Light Steady Braking on a Wet Downgrade
You are driving a 2019 Peterbilt 579 with a 48-foot flatbed loaded with steel, gross weight 79,000 lb, eastbound on I-64 near Morehead at 60 mph. Light rain is falling. A sign warns of a 5% downgrade for the next 3 miles. You are following a box truck at about 3 seconds. To hold speed on the grade, you keep light, steady pressure on the brake pedal. Halfway down, your speed creeps from 45 to 58 mph even though you push harder. The pedal still feels normal, but braking is weak and you smell hot brakes. The engine brake is off, and you are in 10th gear. There is a narrow shoulder and no exit before the bottom. The truck has a working engine brake and a 10-speed manual transmission.
1. What is happening, and what in this setup caused it?
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This is brake fade. You are keeping light, continuous pressure on the brakes down a 5% grade with 79,000 lb, so the drums and linings are being heated continuously without a chance to cool. Once the linings overheat, the friction coefficient drops and the pedal can still feel normal while the truck hardly slows. The 10th gear is too high for this grade, and the engine brake is off, so the service brakes are doing all the work. The 3-second following distance is also too short for a wet road with a heavy load; at 60 mph it is only about 264 feet, and stopping distance will grow. The smell of hot brakes is a warning that you are already past the point where light pressure will hold speed.
2. What should you have done before starting down the grade?
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Before the grade, slow down and select a gear low enough that engine compression holds the truck without continuous braking, likely several gears below 10th. Turn on the engine brake and test it. Use stab braking if the service brakes are needed: apply firmly to reduce speed by about 5 to 10 mph, release, and let the brakes cool as speed builds back slowly. Do not ride the pedal. Increase the following distance to at least 6 or 7 seconds in rain, and check the mirrors and escape options before committing to the grade. The goal is to enter at a controlled speed with the engine and transmission already helping, not to discover halfway down that the service brakes cannot hold the load.
3. What should you do now, and why should you not set the parking brake immediately after stopping?
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Downshift one gear at a time to bring engine compression in, turn on the engine brake, and use firm, short brake applications to get the speed down. Do not continue light steady pressure. If the speed still climbs, get off the road at the first safe place and stop; a runaway on a 5% grade is worse than a delay. After stopping, do not set the parking brake right away on hot drums, because the shoes can weld or the drums can crack. Let the brakes cool, chock the wheels, and inspect before moving. If the brakes are already faded, the parking brake may not hold the load either, so cooling and inspection come first.