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Commercial Driver License Handbook

Section 6: Combination Vehicles

This section is for drivers who need a Class A CDL

This section provides information needed to pass the test for Class A combination vehicles (tractor-trailers, or straight truck and trailer). The information gives you the minimum knowledge needed for driving most combination vehicles.

You should also study Section 7 if you need to pass the tests for doubles/triples.


Driving Combination Vehicles Safely

Combination vehicles are heavier, longer, and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles. In this section, we list some important safety factors that apply specifically to combination vehicles.

Rollover Risks

More than half of truck driver deaths in collisions are from truck rollovers. As more cargo is stacked in a truck, the center of gravity gets higher from the road. The truck becomes easier to turn over. Fully loaded rigs are 10 times more likely to roll over in a collision than empty rigs.

The following two things will help to prevent rollovers: keep the cargo as close to the ground as possible, and go slowly around turns. Section 3 of this handbook talks about transporting cargo safely. Keeping cargo low is even more important in combination vehicles than in straight trucks. A trailer rollover is more likely if the load is to one side. Make sure your cargo is centered and spread out as much as possible.

Rollovers happen when you turn too fast. Go slowly around corners, onramps, and offramps. Avoid quick lane changes, especially when fully loaded.

Steer Gently

Trucks with trailers have a dangerous �crack-the-whip� effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many collisions where only the trailer has overturned.

Steer carefully when you are pulling trailers. If you make a sudden movement with your steering wheel you could tip over a trailer. Follow far enough behind other vehicles (at least one second for each ten feet of vehicle length, plus another second if going over 40 mph). Look far enough down the road to avoid being surprised and having to make a sudden lane change. At night, drive slowly enough to see obstacles before it is too late to change lanes or stop gently. Slow down to a safe speed before going into a turn.

Brake Early

Control your speed whether fully loaded or empty. Large combination vehicles that are empty take longer to stop than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. When the wheels lock, your trailer can swing out and strike other vehicles or it can jackknife very quickly (Figure 6-1). You also must be very careful about driving �bobtail� tractors (tractors without semitrailers). Tests have shown that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor and semitrailer loaded to maximum gross weight.

In any combination rig, allow adequate following distance and look far enough ahead so you can brake early. Do not be caught by surprise and have to make a panic stop.

a diagram of a tractor jackknifed with the rear tractor wheels locked up or spinning and showing line of travel and direction on slide a diagram of a trailer jackknifed with trailer wheels locked and sliding and showing line of travel

Avoid Trailer Skids

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a �trailer jackknife.� This is shown in Figure 6-2. The procedure for stopping a trailer skid is as follows:

  • Recognize the skid. The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it is very difficult to prevent a jackknife.
  • Stop using the brake. Release the brakes to get traction back. Do not use the trailer hand brake to straighten out the rig. This is the wrong thing to do since it is the brakes on the trailer wheels that caused the skid in the first place. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.

Make Wide Enough Turns

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is called offtracking. Figure 6-3 shows how offtracking causes the path followed by a tractor and semitrailer to be wider than the rig itself. Longer vehicles will offtrack more. The rear wheels of the powered unit (truck or tractor) will offtrack some, and the rear wheels of the trailer will offtrack even more. If there is more than one trailer, the rear wheels of the last trailer will offtrack the most. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, other vehicles, etc. However, keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right. If you cannot complete your turn without entering another lane of traffic, turn wide as you complete the turn. This is better than swinging wide to the left before starting the turn because it will keep other drivers from passing you on the right.

a diagram of tractor/trailer off tracking in a 90 degree turn


Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to safe operation of combination vehicles. Coupling and uncoupling incorrectly can be very dangerous. There are differences between different rigs, so learn the details of coupling and uncoupling the vehicle(s) you will operate. General coupling and uncoupling steps are listed below:

Coupling Tractor/Semitrailors

Step 1. Inspect the fifth-wheel

  • For damaged or missing parts.
  • To see that mounting to tractor is secure, no cracks in frame, etc.
  • To see that the fifth-wheel plate is completely greased. Failure to keep the fifth-wheel plate lubricated could cause steering problems because of friction between the tractor and trailer.
  • To see that the fifth-wheel is in proper position for coupling.
    • the fifth-wheel should be tilted down towards the rear of the tractor with the jaws open and the safety unlocking handle in the automatic lock position.
  • To see that the sliding fifth-wheel is locked.
  • To see that the trailer kingpin is not bent or broken.

Step 2. Inspect area and chock wheels

  • To be sure the area around the vehicle is clear.
  • To be sure the trailer wheels are chocked or the spring brakes are on.
  • To see that cargo (if any) is secured against movement during coupling.

Step 3. Position tractor

  • Directly in front of the trailer. (Never back under the trailer at an angle because you might push the trailer sideways and break the landing gear.)
  • Check position, using outside mirrors, by looking down both sides of the trailer.

Step 4. Back slowly

  • Until the fifth-wheel just touches the trailer.
  • Do not hit the trailer.

Step 5. Secure the tractor

  • Set the parking brake.
  • Put the transmission in neutral.

Step 6. Check the trailer height

  • The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed.
  • To see that the kingpin and fifth-wheel are aligned.

Step 7. Connect the air lines to the trailer

  • Check glad hand seals and connect tractor emergency air line to trailer emergency glad hand.
  • Check glad hand seals and connect tractor service air line to trailer service glad hand.
  • Make sure air lines are safely supported where they won�t be crushed or caught while tractor is backing under the trailer.

