It’s a rolling debate among trainers and truckers: is it better to use a constant, light pressure–no more than 10 psi–to keep your momentum in check or “snub and roll,” letting the vehicle pick up speed and then using heavier, intermittent applications to slow yourself down? “From the standpoint of pure physics, it would seem there would be less energy to absorb, and hence lower brake component temperatures, if you use steady pressure,” says Al Wright, the Hope, B.C., author of two definitive books on air brakes, The Air Brake Connection and Air Brakes From the Driver’s Seat II (both published by Wright himself).
But tests show no major difference in brake drum or lining temperatures, he says. In fact, Wright suggests that under most driving conditions, snubbing and rolling is the better way to slow.
Constant pressure, he says, may lead to brake imbalance, given differences in response between tractor and trailer air valves and foundation brakes. Also, the snub-and-roll technique helps your automatic slack adjusters do their job.
“ASAs only adjust when the brakes are applied and released,” he says. “The snub-and-roll method lets the ASAs continually adjust the brakes, keeping the air chamber stroke as short as possible, even as the brake drums expand due to heat.” Wright’s books are distributed by Presto Print in Richmond, B.C. (604/278-2428).
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