Stopping Distance Change Eyed
NHTSA's on a 75-mph path. Could force a move to air disc brakes.
Jim Winsor
Executive Editor
There were several sessions focusing on truck brakes at the recent Truck & Bus SAE meeting in Fort Worth. New brake technology and improved performance are popular subjects at this annual event.
But it's not often a trucking audience is startled during such presentations. In this case, NHTSA's Duane Perrin was outlining government brake research and likely new regulations.
And the number 75 kept popping up in his presentation.
Perrin is the chief of the Crash Avoidance Division, Office of Vehicle Safety Research for NHTSA.
He told truck and supplier brake engineers in the audience what his agency has learned from testing big-rig brakes above 60 mph.
Up until now, 60 has been the top speed, but here was Perrin, demonstrating data with big-rig braking from 75 mph.
As he showed charts and graphs illustrating ongoing truck-braking research, up popped a slide with the subhead "75 mph." It read as follows:
• Stopping distances increase with the square of speed.
• In-stop (brake) fade is more likely from higher speeds.
• 42 states have legal speed limits for trucks above 60 mph.
• Nine states have 75 mph legal speed limits for trucks.
As his presentation continued, we saw charts comparing stopping distances from 75 mph with tractors having several brake systems: traditional S-cam brakes, air disc brakes, and air discs with (ECBS) electronically controlled braking systems. The graphs showed clearly that air discs — with their greater torque input and fade resistance — can stop a big rig or heavy straight truck in one-third less distance.
In a subsequent presentation, Bendix' Ron Plantan showed fade resistance comparing S-cams and air discs both from 60 mph and 75 mph. In a series of 17 stops from 75 mph, as brake heat increased, fade got progressively worse with S-cams. They more than doubled stopping distances after 17 stops. With air discs, there was a minimal distance increase starting at the 10th stop, but no change in distances below that.
There's no question that in the unlikely event a trucker has to make a panic stop from 75 mph, air disc brakes have superior performance. But in the real world, does that ever happen? S-cam brakes and drums simply can't absorb and dissipate the huge amounts of heat being generated. As drums get hotter, they expand. As they expand, S-cams have to move linings out further to reach the drums. The further an S-cam has to turn, the longer the pushrod travel and the more air needed in each brake chamber. It's a losing battle.
Perrin didn't say new rules were coming at this time for 75 mph brake performance. His focus was on upcoming NHTSA rulemaking slated for 2007 to substantially reduce stopping distances from 60 mph. But there was no question in talking with brake engineers who were there that NHTSA is clearly on a 75 mph path. The question is, how soon?
I'll address the likely 2007 brake rule in a subsequent column. For now, chew on these disturbing thoughts: 2007 brings us the next generation of EPA-mandated super-clean diesel engines and ultra-low sulfur diesel fuel in '06. Figures being tossed around include at least a six-cent a gallon increase in diesel and a probable fuel tax increase by then, too.
Couple that to at least a $10,000 increase in big-bore diesel engine prices as installed in truck or tractor chassis with exhaust aftertreatment, etc. Now add in state-of-the-art brakes that by 2007 will include air discs on at least some models, plus ECBS technology.
I get a headache just thinking about 2007 truck prices and I get dizzy when I speculate about 2010. By then, you'll be reading someone else's thoughts. I'll be on my way to a rest home!