Step 8. Supply air to the trailer

  • From the cab, push in the air supply knob or move tractor protection valve control from the �emergency� to the �normal� position to supply air to the trailer brake system.
  • Wait until the air pressure is normal.
  • Check brake system for crossed air lines:
    • shut engine off so you can hear the brakes.
    • apply and release trailer brakes and listen for sound of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.
    • check air brake system pressure gauge for signs of major air loss.
  • When trailer brakes are working, start the engine.
  • Air pressure must be up to normal.

Step 9. Lock the trailer brakes

  • Pull out the air supply knob or move the tractor protection valve control from normal to emergency.

Step 10. Back under the trailer

  • Use lowest reverse gear.
  • Back tractor slowly under trailer to avoid hitting the kingpin.
  • Stop when the kingpin is locked into the fifth-wheel.

Step 11. Check the connection for security

  • Raise the landing gear slightly off the ground.
  • Pull forward gently against the trailer brakes to be sure that the trailer is locked to the tractor.

Step 12. Secure the vehicle

  • Put transmission in neutral
  • Put parking brakes on.
  • Shut off the engine and take the key so someone will not move the truck.

Step 13. Inspect the coupling

  • Use a flashlight, if necessary.
  • Make sure there is no space between the upper and lower fifth-wheel.
  • Make sure the fifth-wheel jaws have closed around the shank of the kingpin. (Figure 6-4)
  • Check that the locking lever is in the �lock� position.
  • Check that the safety catch is in position over the locking lever.

Step 14. Connect the electrical cord and check air lines

  • Plug the electrical cord into the trailer and fasten the safety catch.
  • Check both air and electrical lines for signs of damage.
  • Make sure air and electrical lines will not hit any moving parts.
a diagram of trailer kingpin viewed from underneath the trailer

Step 15. Fully raise the front trailer supports (landing gear)

  • Use low gear range, if equipped, to begin raising the landing gear. Once free of weight, switch to the high gear range.
  • Raise the landing gear all the way up.
  • After raising landing gear, secure the crank handle safely.
  • When full weight of trailer is resting on tractor:
    • check for clearance between the rear of the tractor frame and the landing gear.
    • check for clearance between the top of the tractor tires and the nose of the trailer.

Step 16. Remove and store the trailer wheel chocks

Uncoupling Tractor/Semitrailer

The following steps will help you to uncouple safely:

Step 1. Position the rig

  • Make sure surface of parking area can support weight of trailer.
  • Have tractor lined up with the trailer.

Step 2. Ease the pressure on the locking jaws

  • Shut off trailer air supply to lock trailer brakes.
  • Ease pressure on fifth-wheel locking jaws by backing up gently.
  • Put parking brakes on while tractor is pushing against the kingpin.

Step 3. Chock the trailer wheels

  • Chock the trailer wheels if the trailer doesn�t have spring brakes or if you are not sure.

Step 4. Lower the landing gear

  • If trailer is empty�lower the landing gear until it makes firm contact with the ground.
  • If trailer is loaded, after the landing gear makes firm contact with the ground, turn crank in low gear a few extra turns. This will lift some weight off the tractor. This will:
    • make it easier to unlatch fifth-wheel.
    • make it easier to couple next time.

Step 5. Disconnect the airlines and electrical cable

  • Disconnect airlines from trailer.Connectair line glad hands to dummy couplers at back of cab or couple them together.
  • Hang electrical cable with plug down to prevent moisture from getting in.
  • Make sure lines are supported so they won�t be damaged while driving the tractor.

Step 6. Unlock the fifth-wheel

  • Raise the release handle lock.
  • Pull the release handle to �open� position.
  • Keep legs and feet clear of the rear tractor wheels to avoid serious injury.

Step 7. Pull the tractor partially clear of the trailer

  • Pull tractor forward until fifth-wheel comes out from under the trailer.
  • Stop with tractor frame under trailer

Step 8. Secure the tractor

  • Apply parking brake.
  • Place transmission in neutral.

Step 9. Inspect the trailer supports

  • Make sure ground is supporting trailer.
  • Make sure landing gear is not damaged.

Step 10. Pull the tractor clear of the trailer

  • Release parking brakes.
  • Check the area and drive tractor forward until it clears.

Inspecting a Combination Vehicle

Use the inspection procedures described in Sections 2 and 10 to inspect your combination vehicle. However, there are more items to inspect on a combination vehicle than on a single vehicle.

In addition to the checks already listed in Section 2, complete these checks:

Additional Items for Walkaround Inspection

Coupling system areas:

  • Fifth-wheel (lower):
    • securely mounted to frame
    • no missing, damaged parts
    • properly greased
    • no visible space between upper and lower fifth-wheel
    • locking jaws around the shank, not the head of the kingpin
    • release arm properly seated and safety latch/lock engaged
  • Fifth-wheel (upper):
    • glide plate securely mounted to trailer frame
    • kingpin not damaged
  • Air and electric lines to trailer:
    • electrical cord firmly plugged in and secured
    • air lines properly connected to gladhands, no air leaks, properly secured with enough slack for turns
    • all lines free from damage
  • Sliding fifth-wheel:
    • slide not damaged or parts missing
    • properly greased
    • all locking pins present and locked in place
    • if air powered�no air leaks
    • fifth-wheel not so far forward that tractor frame will hit landing gear, or cab hit the trailer, during turns

Landing gear:

  • Fully raised, no missing parts, not bent or otherwise damaged.
  • Crank handle in place and secured.
  • If power operated, no air or hydraulic leaks.


Section 5: Air Brakes | Table of Contents | Section 7: Doubles and Triples