a steering wheel’s response during a braking event. Parking brake performance could also be affected because of the reduced burnishing of the brakes on other axles.
Luckily, these issues can be addressed with today’s technology. Anti-lock brakes could address the potential of lock-ups caused by higher brake torque, and stability control systems could address other response-related issues.
Regardless of the ruling, the trucking industry will need to comply with these changes within two years.
That will hardly be the end to the new regulations that involve brake performance. Roadside inspectors, for example, can now issue out-of-service violations because of the results from a Performance-Based Brake Tester, notes Accuride technical manager Steve Howse. Rather than simply measuring the travel of a push rod, this equipment will be able to determine if the brakes will actually stop a tractor-trailer within an allowable distance.
A bigger issue may be the recent addition of anti-lock brakes into the Level 1 out-of-service criteria.
“One of the reasons is, the key to all new electronic (stability) technology is the ABS system,” he explains, referring to the expanded standards.
The increasing focus on the condition of ABS components could be a challenge since a lot of the equipment is not working as it should be. A study of 1,000 vehicles found a problem with the ABS warning lights on one in six tractors and one in three trailers. That alone has led the Commercial Vehicle Safety Alliance to ask the US government to require trailer-mounted warning lamps beyond next March.
The light, originally designed as a back-up for in-cab warning lamps, was expected to fade away by that date.
Despite everything that has been put in place, there are bigger changes to come. Indeed, Electronically Controlled Braking Systems (ECBS) and air disc brakes are already emerging as the technology of the future.
Each option has a lot to offer.
The ECBS equipment, for example, offers a number of advantages over a traditional pneumatic system.
“It’s actually a little simpler,” explains Vince Lindley of Volvo Trucks North America. A typical ECBS system includes six pneumatic connections compared to the 12 connections that are required on an ABS-equipped tractor-trailer with Automatic Traction Control. It also has four electrical circuits instead of eight.
The advantages don’t end there. These systems could electronically track lining wear, integrate brakes into stability control systems, and include the load proportioning capabilities that will adjust brakes for a specific application.
“It does appear to provide a real good integrated solution for applications on hybrids,” he adds, referring to how the electronic braking could maximize the amount of power stored by such a system.
Cost, however, is still a challenge. “There are fewer parts under there, but the parts are more expensive,” he admits. “It’s going to be a lot more than a $40 relay valve.”
In defence of discs
Air disc brakes may also be more expensive than their S-cam counterparts, but they have proven their better stopping capabilities.
“They typically have much better fade performance, they’re more reliable through extreme brake applications, (and) they also have a better feel and have less propensity for pulling,” says Mark Melletat of MeritorWabco.
Today’s designs are far superior to the versions that came before them. Earlier disc designs faced problems including linings that wore out too quickly; pistons and caliper slide mechanisms that would bind because of vibration, distortion and corrosion; and rotor checking.
But engineers have found ways to improve the related parts. Changes to saddle mechanisms have addressed the torque on calipers and pins, and slide pins have been sealed to protect them from corrosion.
Weights and costs have also been reduced thanks to single-piston designs, which even weigh less than a high-performance drum. New calipers and carriers can be produced with 18 parts rather than the previous 39 components.
The most apparent changes will be seen whenever a mechanic changes the friction material on a disc brake. A common pad profile has replaced the need to use separate pads on the inboard and outboard sides of the brakes. And the pads can now be changed without moving the calipers.
“We’re seeing significant improvement in lining and rotor life,” Melletat adds. “The pad life has improved significantly.”
The need to extend the life of friction material of every sort is even leading engineers to tackle the common issue of rust jacking. That problem – linked to a corroding brake shoe table – can cause a brake lining to crack and pull apart.
Fleets can even contribute to the issue by power washing equipment, since that will drive de-icing solutions into every crack and crevice.
That will attract moisture to the brake shoe table, lead to the unwanted oxidation, and eventually shorten the life of the lining.
“We are not in a position to eliminate rust jacking. There is no magic wand,” says John Hawker, senior engineer at Bendix Spicer. But there are potential solutions that could slow the process, he suggests.
Engineers are considering the possibility of friction material with a higher density, although that could be more expensive than traditional formulas. Other options have included new materials such as ceramics, different coatings, new adhesives and sealants.
“We’re not stopping,” Hawker says of the engineering work.
The shield’s sacrifice
There has been a long debate about whether or not to use dust shields. The equipment may keep dirt and grime out of the brakes, but it also makes it harder to inspect the condition of linings.
Mark Melletat of MeritorWabco suggests that the shields might have another purpose in these days of corrosive anti-icing solutions.
“It becomes your sacrificial anode,” he noted during a recent meeting of the Technology and Maintenance Council. “Your dust shield ends up catching the magnesium chloride, which absorbs the water. Then it starts rusting the dust shield. And dust shields are a lot less expensive than brake shoes.” •