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Turning up the heat

TORONTO, Ont. - Most people in the trucking industry agree that poor brake maintenance is at the root of the persistently high number of out-of-adjustment brakes found in roadside inspections. But jus...


TORONTO, Ont. – Most people in the trucking industry agree that poor brake maintenance is at the root of the persistently high number of out-of-adjustment brakes found in roadside inspections. But just how much of an effect do out-of-adjustment brakes really have on a truck’s ability to stop? Plenty, according to leading braking system consultant Dick Radlinski.

In an address to delegates at the first North American Brake Safety Conference last month, Radlinski graphically illustrated the direct impact of poor brake maintenance on stopping performance – and, ultimately, safety. To underscore his point, he referred to data from three separate brake safety studies showing that improper brake adjustment leads to lower brake force, increased stopping distances, an increased tendency to jackknife and a reduced resistance to brake fade.

“A heck of a lot of more truck accidents are brake-related than the statistics show us,” Radlinski said

But even if a truck’s brakes appear to be in adjustment during a pre-trip inspection, that doesn’t mean they don’t need to be rechecked at regular intervals during a trip, he said. These same studies show that adjustment measurements are very sensitive to temperature and pressure. To illustrate this point, Radlinski referred to test results that found that while the brake stroke at an application of 85 psi may be 1-1/2 inches when the brake drums are 70 F, the stroke increases to 2-1/8 inches when the drums are 600 F.

“And 600 F is not an unusual temperature for brake drums in city driving or on a downhill grade,” Radlinski said. “So you may think you have a certain level of braking when the drums are cold, and be at a whole other level when they heat up.”

When translated into stopping performance, the effect of heat on the braking system is even more dramatic. Radlinski referred to other test results that showed a loaded truck traveling on a dry road at 60 mph needs 342 feet to stop when its brakes are cool and fully adjusted. With hot brakes, the same truck needs 393 feet to stop.

When the brakes were adjusted out to the current two-inch limit, the stopping distance for the same truck jumped up to 458 feet — an increase of 34 per cent over the fully adjusted brakes. And when the out-of-adjustment brakes were hot, the stopping distance ballooned to 692 feet for an increase of 76 per cent over the hot, fully adjusted brakes.

“These test results show that even with the brakes considered legal at the two-inch limit, you can lose 50 per cent of your braking ability when the brakes are hot,” Radlinski said.

The scariest thing about the effect of poor brake maintenance on stopping performance, Radlinski said, is that drivers usually can’t tell when the brakes are out of adjustment until it’s too late. While the maximum available air pressure is about 100 psi, normal brake applications during typical operation require less than 30 psi. When the brakes are out of adjustment, however, a 100-psi brake application simply does not slow the vehicle enough.

“Drivers in everyday driving are operating down in this low brake pressure range,” Radlinski explained. “But if the brakes are out of adjustment and they attempt to make a panic stop, they find they have just 50 per cent or less of their braking ability. And as you get further and further out on stroke, you lose more and more ability to apply brake force.”

But Radlinski said drivers are asked to take on too much of the responsibility for checking brakes. Either due to a lack of knowledge or simple laziness, he believes many drivers are not doing proper pre-trip inspections.

“Checking the brakes can be a dirty, uncomfortable job, and it’s just not getting done,” Radlinski said. “When it’s cold and snowing or raining, drivers aren’t climbing under there and pulling on slacks to see if they have one inch of free stroke.”

In a perfect world, Radlinski said, all heavy-duty vehicles would be fitted with Electronic Braking Systems because they remove the driver from the brake equation. These so-called brake-by-wire systems are capable of electronically determining the vehicle weight from engine data and comparing that to the amount of brake pressure being applied by the driver. If the pressure is too high for the given brake force and weight, the system can alert the driver to a potential brake problem.

Although EBS has potential, Radlinski said, it still has to gain industry-wide acceptance in North America.

“EBS still has cost issues,” he said. “Fleets are having a hard time seeing the benefits there, and if governments aren’t going to mandate it, we may not see widespread use of EBS for another 10 years.”

In the meantime, Radlinski suggested that one solution to the problem of out-of-adjustment brakes would be to mandate a switch to long-stroke brake chambers. The commercially available equipment extends the useable stroke range by as much as one inch. With that additional range, the brake is still in adjustment for a full half-inch beyond the current limit.

“People are finally starting to realize that long-stroke brakes are a good idea,” Radlinski said. n


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