by Paul Abelson, technical editor
Sometimes it amazes me how outsiders, especially government agencies, want to spend our money. New truck prices are estimated to increase $3,000 to $5,000 starting in October to cover the added cost of new emissions control and such support components as new radiators, fans and shrouds, not to mention insulation. On top of that, we’ll lose around 3 percent in fuel economy, and we’ll either have to service our trucks more often or go to larger crankcases and higher efficiency filters. All these cost more. But that’s from EPA. Let’s look at how the National Highway Traffic Safety Administration (NHTSA) plans to improve our lives and what the costs might be.
NHTSA is the agency that controls safety features on our trucks. They used to specify equipment that must be used, like redundant plumbing systems for air brakes, and trailer ABS warning lights on the dashboards of new tractors. Sometimes they propose specific performance criteria and leave it to the industry to find ways to achieve it. That’s how it is with brake performance. Following a specific set of test procedures involving weight, road conditions and other factors, tractor-trailers must stop from 60 mph in 355 feet. Today, in fair weather, just about every loaded rig can do that safely. We average about 292 feet. But almost all 4-wheelers, including vans and SUVs, stop from 60 in less than 170 feet, and many approach 135 feet. NHTSA wants to lessen that differential, so we don’t rear-end those quick stoppers. They may require future trucks to stop in shorter distances. While nothing has officially been announced, my Washington sources tell me 275 feet has been mentioned as a possible standard. That would mean that significantly more than half of today’s rigs would fail.
Today, only air disc brakes, (ADBs) can meet the possible new standard. Bendix, part of Knorr-Bremse, one of Europe’s more experienced ADB makers, developed a new ADB specifically for the American market. The ADB 225 weighs 30 percent less than its European counterpart, but still delivers stopping performance 26 percent better than s-cam drum brakes and about 40 percent better than NHTSA requirements. In field tests, Bendix ran two trucks, one with conventional drum brakes and one with ADBs, down the eight-mile 7 percent grade just west of the Eisenhower Tunnel on Interstate 70. The brakes were heated by repeated snub braking between 35 and 30 mph. By the time they reached the bottom, s-cam brakes needed a 25 percent increase in air pressure to maintain a 5-feet-per-second deceleration rate. The ADB’s application pressure increased only 4 percent.
The primary reason for ADB’s improved performance is the way each brake absorbs and then dissipates heat. There is a law of physics that energy cannot be created nor destroyed. It can only be converted from one form to another. Brakes are heat engines. They take the kinetic energy of a vehicle and convert it to heat, thus slowing or stopping the vehicle. Stopping from 60 mph can increase drum brake temperature by about 600 degrees Fahrenheit, assuming everything is balanced and properly adjusted. If adjustment is not equal, even though it is within tolerance, or if the load isn’t evenly distributed, some drums may increase to 800 to 1,000 F. Brake drums expand due to heat, about 0.010 inch per 100 F. At 600 F, a drum could grow by 0.060 inches. Multiply that by the mechanical advantage (leverage) between the slack adjuster and the cam, and you could need an extra half inch of brake stroke. The hotter the drum, the more extra stroke you’ll need. Also, if temperatures get too hot, the bond holding the friction material in the brake shoes starts to weaken, and the shoes become less effective.
Now picture disc brakes. A spinning cast iron disc with internal air vents for cooling is grasped on both sides by friction material. The iron heats and expands into — not away from — the calipers applying the friction material. Heat builds quickly, but is quickly transferred to the air that is cooling the disc. ADBs withstand greater stresses, and perform better longer. They resist fading, rather than being susceptible to it.
With higher speeds, the differences are even more dramatic. At 75 mph, s-cam drum brakes stop a NHTSA-spec test vehicle in 517 feet on average. ADB-equipped vehicles stop in 345 feet, a whopping 172-foot decrease. That’s about 10 extra car lengths when you need it most.
