General Knowledge: Mountain Driving

14 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: Energy routing: engine pumping and friction losses as an alternative energy sink, Gear selection before a descent: shifting requires matching engine and road speed, Gravitational force component on an incline: F = m*g*sin(theta), TC 21-305-20 para 13-16, TC 21-305-20 para 13-17, TC 21-305-20 para 13-18, TC 21-305-20 para 6-33, TC 21-305-20 para 6-34, Thermal behavior of friction brakes: lining friction falls as temperature rises

What you need to know

On a descent, gravity is doing work on the vehicle the whole way down.

To hold a steady speed the brakes must get rid of that energy continuously, not just once. The steeper the grade and the heavier the load, the faster energy arrives and the hotter the brakes get. The brakes must absorb that additional energy continuously just to hold your speed.

*Why:* On a slope of angle theta the component of gravity along the direction of travel is m*g*sin(theta). Holding constant speed means net zero force, so the retarding system must supply m*g*sin(theta) continuously. The power it dissipates is P = m*g*v*sin(theta), rising with mass, speed and grade. On level ground this term is zero.

Friction brakes work by turning motion into heat.

On a long descent the heat arrives faster than it can escape, the brakes get very hot, and hot linings grip less well. The pedal still moves but the vehicle slows less, and the effect worsens the longer you keep braking. Continuous friction heats the brakes until the linings grip less well.

*Why:* A friction brake converts mechanical energy into heat at the lining-drum interface. On a sustained descent the input power m*g*v*sin(theta) is roughly constant while the cooling rate rises only with temperature difference, so temperature climbs until they balance. The friction coefficient of brake lining material falls as temperature rises past its design range, so for a given applied force the retarding torque decreases. Thermal expansion of the drum also increases the shoe travel needed.

In a low gear the engine turns faster for the same road speed and absorbs energy by pumping air and overcoming its own internal friction.

That energy never reaches the brake linings, so they stay cooler and keep working. The service brakes then handle only what the engine cannot. The service brakes then absorb less of it, which is what keeps them cool.

*Why:* Total energy to be dissipated on the descent is fixed by m*g*h. It can be split between the service brakes and the driveline. With a closed throttle the engine acts as an air pump and absorbs power that rises steeply with engine speed. A lower gear raises engine speed for the same road speed, increasing that share and reducing the share left for the friction brakes, which is what keeps lining temperature in range.

Engaging a gear needs the engine speed to match the road speed through that gear.

As the vehicle accelerates down the grade, the gear you want may put the engine above its usable range, so it will not go in. You then lose engine braking entirely and are left on the service brakes, which is the worst position on a long descent. You are then left with no gear at all and only the service brakes, on the part of the grade where they are least able to help.

*Why:* For a gear of ratio r, engine speed is proportional to road speed times r. Engaging it requires that product to lie within the engine's operating range. As road speed rises on the grade the required engine speed for a low gear can exceed the maximum, making engagement impossible. The attempt also leaves the transmission out of gear while it fails, removing all engine braking at the moment it is most needed.

A retarder applies braking force through the drive wheels only.

On a slippery surface that can break their traction and start a skid, which is why it is for descending grades on good surfaces and must be off when the road is wet, snowy, icy or slushy. The retarder acts on the drive wheels and can break their traction.

Use the engine retarder for descending grades in city traffic or in any situation where slowing is required. Do not use it on slippery road surfaces (such as rain, snow, sleet, or ice). Using the engine brake on slippery surfaces can cause the vehicle to skid.

TC 21-305-20 para 13-16

The target is holding your speed without touching the service brakes at all.

If you need them to stay at a safe speed, you are in too high a gear, and the brakes will heat up and fade. Gear selection happens as you approach the grade, not once you are on it. The engine should sit at or below the recommended RPM while it does.

Select a gear that allows the engine with the engine retarder applied to control the truck speed with the engine at or below the recommended RPM and service brakes not applied. As the vehicle operator approaches a downgrade, they should progressively select a gear that, when combined with the engine retarder, allows them to maintain an engine speed at the recommended RPM.

TC 21-305-20 para 13-18

One brake application to get back to the right RPM, release the retarder so the downshift can happen, drop a single gear, then bring the retarder back in.

Repeat if needed. Dropping one gear at a time keeps the engine in the range where the retarder actually works. Release the retarder, downshift one gear, then reapply it.

As the engine speed exceeds the recommended vehicle RPM, apply the service brakes once to slow the engine speed to the recommended RPM. Release the engine retarder. Downshift one gear (for example, if in 10th gear, downshift to 9th gear) and reapply the engine retarder.

TC 21-305-20 para 13-18

The retarder works through the driveline, so engaging it mid-shift or in neutral means it has nothing to act against.

The manual also warns that letting engine speed fall below the specified RPM with the retarder applied can seriously damage the transmission.

Never let the engine speed drop below the RPM specified in the vehicle’s TM with the engine retarder applied or it will seriously damage the transmission. Do not engage the engine retarder when shifting or when the transmission is in neutral.

TC 21-305-20 para 13-17

Two hazards that are specific to mountain roads: rock fall from water seeping into the slope above you, and pavement that has buckled because trapped water froze.

Both can appear around a blind curve, so the only defense is a speed that lets you stop for them. Both hazards come from the same source: water getting into and under the road surface. Water under the pavement also freezes and buckles it, so both hazards share one cause.

In wet weather, extra caution is necessary on mountain roads. Rocks, loosened by water seepage, may fall onto the road. Water beneath the pavement may freeze during a cold snap and cause the pavement to buckle.

TC 21-305-20 para 6-33

It turns the engine into a brake, so the work of holding your speed goes somewhere other than the service brakes, which is what keeps them cool enough to still be there at the bottom.

Many military vehicles are equipped with an engine retarder system that lets the engine act as a brake.

TC 21-305-20 para 13-16

Anywhere slowing is wanted, not just on mountains.

Using it routinely is what keeps the service brakes in reserve for the stop you did not plan. In fact anywhere slowing is required, not only on mountain grades.

Use the engine retarder for descending grades in city traffic or in any situation where slowing is required.

TC 21-305-20 para 13-16

The test is whether you need the service brakes at all.

If you do, the gear is too high and the brakes are heating with the whole grade still to come. Engine and retarder alone, with the engine at or below the recommended RPM.

Select a gear that allows the engine with the engine retarder applied to control the truck speed with the engine at or below the recommended RPM and service brakes not applied.

TC 21-305-20 para 13-18

Worse than wet.

The manual does note that a thin layer of soft snow the tires can sink through is manageable, which is a different case from packed snow or ice.

Snow and ice reduce traction even more than a wet pavement does.

TC 21-305-20 para 6-34

Off.

The retarder acts only on the drive wheels, so on a slippery surface it can slow them enough to break traction and start the skid you were trying to avoid.

The engine retarder is most effective between

TC 21-305-20 para 13-16

Check yourself

• What is the reason a downgrade demands more of your brakes?

• What is brake fade?

• What is the effect of an engine retarder on your service brakes?

• Why should the descending gear be selected before starting down a steep grade rather than part way down?

• When must you NOT use the engine retarder?

• What is the goal when selecting a gear for a downgrade?

• How do you respond when engine speed climbs above the recommended RPM on a descent?

• When must the engine retarder be disengaged?

• What are the hazards water creates on a mountain road?

• What does an engine retarder do for you?

• Where is the engine retarder useful?

• How do you know you have chosen the right gear for a descent?

• How does snow and ice compare with wet pavement for traction?

• How do you respond when the road turns slippery with your retarder on?

Answers are in the practice test for this topic.