Brake Rules Could Feature A Drum Role
Steve Sturgess
Senior Editor
Dana charts compare performance of braking system options from conventional 15-inch brakes up to all air discs. The average stopping distances show an advantage for the big drums, (chart 1) and, says Dana's Jim Clark, include figures from some under-optimized systems. A better idea of comparative performance is given by the minimum distances (chart 2), which demonstrate peak performance of each system The all-disc setup offers little improvement over the disc/drum and the 16 1/2 inch front drum gives stopping distances right in line with NHTSA/OEM requirements to meet proposed regulation plus a 10% margin.
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The National Highway Transportation Safety Administration is poised to recommend that heavy duty brakes get a lot better. Confirming that the agency will likely call for a 30% reduction in stopping distances, speakers at the mid-March meeting of the Technology and Maintenance Council of the American Trucking Assns. put together a series of analyses to help fleets understand what would be needed.
But even before they could get to their alternative solutions, Jim Britell, a research engineer for Crash Avoidance Research Vehicle Safety Research for NHTSA, said that in addition to the stopping distances, the rules would likely address the topic of mandated brake maintenance.
At this stage, he said he could offer no specifics about what that maintenance component might be, but anything that addresses in-service criteria would be welcomed by brake suppliers - especially suppliers to the aftermarket.
Ron Armer, president and CEO of friction material manufacturer Brake Pro, says that the safety agency needs to consider whether the real issue is stopping distances of new trucks or stopping distance for trucks in service. He says that his staff is very active in the field and they see a wide variation in brake maintenance practices. Some fleets, he says, are top notch. Others are terrible. He cited poor quality friction materials, inferior rebuilt replacement brake shoes, lack of attention to hardware and fasteners and corrosion caused by aggressive chemicals used on winter roads. And, he said, it doesn't matter how much brake is mandated, poorly maintained vehicles will never stop even to today's regulations, let alone any shorter distance.
Armer said he is also critical of the aftermarket linings that exhibit huge variability in performance as measured by the TMC's recently published Recommended Practice 628A. This rates aftermarket friction material in a relatively simple test based on standards set by FMVSS 121.
That was supported by a comment at the TMC meeting by Fred Hopper, technical sales manager with TMD Friction. He agreed that the upcoming 30% reduction in stopping distances can be achieved with conventional - albeit larger - drum brakes. But he was skeptical that much of the imported and inexpensive brake friction materials would conform to the braking distances likely to be mandated in the already contentious 2007 time frame.
Stopping Distances
Current regulation calls for a stopping distance of 355 feet from 60 mph for a tractor and unbraked trailer at a gross weight of 52,000 pounds. Under the anticipated proposal, this distance would come down to 248 feet under the same conditions. However, Jim Clark, Commercial Vehicle Systems engineering manager at Dana, and a veteran heavy-duty brake expert, says OEMs need a 10% margin below the minimum to be comfortable with variations in performance. That brings the target stopping distance down to 223-224 feet, a distance that is comparable with many passenger vehicles.
According to Clark and fellow panelist, Paul Johnson from ArvinMeritor, that requires an increase in brake torque - and the only place where that can be gained is on the front axle. Tractor drive axles, said Clark, already are at the torque limit, requiring ABS to deliver stopping power in a full application. The front axle, in contrast, conventionally has only a 15x4-inch drum brake because of its 12,000-pound static loading. But it actually sees up to 21,000 pounds in dynamic loading under heavy braking. As such, the front axle can easily load a 16 1/2 x 5-inch drum brake for a significant increase in brake torque to reach the target stopping distances. (see charts).
Ron Bailey is technical sales manager with Bendix Commercial Vehicle Systems. His company has an air disc brake engineered and waiting for conditions to pull it into the marketplace. He agrees that the front axle is the place for the additional braking effort. But he is backing a solution that combines the additional performance of the front-axle disc brake with the already maxed-out brake capacity of the drum-braked drive axles.
That conclusion was suggested at an earlier TMC braking panel where Marty Fletcher, director of technology and training for US Xpress, reported that disc brakes were exceeding expectations and providing very good stopping distances. At that meeting last year, the consensus was that the combination of air discs on the front and drums on the drives provided a great solution to the upcoming stopping distance proposal.
More recently, Paccar divisions Peterbilt and Kenworth both announced at the Mid-America trucking show in late March that they would be offering the Dana 22.5-inch air disc brake as an immediate option for front-axle braking.
The brake selected - while other ADB alternatives are being evaluated - is the Dana ESD225. The air disc brake option gives Peterbilt and Kenworth customers the ability to spec and experience the disc brake solution to anticipated shorter braking distances likely to be mandated by upcoming NHTSA rules.
