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Is there enough air back there?

TORONTO, Ont. - It sounds like one of the math problems that your kids would bring home from school: A tandem-tandem tractor-trailer with a GVW of 80,000 lb. travels 10 kilometres down a six per cent...


John G. Smith

John G. Smith


TORONTO, Ont. – It sounds like one of the math problems that your kids would bring home from school: A tandem-tandem tractor-trailer with a GVW of 80,000 lb. travels 10 kilometres down a six per cent grade, as the driver snubs the brakes every 10 seconds. Assuming the truck has no retarder, and each snub of the brakes requires 20 to 30 psi of air, how much air does the compressor need to deliver?

Before you reach for the calculator, a U.S. study already came up with the answer – these severe conditions would require one cubic foot of air per axle per minute. But what about the added draw of suspension systems, lift axles, tire inflation systems, and air-powered slider systems?

And what about the needs of trailers that incorporate more than a pair of axles?

Challenges such as these have led the Ontario Trucking Association (OTA) and Canadian Transportation Equipment Association (CTEA) to lobby for a way to rate tractors for the trailer configurations they’re designed to haul.

“We want every tractor to come with an air delivery rating,” says Rolf VanderZwaag, OTA’s manager of maintenance and technical issues.

“Ultimately, we want to know if this is a good match or isn’t it.”

Tractors that are originally spec’d to haul lightly loaded tandem trailers, for example, could be a poor match for multi-axle trailers loaded with garbage, which regularly travel the highway from Toronto to Michigan’s landfills.

Fewer than three per cent of U.S. trailers are equipped with three or more axles, but almost 30 per cent of Canadian trailers have multiple axles – such as “Michigan” combinations that regularly travel between Ontario and the U.S., Quebec quads, and multi-axle designs that travel B.C. highways.

While such a rating would go above and beyond the federally mandated Canadian Motor Vehicle Safety Standard rules (CMVSS 121) that govern the minimum standards for brake systems – largely a copy of U.S. standards that were designed with lighter tandem-tandem configurations in mind – the groups have already played a key role in adding components to Ontario’s SPIF trailer designs.

The province’s new five- and six-axle configurations now include split air systems to ensure that leak-related failures can’t interfere with the brakes on more than three axles.

By splitting a system from side to side on a tridem, for example, a tractor-trailer will still maintain 73.7 per cent of its stopping power if it loses half the trailer brakes. Quads would retain 70 per cent. (The idea isn’t that far-fetched. Logging trucks in B.C. often have each axle protected by a dedicated air circuit.)

There is also a red LED located by side marker lamps to warn drivers if air pressure for the service brakes drops below 70 psi.

An OTA study has been credited as the foundation of the recent changes by revealing that spring brake priority systems built to CMVSS 121 standards could unexpectedly leave drivers without trailer brakes.

Missing petcocks

Losses could be caused by missing petcocks that allow air to drain from reservoirs, pinched air lines, or incorrect valve replacements that affect control signals. That’s a particular problem for configurations that haul multi-axle trailers, which leave most of the braking to axles found well behind the fifth wheel.

The next focus of the improvements is to ensure that trailers can’t be “starved” of the air that they need, VanderZwaag adds.

That involves asking three questions about every combination: Do the compressor and dryer produce enough quality air? Can the air be delivered from the tractor fast enough? And how much air does the trailer actually need?

The answers will also involve more than a simple analysis of a compressor’s physical size.

Calculations involving bore, stroke and cycles may give a unit its rating of 16.5 cfm, but it will only produce six to 10 cfm of air under normal operating conditions, and 13 cfm at a maximum of 2,300 rpm.

“During the build-up, the volume coming out of the compressor cannot exceed the air dryer capacity,” VanderZwaag adds. “During the purge, the dryer needs enough time to recharge the desiccant.”

Indeed, the added moisture can be as problematic as the lack of air.

“Moisture aggravates particulate contamination,” says Ed Tschirhart of the CTEA, referring to the resulting “muddy sludge” that can slow the movement of valves.

Two air dryers

VanderZwaag suggests that a four-axle trailer should have two air dryers running in parallel, or a model with a continuous flow.

And while many air system contaminants can be captured by the 0.013-inch gladhand screens mandated in SPIF trailers, additional filters can be added to protect ABS valves from smaller particles, VanderZwaag says.

What can be smaller than that? Think of the desiccant from an air dryer, he suggests, pointing to an image of tiny white pellets clogging one ABS valve.

That has the potential of jamming brakes in an on or off position.

The changes may well go beyond legal minimums, but they could make the difference between having brakes and losing control.


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