ADBs are proven. In Europe, about 80 percent of over-the-road trucks have them. With some models, disc brakes are all you can get today. There are two main reasons. Europeans have been more technology-driven than we are. They were using wedge brakes, a more efficient and more expensive form of drum brake than our s-cam brakes. Their trucks are spec’d to meet the rigors of driving in the Alps, where downgrades more than 10 percent are not uncommon. There was a need for better braking, and ADBs filled the need. Also, the price differential from wedge brakes to ADBs was not as great as from s-cam brakes. Currently, there is a hefty upcharge for ADBs, but when they go into high volume production, the disparity is expected to be minimal.
Some makers believe on a total cost-of-ownership basis, ADBs may already be more cost-effective. They cite the ease of changing caliper pads compared with the labor-intensive job of changing brake shoes. Haldex ADBs, proven in Europe, have been on champion trucks in the tough European Super Truck Racing circuit. With both racing and production ADBs, caliper pads can be changed without disassembling the wheel-end. Wheel-end work is demanding, something that, if not done to 0.001-inch tolerances, can affect tire wear and axle integrity. Bendix’ new ADB system has rotors that can be changed without disturbing wheel-end assemblies.
Electronic Braking Systems (EBS)
Braking can be improved through electronics, but only if the government allows innovation without penalty. The Federal Motor Vehicle Safety Standards (FMVSS 121) currently require redundant pneumatic systems to activate air brakes. If one system goes down, a back up is in operation. One system alone is costly; two, doubly so. For years, Rockwell WABCO, now MeritorWABCO, has been a pioneer in developing Electronic Braking Systems (EBS). As the name implies, the systems use electronics to send the signals to apply brakes. Electronic signals travel at 186,000 miles/second. Pneumatic signals, sent from the brake treadle valve to the rearmost axle and then forward, axle by axle to the front, travel at a maximum speed of 1,090 feet per second. They are slowed every time they encounter a restriction (like a valve or an elbow) in the air lines. With today’s air systems, it could take up to 0.2 seconds for all the brakes to apply. During that time, you could travel 18 feet or more, or about one car length at highway speeds.
Electronic braking systems (EBS) — also known as “brake by wire” — have air tanks right near each chamber. Signals are sent over wires, not air lines. All brakes apply simultaneously and immediately, giving you that extra car length safety margin.
Electronic braking can also be tied to traction control and stability control. When a tire starts to spin, its brake can be applied selectively to stop the spinning and transfer torque to wheels with traction. EBS can detect skids long before they register with the driver, and can apply specific brakes (left steer, right drive, right trailer) individually to counter the skid and keep the rig stable.
So why don’t we have EBS today? It isn’t that the systems haven’t been proven. They have been operating in technology-driven Europe for years, where they’ve been shown to be reliable and effective. Cost is a major factor. NHTSA says it’s OK to have ABS, as long as trucks also comply with FMVSS 121. That means having both EBS and redundant pneumatic ABS. So instead of allowing technology to lower our costs and improve safety, NHTSA rules make us add the cost of EBS without allowing us to subtract the cost of the pneumatic system.
Do EBS systems really work? In October 2000, the major brake manufacturers demonstrated how well they do at NHTSA’s own East Liberty, OH, test facility. There were demonstrations in front of hundreds of Technology & Maintenance Council (TMC) members. Used with ADBs, also demonstrated at the TMC meeting, EBS can improve braking and vehicle control significantly. Air disc brakes currently carry a significant price premium, but as production comes up, costs will come down. They don’t have to equal the price of drum brakes to be cost effective. Savings in maintenance costs may already favor ADBs for total ownership cost over four or five years. But until NHTSA allows EBS as a stand-alone system without the backup pneumatics, it can never be cost effective unless someone figures out how to quantify greater safety. How about it, NHTSA? Isn’t it time to bring FMVSS 121 in line with today’s technology? It sure would make things safer.