The Bendix ADB22X air disc brakes are being offered by many truck manufacturers as a solution for increased brake performance and reduced maintenance costs.
The reduced stopping distance regulations apply to tractors with GVWRs up to 59,600 lbs., with the exception of heavier specialty tractors and lighter two-axle trucks, which have to meet the standard on Aug. 1, 2013.
On August 1, 2011, a new air brake standard took effect for three-axle tractors with Gross Vehicle Weight Ratings (GVWRs) up to 59,600 lbs. This rule applies to tractors invoiced after that date. It will not impact any tractors you had in your fleet before August 1. But the regulation covers the vast majority of new tractors in use today.
The rule calls for a 30% reduction in the stopping distance from 60 mph vs. the previous standards. This means tractors must stop in less than 250 ft. from 60 mph when loaded to their GVWR and 235 ft. when lightly loaded. On August 1, 2013, heavier specialty tractors and lighter two-axle tractors will have to meet the standard. For a small number of very heavy severe-service tractors, the requirement will be 310 ft. under these same conditions.
Implications for straight trucks
This regulation was no surprise. It was finalized by the National Highway Traffic Safety Administration (NHTSA) in March 2007. OEMs have been working with suppliers to develop solutions, which include wider and more capable reduced stopping distance (RSD) drum and air disc brakes.
The regulation is intended to increase safety on public highways by reducing the disparity in braking between automobiles and tractor/trailer combinations. “Due to the nature of an on-highway tractor (80,000-lb. GCW), this regulation makes sense,” says Bernie LaBastide, Navistar chief engineer, brake systems. “It’s not clear at this point that the legislation would reduce collisions or fatalities for straight trucks. Straight truck environments are primarily city routes using slower speeds, etc.”
While straight trucks are exempt from the rule, there will be an impact as manufacturers take advantage of the new braking technologies. For example, Peterbilt announced it will make front-axle air disc brakes standard across its entire Class 8 truck and tractor line. For customers who prefer RSD drum brakes, there is that option, as well.
So expect to see some of the higher performance brakes start showing up on vocational straight trucks. “It only makes sense to use economies of scale by using the new and improved materials where applicable,” says Curtis Dorwart, vocational marketing product manager, Mack Trucks.
“My motto is ‘what is good for a tractor is certainly good for a truck,’” says LaBastide. “We currently offer not only bigger drum brakes, but disc brake options for straight trucks, as well. The benefits of larger drum brakes are more lining material and some reduced operating temperature due to the larger drum lining interface. Most linings react positively (reduced lining wear rate) with the reduced temperatures.”
He adds, “While not as popular as steer axle-only tractors, we do offer and sell all-wheel disc brakes on some straight truck models. Straight truck rear suspensions pose a distinct challenge to allow decent packaging for the air disc brakes. Thus, there is limited availability of air disc brakes on our straight trucks.”
Straight trucks can, however, continue using the standard brake configuration. “The previous brake lining material is still available for vehicles not affected by the new regulations,” says J. Allan Haggai, Daimler Trucks North America.
Bigger and better drums
The switch to RSD drum brakes or air disc brakes is relatively transparent.
“It was our desire to design brakes that were more capable and that would not send any specific signals to the drivers,” says Tom Runels, engineering manager for foundation drum brakes at Bendix Spicer Foundation Brake. “They can drive away and get no sense that there is anything significantly different underneath their vehicle. These brakes are much more capable, and in an emergency, they will be able to use that capability.”
The regulations required more than merely making the brakes larger. “To address the new braking standard, Mack made component changes to its standard drum brakes so they are more robust and generate more brake torque,” says Dorwart. “We’ve also introduced new friction materials on steer axles and some drive axles.”
However, larger drum brakes will be used by most manufacturers. “On a typical Class 8 6x4 tractor, we have met the new standard by upgrading from a type 20 to a type 24 brake chamber, linings and drums with greater width and diameter and larger brake mounting fasteners,” says Haggai. “The brake lining formulas have also been changed.” These larger drum brakes will add an estimated 90 lbs. to a 6x4 tractor.
Navistar is also offering drum brakes as standard equipment. “Currently, air disc brakes are optional on International branded highway tractors,” says LaBastide. “Our standard brakes are air S-cam-type brakes and we offer Bendix air disc brakes as an option. To comply with the new stopping distance regulation, we now offer a larger S-cam brake.”
Navistar selected larger front drum brakes (16.5” x 5”) with a different lining material specifically designed for the new regulation, in conjunction with larger service brake chambers (Type 24). “New lining materials were also applied to the rear to balance out the front brakes,” says LaBastide. “As another option, the customer can order larger rear brakes (16.5” x 8.62”), as well, over the standard 16.5” x 7” rear brakes. The larger front and rear brakes will increase miles between reline by increasing the usable lining volume and reducing the brake temperatures by increasing the braking area surface.”
