Air Disc Brake Dilemma
Air discs with electronics are all the rage in Europe. But here, it's tough to spec them — even if you want them.
STEVE STURGESS
SENIOR EDITOR
The air disc brake has taken Europe by storm. All told, there are more than four million units in operation over there. One supplier alone, Knorr-Bremse, produces more than a million a year. By contrast, there are maybe 70,000 in North America and most of them are on non-commercial vehicles.
And while the air disc with electronic braking controls has become the standard tractor braking system with every truck maker across the pond, the only electronically controlled braking systems (ECBS) operating on this side of the Atlantic are in some high-profile demonstrations, run by technology leaders such as US Xpress of Tunnel Hill, Ga.
In fact, it's tough even to spec air disc brakes if you want them. Most American truck manufacturers don't list the option in the data book, and for some, disc brakes are not even an unpublished option. Mostly, what's available is the Meritor ADB1560 that traces its genesis to the mid-'80s. While we've heard a lot about the second-generation types currently in the demonstration testing like Penske's program with Bendix starting this month, the much-touted, ready-for-North-America, air disc brake (ADB) has yet to make much impression in fleets.
The same goes for ECBS — though you can order electronic braking from Freightliner at a considerable option cost. It is available mostly on a fleet trial basis and usually through the supplier — Bendix or Meritor WABCO — working with the OEM.
It's a chicken-and-egg situation. Fleet experience with earlier disc brakes was not good. The products were big, heavy, expensive, unreliable and maintenance intensive — everything a fleet manager wants to avoid.
Getting the air disc's image turned around will take work. Ron Bailey, technical sales manager at Bendix, probably summed it up best in noting that the industry is really not selling the product well by even referring to it as ADB. By doing so, the much-improved products of today are associated in buyers' minds with the products that made an all-too-soon appearance in the early '80s.
Today, the field has narrowed to Bendix, now a subsidiary of European disc brake giant Knorr-Bremse; Dana, which has taken over the braking business from partner Eaton; and ArvinMeritor, with brakes from Meritor and controls from Meritor WABCO.
Propelling the discussion and acceptance of ADBs, all three were represented in an "Integrating the Technologies" panel under the title Government and Industry Brake Research and Regulations held at last November's Society of Automotive Engineers Truck & Bus meeting in Detroit.
Also on the panel and giving the fleet perspective was Marty Fletcher, who is technical and training director for US Xpress. He had only good things to say about the 100-truck test with 50 ADB/ECBS tractors and 50 control trucks, with the air discs showing no maintenance issues, improved performance and, at 225,000 to 250,000 miles only around 25% wear to the brake pads. At that level, they could go to a million miles, he said. However, Fletcher remains guarded. He says he wants to see what happens to braking performance when the pads get beyond 50% worn.
ECBS has not been without fault, with several data-link failures early in the test. But because the systems have a two-circuit air brake redundancy, Fletcher reported that, "the revision to air braking with acceptable pedal pressure" avoided any danger.
This uncertainty, plus the high cost of ECBS — or brake-by-wire — is one of the issues holding back the deployment of the European model in North America.
In part it's cultural: the European truck manufacturers are mostly vertically integrated, and the trucking customer is used to having the truck OEM specify most of the running gear, hurrying the pace of technological change in the marketplace. And European braking systems have historically been heavier and more expensive than the systems developed by American S-cam brake suppliers. That has meant Europeans have been able to enjoy the enhanced performance of the combined air disc and brake-by-wire at no cost premium, a big plus considering the crowding on most European freeways. In the Alpine countries, there are extremely long grades that double the steepest descent found on American freeways.
Regulation may be what will drive the use of the ADB — with or without ECBS — here. The National Highway Traffic Safety Administration is expected to issue a recommended performance rule that would call for 30% shorter stopping distances for heavy trucks by 2007. Today's braking regulation requires heavy vehicles to stop in 355 feet from 60 mph. According to Bendix testing, a typical drum-braked tractor with unbraked control trailer at 56,470 pounds GVW can pull up in about 292 feet, or 17% shorter. That won't be enough to meet the more stringent standard without further development of the basic S-cam brake.
