As you may know, the National Highway Traffic Safety Administration last year proposed a rule that will require a 20- to 30-percent reduction in tractor stopping distance from 60 mph. The choice of hardware to get there is up to you.
“We anticipate they’ll go with 30 percent,” says Paul Johnston, senior director of compression and braking for Meritor WABCO Vehicle Control Systems. “Implementation of the rule is expected in 2009, and in 2011 for specially configured rigs.”
That means stopping distance limits will shrink from the current 355 feet to about 284 feet. OEMs likely will reduce that number further to 249 feet as a safety margin.
According to Joe Kay, ArvinMeritor’s brake engineering manager, today’s tractors already can do better than the current limit, with a stopping range from 275 to 295 feet, but there’s room for improvement. With high-performance drum, or cam, brakes (16.5 x 5 or 6 inches, front, and possibly 16.5 x 8.6 inches, rear), he says, brake torque could be increased by 20 percent. A combination of front air disc brakes up front with drums elsewhere will yield a 28 percent improvement, while discs all around will net a 38 percent reduction. That wouldn’t be too far from passenger-car stopping ability, so it would help level the playing field in daily traffic.
Of course, advantages of drum brakes will remain, says Kay, even with high-performance versions, including relatively low cost, known maintenance procedures and parts commonality. A plus with high-performance drums is that, since there will be more friction-surface area, a 20- to 100-percent increase in lining life is anticipated.
But the advantages of discs can’t be ignored. The braking torque they produce is far better than drums, and their performance isn’t nearly as affected by variations in speed and temperature. Also, because of their smaller friction-surface area and wiping action, they do a good job of removing water, and are virtually immune to water-induced fade. They’re much easier to replace, and they minimize braking variations between the left and right sides of a vehicle by applying nearly identical brake torque on both sides. That, in turn, decreases stopping distances and improves vehicle stability. Think about it. Would you want to go back to driving a car without at least front disc brakes?
Not that there aren’t some considerations if you decide to go the disc route, says Gary Ganaway, ArvinMeritor’s director of CVS Systems and product strategy. Proper wheel alignment is more critical, he says, due to quicker response and higher brake torque. Also, front axles and suspensions may need to be beefed up for the same reasons, and tires may be more susceptible to irregular wear. And if you’re stopping more quickly, you’d better have things held in place; load securement could be an issue.
But the biggest obstacle to widespread use of discs is price – an estimated $1,000 to $2,000 premium. The air disc is a product that hasn’t sold well yet because it’s expensive – and it’s expensive, in large part, because it doesn’t sell well, and there are no production economies of scale. Perhaps the new stopping-distance rule will help jump-start interest.
Whatever option you choose, you’ll have better brakes.