Despite having been around for the last century, drum brakes have really evolved from a technical and performance standpoint. “They are significantly different than they were 10 to 15 years ago,” says Gary Ganaway, director of drum brakes, marketing and global development, Bendix Spicer Foundation Brake. “They are much more capable. Be that as it may, there are still some fundamental differences that favor air disc brakes. The way we position them in our product line is the drum brake is very economical. It is a basic product that allows the fleet and the OEM to meet all of the requirements. The disc brake adds an added level of performance and feel.”
There will be a weight penalty for increasing the brake size to meet the legislation. “The larger rear drum brakes will net you an additional 60 lbs. on a tandem rear tractor,” says LaBastide. “This is moving from the 7-in. brake to the 8.62-in. wider brake with the same diameter.”
But the weight penalty is relatively minor compared to the cost penalty for air disc brakes. “Traditionally, air disc brakes have been priced as a premium to the S-cam brake,” says LaBastide.
Advantages of disc brakes
Despite the initial cost difference, some customers value the benefits offered by air disc brakes.
The challenge comes from load transfer during braking. As the tractor decelerates, the load is transferred toward the front axle. As a result, the front steer axle brakes became a primary focus. This is where the increased performance of an air disc brake can have the greatest impact. “You get your biggest bang for the buck putting it on the front axle,” says Ganaway.
“We see a growing appetite for air disc brakes and expect customers to continue migrating toward that spec in the future,” says Dorwart. “Air brakes can provide an increase in service life over drum brakes in the same application, which can make for a positive return to offset the initial price increase. But be mindful that the duty cycle, vehicle weight, etc., will have a bearing on the life-cycle performance. With the wide array of applications, disc brakes are better suited for some operations than others.”
At Waste Expo in May 2011, Mack launched its initial offering of air disc brakes engineered specifically for vocational trucks. “The balance between friction material and rotor material has been optimized for the stop-and-go needs of [vocational applications], providing improved stopping distance and handling,” says Dorwart. “The air disc brakes are easier to service and the friction materials have been designed to last twice as long as the material in S-cam drum brakes.”
Maintenance is one of the key benefits of air disc brakes. “From a maintenance standpoint, the time it takes to change air disc brake pads once the wheels are off is only about one-fourth the time it takes for a drum brake,” says Ganaway. “The other advantage is the disc brake is sealed for life. It doesn’t require any periodic lubrication. With most drum brakes, you are still required to do that.”
LaBastide adds, “Air disc brakes have a substantially higher initial cost, but are easier to maintain, assuming you are only doing maintenance like pad changes. Most air disc brake calipers are sealed and non-serviceable, so if the caliper internals fail, you need to replace the complete caliper assembly. So far, the latest generation of air disc brakes has performed very well.”
The increased cost of air disc brakes must be factored into the life-cycle cost. “When you factor in the added acquisition cost and you total that against increased service times and overall maintenance costs, the longer the owner holds onto the vehicle, the more favorable the disc brake solution becomes,” says Ganaway.
The weight difference between air disc and drum brakes is almost insignificant. “This can vary depending upon the equipment, but they are a few pounds lighter to weight-neutral compared to drum brakes,” says Dorwart.
According to Haggai, the air disc brake system performs well, but may not be the best option over a properly spec’d drum setup for vocational customers. “In most applications, the vocational user may not realize a measurable return on the additional cost of the disc system,” he asserts.
The air brake system is versatile, but you need to consider your application. “Air disc brakes can virtually be used in any application, including the construction vocation,” says LaBastide. “One air disc brake manufacturer utilizes one brake for all applications and different rotors. Another manufacturer utilizes different brake sizes and lining material and customizes those attributes to each application.”
Some precautions need to be taken on muddy jobsites or when working around wet concrete. “You have to use caution when using air disc brakes in these environments by hosing down the wheel ends,” says Ganaway. “The air disc brake is a robust product, but it is not a maintenance-free product. Those types of application environments just enhance the need to ensure some components don’t have dirt ingress or concrete ingress.”
Benefits outweigh costs
The RSD brakes are going to add a little cost and weight up front, but increased braking performance and service life may offset any potential drawbacks.
“In an industry that watches cost and weight very closely, this regulation may have some impact,” Haggai says. “However, increased braking performance and lining life may provide dividends in the long run.”
The first phase of the regulations should have minimal effect on the industry. “I don’t expect much impact on the purchase plans of fleets,” says LaBastide. “The cost penalties have been minimal except with the air disc brakes, which again are optional for Phase I of the regulations. Phase II is a different story though. I expect to see more air disc brakes and even six-channel ABS being required on those units in the Phase II categories.”