Drums Not Dead
But don't rule out the good old drum brake from maintaining its popularity - even in the new stopping environment. Bigger, wider drums, and high performance linings can boost front brake torque to the point where the proposed braking distances are quite easily achievable.
One of the unknowns, though, is just what NHTSA will do about the escalating speed of trucks. Current regulations call for stopping from 60 mph. But 42 states have legal speed limits for trucks above 60 mph. Nine states have 75 mph legal speed limits for trucks. Not surprising, then, that the agency may set braking distances that take these into consideration.
This is likely to hasten the acceptance of a front-axle air disc solution, but it still may not be in the 2007 time frame. In his January column, Jim Winsor pointed out the unfortunate timing of the proposal: 2007 is about the same time truck prices will escalate to accommodate new engine emissions levels. If the industry can get by without having to redesign the whole front of the chassis and the wheel-ends to accommodate the packaging requirements of the air disc, there will be a sigh of relief from users and truck manufacturers alike.
Brake Pro's Armer says drum brake performance is well up to the front-axle task with the right choice of lining. Brake Pro specializes in tough applications such as bus, refuse, logging and vocational vehicles and has developed braking friction materials that can reliably handle higher braking torques, he says. They can also cope with elevated temperatures, which greatly extends the application of the drum brake.
According to Armer, new technology allows for better performance and more aggressive stopping without the accelerated drum wear that used to be the tradeoff. The technology he calls cohesive friction is not isolated to his company's products, though. For instance, you can find products from Carlisle Motion Control and Abex that feature this "thin film" technology.
The formulation of the friction materials includes chemicals that bleed out of the block under heavy braking. They coat the drum and the lining with a film only one or two microns thick. This actually raises the friction between the two like surfaces and, at the same time, also protects the drum surface. This reduces wear and noise, a real plus in severe duty applications like buses and garbage packers.
Maintenance Is Key
But while brake manufacturers all agree that new combinations of available brakes will be needed to meet the regulation, the rules and the reality will likely be far apart. The issue is the wide variation in brake maintenance. The disparity in brake maintenance programs and the variability in quality and consistency of offshore brake linings - especially those from South America, China and India - mean that trucks in service may well fail to meet the criteria of even today's regulations.
In their defense, fleet maintenance people have had a tough time of it, because it has taken many years to come up with even a standard friction rating system.
Such a system exists now. The Technology and Maintenance Council's RP628A uses an FMVSS121-like testing process. But while that gives a little guidance so that fleets can attempt to match replacement linings with original equipment, it is no absolute guarantee.
According to Armer, the brake products in the aftermarket are of variable quality, so that where a lining might match its rating one month, a short while later it could be way off.
The maintenance of brakes is made more difficult because there are foreign-made products that are best described as will-fit; they look like they'll do the job, they're rated to do the job, but the quality is so poor there's absolutely no performance guarantee, he says. These products account for 10-12% of the aftermarket volume, with the percentage rising. This places a real premium on the recognizable brands of the long-standing and familiar competitors in the aftermarket.
NHTSA is getting into this lining rating issue. Oak Ridge National Laboratories has had a contract to attempt a rating and marking system that will tie together OEM and aftermarket, making its final recommendation to NHTSA this year.
During the Government Relations Report at the TMC meeting, Peter Blau from the Laboratories' Metals and Ceramics Division said that a "Truck Brake Lining Identification" code would help users get the original equipment friction ratings. This would be of value to fleet maintenance managers, but it could also be valuable to second users of the equipment who want to restore foundation brakes to as-new lining materials.
To this end, the objective is to come up with a sticker that could possibly be located in the door jamb. This would include information on the manufacturer of the original linings, the application code, an identifier and a key to the torques measured in a beefed-up FMVSS 121 type test.
In the interim, the maintenance manager's best bet for consistent brake performance is going with established and familiar brands. Some of these are the tier-one suppliers to the OEMs, while others are specialists in brakes and friction materials. All can help the fleet with brake-maintenance issues and often the specialists can bring significant resources to the problems.
But sometimes the experts in the aftermarket can bring more to the table. One supplier points to the 23 different mixes of materials in his company's extensive heavy and severe-duty offerings. It's his contention that as a specialist, he can tailor the replacement braking to the application even better then the OE component supplier. As an example, a fully dressed drive axle might find itself in an urban or heavy P&D application, or it might just as well see application in an over-the-road tractor. The duty cycles are totally different and the independent supplier might well be better able to help the fleet with the aftermarket selection of the friction material that will best suit the application.
Brake suppliers are all able to provide technician training so that brakes are up - not just to inspection, but to the performance that enabled the OE manufacturer of the vehicle to certify and meet the braking standard FMVSS 121 - to today's standard or far stricter requirements tabled for a few years in the future.
