Assessing Grades and Selecting the Right Speed
Mountain grades are rated by their steepness (percentage or degree) and length. A 6 percent grade means the road drops 6 feet for every 100 feet of horizontal distance. Common mountain pass grades range from 4 to 8 percent, with some reaching 10 percent or more. The combination of grade steepness and grade length determines how much braking energy must be managed during descent.
The critical principle of mountain descent is: select your speed and gear at the top of the grade, before you start down. Once you are on the grade and gaining speed, every second of delay in getting the truck under control makes the situation worse because speed generates more heat and reduces braking effectiveness.
The basic rule for gear selection: use the same gear descending that you would need ascending the same grade. If you would need to be in 6th gear to climb a grade, you need 6th gear to control your descent. This gear provides enough engine braking to control speed without relying heavily on the service brakes.
Speed entry at the top of the grade should be 5 to 10 mph below your target descent speed. This gives you a buffer as the truck begins to accelerate on the downgrade. If your target descent speed is 30 mph, enter the grade at 20 to 25 mph. It is much easier to let the truck accelerate slightly to your target speed than to slow down after you have already exceeded it.
Understanding and Managing Brake Temperature
Brakes convert kinetic energy (the truck's motion) into thermal energy (heat). On a long descent, the amount of energy the brakes must absorb is enormous. A fully loaded 80,000-pound truck descending 2,000 vertical feet must dissipate approximately 200 million foot-pounds of energy. If the brakes absorb too much of this energy, they overheat and fade.
Brake temperature builds cumulatively. Each brake application adds heat, and if the heat is not dissipated between applications (through airflow over the drums or rotors), the temperature climbs progressively higher. Brake fade begins at approximately 500 to 600 degrees Fahrenheit and is complete by 800 to 900 degrees, at which point the brake linings cannot grip the drums regardless of how hard you press the pedal.
The snub braking technique is the recommended method for controlling brake temperature on long descents. Apply the brakes firmly to reduce speed by 5 to 10 mph below your target speed, then release the brakes completely and let the truck accelerate back to your target speed before braking again. This technique allows the brakes to cool between applications. Each brake application is short and firm rather than continuous and light.
The worst technique is continuous light braking, also called riding the brakes. Holding light brake pressure continuously generates constant heat without the cooling periods that snub braking provides. The brake temperature climbs steadily until fade occurs. This is the most common mistake that causes runaway truck incidents on mountain grades.
Maximizing Engine Brake Effectiveness
Engine brakes (Jake brakes, exhaust brakes, compression release brakes) are the primary tool for mountain descent because they provide retarding force without generating brake heat. A properly functioning engine brake can provide 60 to 80 percent of the retarding force needed to control descent speed on most grades.
Engine brake effectiveness depends on engine RPM. Higher RPM produces more retarding force. This means selecting a gear that keeps the engine in its effective RPM range (typically 1,500 to 2,100 RPM) at your target descent speed. Too high a gear allows the engine RPM to drop below the effective range, reducing engine brake force.
Exhaust brakes (found on some engines as a supplement to compression release brakes) restrict exhaust flow to create additional backpressure that resists engine rotation. The combination of compression release and exhaust braking provides the maximum engine retarding force. Use the highest engine brake setting on steep grades.
Engine brake limitations include reduced effectiveness in lower gears at lower RPM, noise (some areas restrict engine brake use due to noise), and the inability to fully control descent on very steep or very long grades without supplemental service brake use. The engine brake does not replace the service brakes; it supplements them to reduce the total heat they must absorb.
Runaway Truck Ramps: Knowing When to Use Them
Runaway truck ramps (also called escape ramps or arrester beds) are specifically designed to safely stop trucks that have lost braking control on mountain grades. They consist of a long, uphill bed of deep, loose gravel or sand that absorbs the truck's kinetic energy and brings it to a controlled stop. Some ramps also use arrester cables or catch nets.
Use a runaway ramp without hesitation if your brakes have faded and you cannot control your speed. The ramp will damage the truck (gravel packs into the brakes, undercarriage, and drivetrain), but it will save your life, the lives of other motorists, and prevent a catastrophic collision at the bottom of the grade. No truck is worth dying for, and no load is worth killing someone.
Know the locations of runaway ramps on every mountain grade you descend. They are marked with signs counting down to the ramp entrance. On major mountain passes, ramps are positioned at intervals along the descent. Missing a ramp because you did not see the sign or hesitated too long is a life-threatening mistake.
The decision to use the ramp must be made early. As your speed increases beyond your ability to control it, the ramp becomes harder to hit because you are approaching it faster, you have less steering control, and the entrance angle becomes more critical. When you realize your brakes are fading and speed is climbing, commit to the next available ramp immediately.
Planning Mountain Descents in Advance
Research every mountain grade on your route before you encounter it. Key information includes the grade percentage, the length of the descent, the location of runaway ramps, the recommended speed and gear for your weight, and any restrictions or advisory signs.
Weight affects everything on a mountain descent. A loaded 80,000-pound truck generates far more kinetic energy than an empty 35,000-pound truck on the same grade. The gear and speed that controlled your descent at 35,000 pounds may be completely inadequate at 80,000 pounds. Adjust your descent plan based on your actual weight.
Weather compounds mountain descent difficulty. Rain reduces tire traction and makes brake drums wet (reducing friction). Snow and ice reduce traction to a fraction of dry-road levels. Fog reduces visibility of warning signs and ramp entrances. Wind can push the truck laterally on exposed mountain roads. Check weather conditions for mountain passes before your descent and be prepared to wait for conditions to improve.
Famous challenging mountain grades for truckers include Donner Pass (I-80, California), Eisenhower Tunnel approach (I-70, Colorado), Snoqualmie Pass (I-90, Washington), Lookout Pass (I-90, Montana/Idaho), Cabbage Patch (US-6, Utah), Afton Summit (US-89, Wyoming), and Teton Pass (WY-22, Wyoming). Each has specific descent challenges and recommended techniques that experienced mountain drivers know from repetition.
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