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2.16 – Mountain Driving
In mountain driving, gravity plays a major role. On any upgrade, gravity slows you down. The steeper the grade, the longer the grade and/or the heavier the load — the more you will have to use lower gears to climb hills or mountains. In coming down long, steep downgrades, gravity causes the speed of your vehicle to increase. You must select an appropriate safe speed and then use a low gear and proper braking techniques. You should plan ahead and obtain information about any long, steep grades along your planned route of travel. If possible, talk to other drivers who are familiar with the grades to find out what speeds are safe.
You must go slowly enough so your brakes can hold you back without getting too hot. If the brakes become too hot, they may start to “fade.” This means you have to apply them harder and harder to get the same stopping power. If you continue to use the brakes hard, they can keep fading until you cannot slow down or stop at all.
2.16.1 – Select a “Safe” Speed
Your most important consideration is to select a speed that is not too fast for the:
- Total weight of the vehicle and cargo.
- Length of the grade.
- Steepness of the grade.
- Road conditions.
- Weather.
If a speed limit is posted or there is a sign indicating “Maximum Safe Speed,” never exceed the speed shown. Also, look for and heed warning signs indicating the length and steepness of the grade.
You must use the braking effect of the engine as the principal way of controlling your speed. The braking effect of the engine is greatest when it is near the governed RPMs and the transmission is in the lower gears. Save your brakes so you will be able to slow or stop as required by road and traffic conditions.
2.16.2 – Select the Right Gear Before Starting Down the Grade
Shift the transmission to a low gear before starting down the grade. Do not try to downshift after your speed has already built up. You will not be able to shift into a lower gear. You may not even be able to get back into any gear, and all engine braking effect will be lost. Forcing an automatic transmission into a lower gear at high speed could damage the transmission and also lead to loss of all engine braking effect.
With older trucks, a rule for choosing gears is to use the same gear going down a hill that you would need to climb the hill. However, new trucks have low friction parts and streamlined shapes for fuel economy. They may also have more powerful engines. This means they can go up hills in higher gears and have less friction and air drag to hold them back going down hills. For that reason, drivers of modern trucks may have to use lower gears going down a hill than would be required to go up the hill. You should know what is right for your vehicle.
Multiple-Choice Questions:
When going down a mountain, what is the principal way of controlling your speed?
- The weight of the cargo
- The braking effect of the engine
- The foot brake
- The trailer brake hand valve
Which of the following is true about gear selection when going down a mountain with a heavy load?
- With modern trucks, it's easy and safe to shift while going downhill. You can "adjust as you go" to the steepness of the mountain and road conditions. Shift up and down freely.
- Stay in high gear and start down the mountain. Once you get a feel for the steepness of the grade, shift to the proper gear.
- Shift the transmission to a low gear before starting down the grade. Do not try to downshift after your speed has already built up.
- Start in high gear but downshift as you descend until you're at 25 mph. This is the safest speed for any truck to go down a mountain.
With older trucks, a rule for choosing gears is to use the same gear going down a hill that you would need to climb the hill. However, new trucks have low friction parts and streamlined shapes for fuel economy. They may also have more powerful engines. This means they can go up hills in higher gears and have less friction and air drag to hold them back going down hills. For that reason, drivers of modern trucks may have to use lower gears going down a hill than would be required to go up the hill. You should know what is right for your vehicle.
You must go slowly enough so your brakes can hold you back without getting too hot. Describe what happens if they get too hot:
- They may start to "grab" or "bind." This means the braking force will fluctuate and the drums can begin to wobble from the inconsistent force making it hard to keep your speed down.
- They may start to "fade." This means the air pressure going to the brake chamber builds to the point that the foot valve isn't operating properly and brake application becomes inconsistent.
- They may start to "contract." This means the S-cam will twist in the chamber to the point that the shoes can not retract all the way from the drums which causes brake drag.
- They may start to “fade.” This means you have to apply them harder and harder to get the same stopping power. If you continue to use the brakes hard, they can keep fading until you cannot slow down or stop at all.
What may happen if you try to downshift after your speed has already built up while going down a mountain?
- The engine will rev higher and may go above the governed limit, which is fine as long as you're in the higher gear range. Over-revving the engine may cause damage in the low gear range, but not in the high range.
- You may not be able to shift into a lower gear. You may not even be able to get back into any gear, and all engine braking effect will be lost.
- The clutch will not operate properly at higher RPMs so you won't be able to get the truck out of gear to downshift.
- You will more easily be able to shift into a lower gear, increasing the engine braking force.
Shift the transmission to a low gear before starting down the grade. Do not try to downshift after your speed has already built up. You will not be able to shift into a lower gear. You may not even be able to get back into any gear, and all engine braking effect will be lost. Forcing an automatic transmission into a lower gear at high speed could damage the transmission and also lead to loss of all engine braking effect.
With older trucks, a rule for choosing gears is to use the same gear going down a hill that you would need to climb the hill. What is the rule for most modern trucks?
- Drivers of modern trucks may have to use lower gears going down a hill than would be required to go up the hill
- Drivers of modern trucks may have to use higher gears going down a hill than would be required to go up the hill
- Drivers of modern trucks can rely on the foot brake to get them down without worrying much about gear selection. The brakes nowadays won't fade or overheat like the old ones did.
- There is little difference between older and newer air brakes, so the same rule applies to modern trucks as it does to older trucks
With older trucks, a rule for choosing gears is to use the same gear going down a hill that you would need to climb the hill. However, new trucks have low friction parts and streamlined shapes for fuel economy. They may also have more powerful engines. This means they can go up hills in higher gears and have less friction and air drag to hold them back going down hills. For that reason, drivers of modern trucks may have to use lower gears going down a hill than would be required to go up the hill. You should know what is right for your vehicle.
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