CDL · General Knowledge · 6 min read
Controlling Speed on Steep Grades
- Identify how gravity affects commercial vehicles on hills and select an appropriate safe speed using route, vehicle, and weather factors.
- Explain the principles of engine braking, transmission gear selection, and how modern trucks differ from older trucks on downgrades.
- Apply the proper downgrade braking technique and describe how escape ramps protect drivers, cargo, and equipment.
Gravity and Selecting a Safe Speed
Gravity plays a powerful role whenever a commercial vehicle travels on hills. When you drive up any upgrade, gravity naturally works against the vehicle, slowing it down. When you head down long, steep downgrades, gravity causes vehicle speed to increase. Because commercial motor vehicles carry significant weight, gravity pulls them downhill with tremendous force. Climbing hills requires lower gears when carrying heavier loads, and steeper and longer grades also demand lower gears to reach the crest.
Before starting down any steep grade, you must select an appropriate safe speed, get into a low gear, and prepare to apply proper braking techniques. To choose the right speed, you must plan ahead and obtain information about long, steep grades along your route. If possible, talk to drivers who are familiar with those specific grades to find out what speeds are considered safe. For CDL knowledge-test preparation, know the factors the manual lists for selecting a safe downhill speed.
Your safe downhill speed must allow you to travel slowly enough so your vehicle brakes can hold the vehicle back without getting too hot. To determine that safe speed, you must evaluate several critical factors: the total weight of the vehicle and cargo, the length of the grade, the steepness of the grade, current road conditions, and weather conditions. A heavily loaded tractor-trailer descending a steep, wet grade requires a much lower speed than a lighter truck on a dry, gentle slope.
Always observe highway signage while driving in hilly terrain. You must heed warning signs that indicate the length and steepness of a grade ahead. Furthermore, you must never exceed a posted speed limit or a posted Maximum Safe Speed sign on any grade. If a speed limit or Maximum Safe Speed is posted, never exceed the speed shown.
Common mistakes
- Select a low gear before the downgrade: once speed builds, you may be unable to shift into a lower gear or even get back into any gear, losing all engine braking effect.
- Using the brakes too much instead of relying on engine braking can generate excessive heat, make brake linings wear faster, and cause brake fade or failure.
- Do not assume the climbing gear is right for descending in a modern truck; its low-friction parts and streamlined shape may mean it needs a lower gear going down.
- Check brake adjustment frequently: brakes that are out of adjustment stop doing their share of the work, and the other brakes can overheat and fade.
Key points
- Gravity slows vehicles on upgrades and causes speed to increase on downgrades, requiring low gears and planned safe speeds.
- Safe downhill speed depends on total vehicle and cargo weight, grade length and steepness, road conditions, and weather conditions.
- Never exceed posted speed limits or Maximum Safe Speed signs, and always shift into the correct low gear before starting down a grade.
- Modern trucks may have low-friction parts, streamlined shapes, and more powerful engines, so they may require a lower gear to descend than to climb.
- Braking friction creates heat, leading to brake fade and failure; every brake must be properly adjusted to do its share of the work.
- The proper downhill braking technique uses engine braking as primary control, supplementing with firm three-second brake applications to drop speed 5 mph below safe speed.
- Escape ramps use beds of loose, soft material, sometimes combined with an upgrade, to safely stop runaway vehicles and save lives, cargo, and equipment.
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