Air Disc Brakes In Your Future?
They're generally not available & expensive, but you can't beat their performance.
JIM WINSOR
EXECUTIVE EDITOR
It's a fact that in North America we're not seeing air disc brakes except on specialized premium vehicles such as custom fire apparatus and top-of-the-line intercity motor coaches. Air discs were made standard on the MCI Renaissance motor coach in 1994, North America's first all-wheel air disc braked vehicle. By contrast, in Europe these days, 70% of heavy vehicles are built with air disc brakes.
A few U.S. fleets took a run at air discs in the early '80s, primarily for improved braking and safety. However, that generation of air discs was expensive, bulky, not generally available, they were difficult to service and had relatively short brake pad life. Except for a few die-hard users, air discs went away. But it's now 2002. Brake suppliers have developed a whole new generation of air discs, basically European-designed and manufactured for the large European markets. U.S. versions are imported but are not readily available except as noted above.
It appears that U.S. truck fleets are not yet ready to buy a clearly superior brake. There are good reasons. At the summer meeting of the Technology and Maintenance Council of the ATA, attendees heard a thorough update on air discs, including a prediction of what will put air discs on our future big vehicles. (Quick answer: NHTSA will. See below for details.)
Probably the main reason air discs haven't yet caught on here is costs. Where available, they run about twice the price of today's popular 16 x 7-inch S-cam air brake, the industry standard. Furthermore, the venerable S-cam systems continue to be improved, especially brake lining technology. And S-cams are nearly service-free these days in the majority of on-road applications. With this kind of a track record, getting U.S. fleets to pay more if they're happy with what they've got is a tough sell. It should be noted that hydraulic disc brakes have become quite popular on medium and heavy straight trucks.
Looking ahead, there's no question that U.S. heavy trucks and tractors will need higher torque braking capacity. One reason air discs have taken over in Europe is their use of powerful high torque front brakes, something U.S. truck drivers have an aversion to. In fact, it wasn't much more than 20 years ago that U.S. truckers routinely backed off front brakes and 30 years ago that U.S. truck builders deleted front brakes entirely on request.
As the years roll by, our rigs have lower and lower rolling resistance. Aerodynamics, radial tires, fan clutches and lower rpm engines have all reduced the natural retardation a truck or combination has. This means today's brakes have to work harder to replace that built-in retardation. Brake engineers are now saying it takes 250 brake horsepower to hold a rig at 50 mph on a 3% downgrade. This is twice the braking power required 25 years ago.
The National Highway Traffic Safety Administration (NHTSA) has a goal of reducing trucking fatalities 50% by year 2010. Part of that program calls for 30% shorter stopping distances for big/heavy vehicles. It's a nine-step program and it will require significantly higher brake torque and reduced brake fade to make this happen. We are told that NHTSA's "master plan" calls for announcing its program in a Notice of Proposed Rule in 2005 and with shorter stopping distances kicking in on new vehicles built in 2007.
I'm told that for starters powerful air discs on front axles will probably be required. It's not certain yet if they'll be needed on drive axles or trailers. Part of that answer lies in how much brake fade will be tolerated. Air discs are virtually fade proof. Brake engineers tell me that a "typical" 18-wheeler running today takes 360 feet to stop from 60 mph fully loaded. "Good" rigs can do it in 270 feet. "Perfect" rigs under ideal conditions can do it in 213 feet. There's no question that air discs all around will do better and will do it without brake pull or brake fade.
So what comes first? Probably optional front air discs, especially for heavy axles and/or frequent start/stops as in refuse trucks. All the U.S. truck manufacturers already have air disc development and test programs. And all our brake suppliers are in the game: Arvin Meritor, Bendix, Dana and Haldex.
