Future trucks may finally benefit from two major improvements in foundation brakes, according to Larry Strawhorn, vice president of engineering, American Trucking Associations. Strawhorn cited work in progress by the National Highway Traffic Safety Administration at last month’s meeting of ATA’s Technology & Maintenance Council.
Frictional characters
Once upon a time, fiction taught us that OE and aftermarket brake linings displaying the same friction-rating edge code – such as FF, EE, FE or EF – would provide virtually the same braking force.
However, in the late ’80s, when it became widely recognized that identically marked brake linings could produce brake-torque variations of up to 40 percent, ATA petitioned NHTSA to initiate rulemaking that would ensure accurate ratings and consistent performance.
Well, the wheels of government turn slowly (dragging brakes, perhaps?), and when NHTSA missed its target of December 2001 to finish its action relating to brake-lining friction ratings, Congress provided the administration funding to proceed.
And the Department of Transportation has set aside $300,000 in its 2002 appropriations package for research into brake lining friction. Reportedly, NHTSA will use the funds to complete materials research in an attempt to finish its rulemaking.
Brake of the future
“The air disc is the brake of the future,” as the quip goes, “and it always will be.” Except now, NHTSA is planning on rulemaking for the brake system of 2010, which would require a 30 percent decrease in truck panic stopping distance, to about 200 feet from 60 mph for combination vehicles.
And the administration is convinced that high-torque, disc brakes, at least on the steer axle, will be required to make such short stops.
But why haven’t air discs caught on already? How could 80,000 pounds of rolling stock rely on old-fashioned, heat- and water-fade-prone drum brakes?
Most folks with cars made after the ’60s know that disc brakes are inherently superior. Discs provide much more consistent braking torque over a wide range of temperatures and speeds. They resist heat fade since, as brake pads and rotors get hot, they expand into each other, while, when drums and linings get hot, the drum expands away from the linings.
Also, discs are virtually immune to water fade. Drum brakes, with their larger surface areas, apply fewer psi of force between lining and drum during a stop. This, added to the drum’s water-retaining shape, promotes hydroplaning between shoe and drum under wet conditions.
Disc brakes, with their smaller friction surfaces and high clamping forces, do a good job of “squeegying” water from rotors, and so display little reduction in stopping capability when wet.
One stumbling block to air-disc acceptance is compatibility with drum brakes. Invariably, during a long, hard stop on a mixed-brake heavy vehicle, drum brakes experience heat fade, and their brake torque drops off sharply. Meanwhile, the disc brakes keep right on stopping, as well as absorbing extra work bequeathed them by the dying drums.
The overworked discs get red-hot, and the resulting rotor cracking and pad de-bonding appear to be disc-brake problems, but they’re really a testament to the disc’s ability to outwork drums.
Then there’s cost. Our extremely cost-conscious trucking community is reluctant to pay more, even for a superior brake. And discs go for about 2 1/2 times as much as drum brakes.
Of course, part of the reason they’re so expensive is that they don’t sell very well, and are therefore a low-production item with no economy of scale. It’s a chicken-or-egg thing.
But, hey, if you want to make an omelette, you’ve got to break a few eggs. And NHTSA’s recipe could be the push we need to bring our brakes up to world standards.
