When snubbing is a good thing

Country music artist C.W. McCall dealt humorously with a runaway truck in his song “Wolf Creek Pass.” But the reality is as serious as truck driving gets. Reminding a driver of the facts about downhill braking could save a few minutes of extreme panic. It could even save a life.

Modern trucks may have ABS, but most of the brake system is mechanical technology. And ABS doesn’t do a thing to help keep brakes cool in a long, gradual descent.

A driver’s No. 1 asset in running down a long grade is the engine brake. Drivers should first bring the vehicle down to a conservative speed in the same range as that which they could maintain in an ascent, if it’s a gradual grade. The next step is to shift to a gear that puts the engine within 200 to 300 rpm of the red line approved for engine brake use, and engage the brake on all cylinders as needed. Finding the right gear is important as the retardation effect increases greatly with rpm.

Drivers also need to understand how truck brakes are activated when they press down on the treadle valve. What the treadle produces is just an air signal that travels the length of the rig. The bulk of the air is stored in tanks near each axle. The air signal from the treadle valve opens a relay valve near each axle to supply air rapidly, as signaled, to the brake chambers. The problem of downhill braking is getting all the relay valves to open.

With 10 brake drums, a modern tractor-trailer rig has enough total metal exposed to outside air to throw off the heat generated by braking. Heat is actually thrown away as fast as it is generated, and the brakes continue to operate efficiently. Problems occur if only six or eight of the 10 drums are working. This can happen if the crack pressures aren’t all the same and the driver brakes steadily at an apply pressure between those crack pressures. Both improper design and imperfect maintenance can easily skew the crack pressures, a problem referred to as a lack of brake system balance.

The result of having too few axles working is excessive heat in the drums doing more than their share of the work. At high temperatures drums expand away from the linings, reducing application force. Also linings lose their grips on drums at elevated temperatures. This is called heat fade, and it can result in a runaway truck.

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Drivers must avoid this situation by snubbing – applying the brakes hard enough to slow the vehicle at the gentle rate typical of pulling up to a traffic light, rather than just holding a steady speed. This means losing 5 to 6 mph in about three seconds and then releasing the brakes until the speed has been regained. After that, they repeat the cycle. If the driver has an air application gauge and knows how to read it, this means an applied pressure of 20 to 30 psi.

Some drivers may complain that this action will deplete the air tanks, but it won’t if they do it right. Snubbing means all the relay valves will be well beyond their crack pressures. All the drums will be working. And if the brake system is in sound condition, the truck will not experience significant brake fade.

“Snubbing is still the best approach as you can never be sure what the balance is,” says brake expert Dick Radlinski, president of Radlinski Associates. “It will also give the driver more feedback if the brakes are not responding (i.e. fading).” Snubbing is supposed to be covered in CDL training and exams. But don’t count on that. Besides, drivers can and do forget the basics.