Downgrade With Soft Pedal and Hot Lining Smell
You are hauling 44,000 pounds of bottled water in a 53-foot van. The gross combination weight is 78,500 pounds. The route descends a 6% grade for 2.5 miles to a curve posted 40 mph with a yellow rollover warning. The temperature is 34°F, and the pavement is wet with patches of black ice in the shade. You are in 9th gear at 45 mph with the engine brake on high. The tractor has disc brakes; the trailer has drum brakes. Your front axle gauge reads 110 psi and the trailer gauge 105 psi. Halfway down, you smell hot brake lining and the pedal feels soft, but the engine brake still holds 45 mph. A box truck is 300 feet behind you. The curve is 800 feet ahead.
1. What should your plan have been at the top of the grade, and why is 9th gear at 45 mph already a problem?
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Before starting down, I would check the brakes, set the engine brake, and select a gear low enough that engine braking alone holds a safe speed without riding the service brakes. At 78,500 pounds on a 6% grade, 9th gear at 45 mph is too high and too fast, especially with a 40-mph rollover curve ahead and wet or icy pavement. The combination already has high rollover risk from the load and the curve. I would have started down at a lower speed, likely well below 45 mph, in a lower gear, and used staged or snub braking only to control speed. The engine brake should do most of the work, and I would not enter the grade relying on the service brakes to hold 45 mph.
2. Halfway down, the pedal feels soft and you smell hot lining. What do you do?
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Treat this as brake fade and stop before the curve. Do not wait for the curve or for the box truck to pass. Keep the engine brake on, downshift if possible without overrevving, and use light, steady braking if the pedal still works. Look immediately for a safe pull-off or runaway truck ramp; if none is available, reduce speed as much as possible before the curve and be prepared to stop. Do not pump the brakes rapidly or hold hard pressure until the brakes fade completely. Once stopped, set the parking brake, chock the wheels, and let the brakes cool. Do not continue down the grade until the cause is found; hot lining and a soft pedal mean the service brakes may not be able to stop you at the curve.
3. Why is the trailer hand valve the wrong tool here?
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The trailer hand valve applies only the trailer brakes. On a slippery downgrade, that can cause the trailer wheels to lock and swing out, starting a jackknife. It also does not give you the balanced braking you need from the tractor and trailer together. The correct approach is to use the engine brake and a low gear, with careful service braking as needed. If the trailer hand valve is used at all, it should only be for testing the trailer brakes at low speed, not for controlling speed on a grade. In this situation, with a soft pedal and a curve ahead, the priority is to slow down and find a place to stop, not to use the hand valve to hold back the load.