Braking a Triple on a 6% Grade
You are driving a 75,000-pound set of three 28-foot trailers down a 6% grade on I-80 near the Delaware Water Gap. The posted limit is 45 mph and you are at 58 mph. You have been riding the service brakes to hold speed because the engine brake is set on low. The tractor’s brakes begin to smell hot, and the trailer brake hand valve does not seem to hold the set on the grade. The next runaway truck ramp is 1 mile ahead. Traffic is light behind you. Your front trailer brake pressure gauge shows normal; the rear trailer brakes were adjusted yesterday. The road curves right at the bottom. The shoulder is narrow and there is no exit before the ramp.
1. What braking technique should you have used from the top of the grade, and what should you do now?
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From the top I should have selected a lower gear, set the engine brake on high, and used snub braking: apply the service brakes firmly enough to drop about 5–10 mph, release completely, let the engine brake hold, and repeat before speed builds. I should not ride the brakes at 58 mph on a 6% grade.
Now: get off the service brakes immediately, engage the engine brake on high, downshift to a gear that holds the set near 45 mph, and use short, firm snubs only as needed. If speed is still climbing, or the brakes are faded, use the runaway ramp one mile ahead rather than trying to save the stop. Do not continue to the curve at 58 mph.
2. If the rear trailer brakes fail completely, how should you bring the combination to a stop?
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If the rear trailer brakes fail completely, the trailer hand valve will not help-it controls trailer brakes that are not working. I would keep the tractor brakes smooth and progressive, use the engine brake and a lower gear, and avoid a hard, sudden stop that can lock the tractor and let the trailers swing. I would brake earlier and longer, keep the combination straight, warn traffic, and use the runaway ramp if I cannot control speed. If I must stop on the grade, I would choose the flattest, straightest spot and use the parking brakes only after stopping.
3. Why is continuous light brake application especially dangerous with three trailers?
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Continuous light brake application is dangerous because it heats the drums and glazes the linings, causing fade just when you need braking. With three trailers, the tractor brakes and front trailer brakes can do more than their share; the rear trailers can push, and if one set brakes harder than another the trailers can swing or jackknife. A light, steady application also gives you no reserve for an emergency. Snubbing keeps the brakes cooler and gives a margin.