The biggest difference between the old brakes and the new DX models, according to Szapacs, is the way the brake actually operates on the slide pins that allow the caliper to slide freely on the rotor.

Szapacs pays more up-front for the air disc brakes, but his trucks have more stopping power and maintenance costs on the DX 195s are, he says, "virtually nonexistent." Most of the units easily go 500,000 miles with no pad replacements or relines, and no rotor issues. Air Products trades its tractors out at 600,000 miles, and it's not uncommon for those trucks to be sold with their original pads and rotors up front.

In fact, the company said all-disc truck tractors have consistently shown the ability to stop within today’s regulatory standard for passenger cars.

The impetus behind the NPRM on truck stopping distances is the Motor Carrier Safety Improvement Act of 1999, which was designed to reduce highway fatalities by 50 percent over the following 10-year period. NHTSA believes it can achieve this goal by reducing the tractor stopping distance requirement. Among its findings are that the difference in stopping distances between large trucks and passenger vehicles are a leading factor in the intensity of highway accidents. Currently, FMVSS provisions set the stopping distance for heavy trucks at 355 feet and at 216 feet for passenger vehicles. A 30 percent reduction would lower the heavy truck threshold to 249 feet.

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.

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-braked 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.