Six Percent Grade: Fade, Decisions, and the Escape Ramp
You are westbound on a 6 percent mountain grade in western Maryland, five miles long, signed "Trucks use lower gear." Your combination weighs 74,000 lb. You topped the grade at 40 mph in 8th of 10 gears, near the engine's governor. In the first mile you used the service brake in short applications to hold 40 mph. By the second mile you needed much longer applications to hold the same 40, and by mile three you are at 55 mph and climbing with the pedal on the floor. You can smell hot lining and the applications feel weak. The next exit is a mile ahead; an escape ramp is 1.5 miles ahead on the right. Traffic behind you is light.
1. Explain what is happening to the brakes, and what the correct technique would have been at the top of the grade.
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The brakes are fading. Continuous or repeated braking converts speed into heat in the drums and linings faster than they can shed it. Hot lining loses friction, so the same pedal pressure produces less stopping force, which is why you need longer applications to hold the same speed. The smell of hot lining plus weakening applications at 55 mph on a 6 percent grade is a fade in progress, not a maintenance nuisance.
The mistake at the top was using the service brakes to hold speed. The correct technique is to pick a gear low enough that engine braking holds the load at a safe speed with little or no service brake, and to be in that gear at the top, at a speed below the gear's governed limit. Then use snub braking: a firm application that drops roughly 5-6 mph, then off the pedal, letting the brakes cool while the engine holds the speed down. Short, hard applications separated by cooling time are far better than light continuous pressure.
The sign was the warning. If the grade says trucks use a lower gear, the decision is made before the descent starts, not in the third mile.
2. You are at 55 mph and still accelerating. What do you do now, in what order?
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First, get off the pedal if the grade and speed allow it. The brakes are only going to get worse with more heat, and they need cooling time to recover any effectiveness.
Try to downshift, but only if the engine will not over-rev. If road speed is too high for the next gear down, the shift is not available, so keep it in gear and keep looking for an out.
Decide early. The exit a mile ahead only helps if you can reach it and stop. The escape ramp at 1.5 miles is built for exactly this situation; if your speed is still climbing, take it. It is a designed, graded run-out and far better than arriving at the bottom of the grade at 70 mph with no brakes.
Do not make it worse: do not coast in neutral or shut the engine off, do not use the trailer hand valve to slow down, and do not pull the parking brake at speed. Locking the rear wheels can put you into a jackknife. Use the flashers and the horn to warn traffic while you work the problem.
3. You get stopped safely. What do you check before continuing, and what would you do if there were no escape ramp?
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Set the parking brakes, chock, and let everything cool before you touch it. Drums and linings stay hot for a long time, and a hot drum can crack if it is cooled suddenly. Then inspect: lining thickness and glazing, drums for cracks and heat checking, air lines for leaks or chafing, and dragging brakes.
Check the slack adjusters with the brakes released. Pull each one by hand where the push rod attaches; if it moves more than about 1 inch, that brake is out of adjustment and was not doing its share on the way down, which may explain why the fade came on so fast. If anything is wrong, or you are not sure, call for service, because you are about to continue down the same hill.
With no escape ramp, the decision is still to stop. Look for a graded shoulder, an uphill side road, a gravel lot or the least bad place to leave the road, and do it early while you still have steering and some braking, not at the bottom with none. A controlled departure into soft ground beats a runaway.