Braking Point: There’s options for NHTSA’s stop distance rule

TORONTO — After more false starts than an old cabover on a February morning in Saskatoon, the U.S. National Highway Traffic Safety Administration (NHTSA) issued a long-awaited new braking standard that cuts allowable tractor stopping distance by 30 percent.

With the goal of narrowing the gap between truck and car braking abilities, the rule requires that a truck traveling at 60 mph at its full gross vehicle weight comes to a complete stop in 250 ft — down from 355 ft — although new technology can practically get it down to about 210 ft.

It applies to truck-tractors only, and does not include straight jobs, trailers and buses.

NHTSA says the overwhelming majority of the North American truck fleet — three-axle tractors weighing less than 59,600 pounds — must meet the new stopping distance by Aug. 1, 2011. Two-axle tractors and severe service units weighing over 59,600 pounds have until 2013.

If you’ve been following the issue over the last, oh say, half-decade or so, you know by now that the rule won’t require an overhaul of your fleet’s entire braking system as some originally predicted. 

NHTSA is not mandating a blanket shift to air disc brakes — the system Europe systematically converted to about 20 years ago — as larger s-cam drum brakes will effectively meet the standard in most cases. (Every application is different, of course, and some lightweight applications won’t require any change on the steer axle at all).

Typically, many trucks come with 15 or 15.5-inch brake drums today and that’ll go up to 16 or 16.5-inch brakes.

As suppliers like ArvinMeritor and Bendix note, these larger drums have already been in use on many trucks for years.

In a nutshell, brake makers and truck OEMs will meet the standard by adding brake torque, possibly just on the steer axle alone.

MAKING THE CUT: New NHTSA brake standards
will cut back the stopping distances by 30 percent,
drastically improving the safety level of panic stops.

ArvinMeritor’s Paul Johnston says you can gain 20-percent brake torque with bigger cam brakes, 28 percent with discs up front and S-cams out back, or 38 percent with air discs at all tractor wheel positions.

Either way, the cost to you is going up, although the chasm in price between larger drum brakes and discs is quite significant. It’ll run you $211 US per three-axle tractor to beef up the drums, not counting a 100-pound weight penalty. For disc brakes, add about $1100 per truck.

Despite the steep price tag, it’s expected that the rule will slowly accelerate market penetration of air disc brakes, particularly on the steer axles of 4×2 tractors.

"The cost of air disc brakes will come down as volume and manufacturing efficiencies picks up. Right now it’s low volume from us and our competitors," says Aaron Schwass, Bendix’s Foundation Brake Product Line director. "Also, the cost of drum brakes will go up a little.

"Disc brakes will never cost the same as drum brakes, nor should they because disc brakes are an advanced technology with additional value, but the gap will close eventually."

So, just what are the benefits for spec’ing discs? How about shorter stopping distances, no fade, truly automatic adjustment, and increased side-to-side braking balance, for starters. That’s mainly why bulk hazmat fleets and those that operate in mountainous or severe weather terrain have been the early North American adopters.

On the question of balance, there’s still some lingering concern that a mixed drum/disc tractor-trailer unit or, perhaps, big brakes sitting ahead of ordinary s-cams could throw things out of whack during panic stops.

Manufacturers are aware of the issue, but once the appropriate adjustments are made to the air brake system, they don’t anticipate any problems.

ArvinMeritor’s Joe Kay, chief engineer of the Foundation Brake and Wheel-end Business, said North American suppliers have paid close attention to Europe’s past learning curves. Generally, he says the tractor will incur more of the braking workload (about five percent) while the trailer will experience a proportionate reduction in workload.

The bottom line: Much to the relief of the average trucking operator, you won’t necessarily need to spec air disc brakes on your tractors to meet the standard.

Unless, of course, you want to — which isn’t a bad idea anyway.  

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