But equipped with the air discs on steer and drive axles, the same rig can stop in 215 feet, or 39% shorter. Not only does that exceed the proposed performance target, but it is actually only about five feet more than a typical passenger car. As Bendix' Bailey points out, if the purpose of the regulation is safety, it makes a lot of sense to regulate trucks and cars to have comparable stopping performance.
And while there is no anticipated performance standard at 75 mph, there are plenty of states where trucks run at this speed. The disc brake's improved ability to covert kinetic energy to heat — without fade — is even more obvious here. In the Bendix tests, the drum-brakes tractor-trailer stopped in 517 feet while the disc-equipped unit pulled up in 345 feet — just over the length of a football field — maintaining its better than 30% performance improvement and stopping 172 feet shorter.
The fade resistance of the technology is impressive, especially since the drum brake shows its limitations in long grades more than anywhere else. Bendix testing on the I-70 grade west of the Eisenhower Tunnel in Colorado shows this in detail. On the 8-mile, 7% downgrade regular snubs of the brakes to reduce speed from 35 to 30 mph resulted in a brake pressure rise of 25 psi to maintain a 5ft/sec2 deceleration. With a disc-braked vehicle, the rise was only 4psi. At an offramp completing the downgrade, with the brakes hot, it took twice the air pressure to achieve a 12 ft/sec<+>2 deceleration — not a panic stop. With the brakes still hot, a maximum pedal effort stop produced only 20 ft/sec<+>2 where the disc brakes achieved 24 to 25 ft/sec<+>2 and were in full ABS antilocking mode.
The fleet most often associated with air disc brakes is Air Products, where Ron Szapacs is maintenance specialist. The fleet has long — and at times frustrating — experience with first-generation ADB and consequently reverted back to S-Cams. On its 2000 orders, however, the fleet began using the Meritor DX195 on the steer axle only, because the 225 wasn't available for the drives. "The main reason we went back to the ADB on the steer axle is the S-Cam brake system has more negative effects on the steering than ADB systems," says Szapacs. He echoes a point made several times during the SAE meeting that the self-energizing of the leading/trailing drum brake aggravates steering effects of brake drums out of round and out of balance. Szapacs says this causes shaking and vibrating in the steering.
Szapacs adds another maintenance benefit: "(A drum brake's) additional maintenance cost — and also a safety issue — is in a leaking wheel seal that allows the oil to saturate the brake lining causing a pulling in the steering. Not only does this require a new seal, but new lining, and it's not a good practice to reline one side only. A disc brake wheel seal leak does not allow oil to contact the brake pads and has no effect on brake performance," he said.
He also believes their performance is a plus in ABS situations. "ABS brake systems function a lot more on the steer axle than the drives. The ADB uses less air when applying and releasing the brakes."
In fact, Szapacs' solution is likely the one that will be the first deployment of air discs in North America: ADBs on the steer axle, S-cam drums on drive and trailer. All-disc tractors present major timing issues with older drum-braked trailers, especially as Dana's chief engineer Jim Clark points out, many do not even have automatic slack adjusters.
Clark says the National Highway Traffic Safety Administration would be smart to consider rulemaking to require retrofit ASAs on older trailers; to reduce the amount of marginal equipment operating with brakes out of adjustment. A rule for ASA retrofits would cause owners to really improve trailer brakes or, if they weren't prepared to spend the dollars, retire the "oldies."
Clark says NHTSA's focus is not initially to push for ECBS but to go for rulemaking, perhaps one rule for shorter stopping that would hasten the acceptance of front axle air discs without impacting trailer balance.
ADBs might catch on in a big way once fleets and drivers experience the benefits.
Air Discs continued...
