Combination Vehicles: Combination Operation

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

Sources: 49 CFR 393.43, 49 CFR 393.49, TC 21-305-20 para 13-14, TC 21-305-20 para 13-15, TC 21-305-20 para 13-26, TC 21-305-20 para 13-28, TC 21-305-20 para 13-29, TC 21-305-20 para 13-30, TC 21-305-20 para 13-31, TC 21-305-20 para 13-33

What you need to know

The hinge at the fifth wheel means the trailer follows its own path through a turn and responds in the opposite sense when backing.

Overall length also changes how long you need to pass and clear a turn.

When driving a towing vehicle and semitrailer, keep in mind the overall length of the unit when passing other vehicles and when turning. Remember, the unit is hinged in the middle and therefore turns and backs differently than a truck

TC 21-305-20 para 13-28

Off-tracking means the trailer wheels cut inside the path of the tractor.

Keeping the front out toward the centreline on a right-hand curve gives the trailer wheels room so they do not drop off the pavement edge.

When driving a tractor/semitrailer, off-tracking must be allowed for. Therefore, when entering a right-hand curve, position the front of the vehicle as close to the centerline as possible (without letting the semitrailer tire cross the centerline) to keep the semitrailer tires from running off the pavement.

TC 21-305-20 para 13-29

The trailer cuts inside again, so on a left-hand curve that means toward the centreline and into oncoming traffic.

Running the tractor out near the shoulder buys the trailer the room it needs. Without running off the road.

When driving into a left-hand curve, move as close as possible to the shoulder of the road (without running off the road) to keep the semitrailer tires from crossing the centerline.

TC 21-305-20 para 13-29

Braking in a curve asks the tires for braking grip and cornering grip at once, and there is only one budget.

On a combination it can also break the trailer wheels loose and start a jackknife.

Do not brake while in a curve. Brake before entering the curve.

TC 21-305-20 para 13-29

To send the trailer left you first turn the wheel right, which is the opposite of a straight truck.

Then you follow the trailer with the tractor to straighten the combination.

Back a semitrailer by reversing the direction that is used to back a truck.

TC 21-305-20 para 13-30

Backing to the left keeps the trailer on the driver's own side of the cab where it can be watched directly.

The manual says to use it whenever possible, because a blind-side maneuver relies on mirrors alone. That keeps the maneuver on the side you can see out of your own window.

Turn the steering wheel to the right until the trailer is headed in the desired direction. Then turn the tractor wheels to the left to put the tractor on the same line of travel as the semitrailer. This method is known as backing sight side

TC 21-305-20 para 13-31

Locked rear tractor wheels cannot resist sideways force, so the back of the tractor comes around and the trailer pushes it.

Steering in the direction of the skid points the tractor where it is actually going. Steer in the direction of the skid to recover.

Tractor jackknifing occurs when the rear tractor wheels lock and skid to the left or right. To gain control, steer in the direction of the skid.

TC 21-305-20 para 13-14

A rolling wheel can generate the sideways grip that pulls the trailer back behind the tractor; a locked one cannot.

The manual warns control must be regained immediately or not at all. That gets the wheels turning, and a rolling wheel can pull the trailer back in line.

Semitrailer jackknifing occurs when the semitrailer wheels lock and skid to the left or right. Control must be regained immediately if it is to be regained at all. To regain control, quickly release the brakes to get the wheels turning.

TC 21-305-20 para 13-15

Braking the trailer alone while the tractor keeps rolling is exactly the condition that produces a trailer skid and a jackknife.

Its only legitimate uses are holding against roll on an incline and coupling. Which is exactly how you make the trailer skid or jackknife.

DO NOT use the semitrailer hand brake in driving because this can cause the trailer to skid or jackknife.

TC 21-305-20 para 13-15

How the load sits changes how much vertical force is on each set of tires, and therefore how much braking each can deliver before locking.

The same brake application stops a differently loaded combination differently.

The distribution of weight will affect stopping the vehicle.

TC 21-305-20 para 13-28

393.49(a) requires one application valve to operate all service brakes, then states expressly that this does not prohibit an additional valve for trailer brakes.

That additional valve is the trailer hand brake, whose use while driving the Army manual warns against.

This requirement must not be construed to prohibit motor vehicles from being equipped with an additional valve to be used to operate the brakes on a trailer or trailers

49 CFR 393.49

393.43(a) requires the towing vehicle to retain the ability to stop itself on its service brakes after a breakaway.

The tractor protection valve is what delivers this, by sealing the tractor's air from the open trailer lines.

Every motor vehicle, if used to tow a trailer equipped with brakes, shall be equipped with a means for providing that in the case of a breakaway of the trailer, the service brakes on the towing vehicle will be capable of stopping the towing vehicle.

49 CFR 393.43

Levers down is open and levers up is closed on the lever type.

The handle type is the opposite convention, vertical up being open, which is why you check the actual valve rather than assume.

Open the lever shut-off valves; place the levers in the down position.

TC 21-305-20 para 13-26

Para 13-33 gives the reason: air leaks away over time and the brake releases.

This is the same logic behind FMVSS 121 S5.6.3.2 requiring a MECHANICAL means to hold a parking brake after all air bleeds to zero. Which is why it is never used for parking.

Do not use the trailer hand brake control as a parking brake. Using the semitrailer hand control to park can cause all the air to leak out.

TC 21-305-20 para 13-33

Check yourself

• What is the key handling difference between your tractor-semitrailer and a straight truck?

• Entering a right-hand curve in your tractor-semitrailer, where should the front of the vehicle be placed?

• Entering a left-hand curve, where should your tractor be placed?

• What is the manual's rule on braking in a curve with your combination vehicle?

• How does steering input when backing your semitrailer compare with backing a straight truck?

• Backing sight side means ______.

• What is tractor jackknifing and how is control regained?

• How do you recover when the semitrailer wheels lock?

• Why must your trailer hand brake not be used while you drive?

• What does the manual say weight distribution affects in your combination vehicle?

• Does the rule permit a separate trailer brake valve in addition to the main brake control?

• What must your towing vehicle still be able to do if your trailer breaks away?

• On your tractor with lever-type air shut-off valves, which position is open?

• Why must your trailer hand brake not be used to park?

Answers are in the practice test for this topic.