Science is safer than intuition


While thumbing through the latest edition of CCJ’s Air Brake Book, I realized that it contains two important braking tips that may not be widely circulated.

While we’d like to see copies of our book everywhere air-braked vehicles are domiciled or maintained, this potentially lifesaving information is too important not to share with all fleet managers and drivers. The recommended actions may at first seem counterintuitive, but they make perfect sense once the dynamics are understood.

The first tip concerns downhill braking. For many years, popular wisdom held that the best way to avoid brake fade while descending a long grade is to employ light, steady braking. This myth was even propagated officially by the first edition of the federally funded Model Driver’s Manual For Commercial Vehicle Driver Licensing (CDL Manual).

The problem with light, steady braking lies with the crack pressure of brake system relay valves. That’s the pressure at which the valves open in response to signal pressure from the brake pedal. On an ideally brake-balanced tractor-trailer rig, the trailer relay valve will crack at a pressure low enough that the brakes begin to apply at the same time – and with the same amount of force – as the tractor brakes.

But not all brake systems on the road are perfectly balanced, especially if brake repairs have involved installation of aftermarket relay valves. So, if the light, steady braking is enough, for example, to activate the brakes on the tractor but not on the trailer, the tractor’s brakes can easily fade and become ineffective. At that point, a higher-pressure application might save the day by firmly engaging the trailer’s brakes. In a worst-case scenario, however, the overburdened trailer brakes could fade, and you have the makings of a runaway accident. A similar risk is presented by lightly dragging the air brakes on a straight truck or bus with brakes that are badly balanced.

By contrast, intermittent, higher-pressure braking – known as snubbing – should ensure that all brakes engage and do their share of the stopping work. “Since most drivers don’t know how well balanced their vehicles are, the snub method is the safest approach,” says Dick Radlinski of Link-Radlinski. “The driver should select a gear in which, at governed engine speed, the vehicle tends to remain at or below the posted speed limit for trucks. If speed increases, he should then snub the brakes moderately, slowing the vehicle to 5 to 6 mph below his target speed. Next, he should release the brakes and let the vehicle accelerate back up to governed speed. Subsequently, the snub-and-release sequence should be repeated, as required, until the grade levels off.”

The second tip involves braking – or not – after a tire blows. The CDL Manual’s advice is not to brake at all until the vehicle slows down. That’s correct, but that advice fails to mention that it can be dangerous to even remove your foot from the accelerator too quickly.

According to Michelin and the safety department of the American Trucking Associations, the safest reaction to a blowout is to maintain or increase road speed for 8 to 10 seconds to regain directional stability. Then back off the throttle and avoid braking as long as possible.

The reason is that a moving vehicle presents a straight-ahead force, directly proportional to the vehicle’s speed. When a tire blows, a smaller, side force is introduced, which tries to pull the vehicle toward the side where the blowout occurred. As long as the straight-ahead force is greater than the side force, there’s no problem. And the best way to keep the good force with you is to increase, or at least maintain, speed.

Once the vehicle is stable, drivers are advised to make necessary steering corrections and not to panic – even if the tire comes off the rim. While there are heavy traffic situations in which this technique isn’t practical, drivers should use it when possible. I’ve tried it. It works.