Stopping a truck is no small task, and it’s nearly miraculous that conventional braking systems do it as well as they do. With advances in aerodynamics and reductions in other ‘natural’ retardation sources like tire rolling resistance and driveline friction, we ask almost twice as much of our brakes as we did in 1980.

The challenge is about to get bigger, though the new stopping-distance regulations we’ll see this month are actually quite manageable. It does mean that the new truck you spec in 2009 or thereabouts will need at least bigger S-cam brakes up front, or it may well have air discs. In fact, the era of the air disc brake, already seen on nearly all new trucks in Europe, may finally be launched here in the New World. And it will come, as many such things do, by way of American regulation.

A couple of years back the U.S. National Highway Traffic Safety Administration (NHTSA) proposed a stopping-distance reduction of 20-30 percent for three-axle highway tractors. ArvinMeritor brake chief Paul Johnston is betting on 30 per cent in the final rule (see the note below if you want to look at the regulation in detail). The likely implementation date is 2009, he adds, and probably 2011 for what are called ‘specially configured’ tractors-meaning anything other than the basic three-axle vehicle that makes up 80 percent of what’s produced.

So what does a 30-percent reduction mean? The present NHTSA rule, mirrored here in Canada, demands that an air-braked three-axle tractor pulling an unbraked trailer (for testing purposes only, not as an attempt to mimic real life) must be able to stop in 355 ft from 60 mph. In fact, truck makers test for stops in about 280 ft. Assuming a 30-percent reduction, that same tractor will have to stop in 248 ft as of 2009, with a 10-percent compliance margin. In practice, OEMs will aim for — and reach — 220 ft, which is getting very near to car territory.

How will they do it? By adding brake torque, certainly on the steer axle, and possibly even on that axle alone. That could simply mean bigger 16.5-in. cam brakes, maybe a combination of cam and disc brakes, or possibly disc brakes all round. 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.

All of those options will cost more, he admits, though the number is very unclear at this point — NHTSA guesses $300 to $1,000 per axle — and there may be a small weight penalty, but regardless of which combination you spec, you’ll see significantly longer brake life to offset the downsides.

That means 25 percent to as much as 100 percent, and it’s those resulting lower life-cycle costs that have driven European truck operators to air discs. Note that a 16.5-in. steer-axle drum brake also provides a 65-percent increase in wearable lining volume, compared to the current 15-in. brake, which means longer life for that combination as well.

The stop distance benefit with air disc brakes is dramatic
when road speeds rise. The problem with drums is in stop fade.

Incidentally, some 37 percent of trucks are already equipped with 16.5-in. brakes up front, and fully 98 percent of trailer axles. At present, air discs make up a lowly one or two percent of steer-axle brakes.

Bendix Spicer has done much testing of trucks in real-life service, of course, and has some very interesting figures to show. It projects that dry-freight vans in the 60-80,000-lb GVW category can get over two million miles on front disc pads and nearly 1.2 million on the drives, compared to 500,000 miles on ordinary drum-brake linings. The rotors should be equally long-lived – 2.4 million miles up front and 1.5 million out back.

The company’s projections for a 117,000-lb bulk hauler are significantly different, as the accompanying chart shows, but the disc advantage is still clear.

Will there be balance problems if big braking capability is sitting ahead of an ordinary cam-braked trailer? Possibly, and this is an issue being addressed, but they would only appear in panic stops. Given that the vast majority of braking applications are at 25 psi or less, most drivers will never know the difference because in those circumstances the brakes will feel and act more or less as they always have.

So does it make sense to test stopping ability from just 60 mph? Only from a regulatory standpoint, as a means to establish standards, but many experts suggest it would probably make more sense these days to test at 75 mph.

The disc advantage is even more apparent at 75 mph, and dramatically so. Using that unbraked test trailer and a GVW of 56,500 lb, ArvinMeritor tests show that a tractor with current-spec drum brakes can stop in 267 ft from 60 mph.

And from 75 mph? You don’t want to know. Actually, with those extra 15 mph the stopping distance more than doubles to a whopping 541 ft, and there are some tests that note a figure of 618 ft.

Put discs all around the tractor and the 60-mph stop is accomplished in 204 ft, or 324 from 75. Note that bigger drum brakes also record decent numbers (see the chart above).

The bottom line here is that you won’t necessarily need to spec air disc brakes on your 2009 and later tractors because a high-performance S-cam will quite possibly meet the standard. Every application is different, of course, but do you gain enough by spec’ing discs? The upside is pretty compelling.

Current air disc brakes win the life battle
hands down when GVW is low.

There are five key advantages: shorter stopping distances, virtually no fade, truly automatic adjustment, much increased side-to-side braking balance, and very quick, very easy friction changes.

The fade issue is especially important if you drive the mountains. Rather than seeing the drum expand away from the friction lining as heat builds up on a long downgrade or in a panic stop, which you have with an S-cam brake every time, the opposite happens with an air disc the rotor expands into the friction pad so you get constant braking torque all the way down that nasty seven-percent grade with the right turn at the bottom.

If your drum brakes aren’t properly adjusted, fade could leave you with no braking power at all halfway down that hill. It happens all the time. But adjustment really isn’t an issue with discs, and the more they heat up, the closer that rotor gets to the friction.

Is there a downside? Sure, starting with up-front costs as already mentioned. The only other issue of any consequence is that, with all three air discs presently on the market, changing the rotor means removing the hub, which makes that a longer job than the equivalent changing of a drum. But unlike early examples of air discs in the 1980s, today’s rotors are far more robust. When you factor in the simple pad changes, life-cycle costs clearly favor discs in most cases.

And what about the history of air discs over here? Back in the 1970s it was a rocky road, with reliability, service and parts-supply problems. But dramatic advances in the technology and many, many miles of real-world operation in Europe have changed things entirely. More than two million air discs have been made in Europe, and much of that technology has migrated west, so today’s product is more refined, more robust, and generally ready for prime time.

So are you bound to have air discs on the trucks you run in the future? It’s probably a safe bet for many of you, and there are some fleets — especially those hauling hazmat tanker loads — that have already made the switch. More and more operations will follow that lead over the next few years. It’s simply inevitable.