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Section 7: Doubles and Triples

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Section 7: Doubles and Triples

This Section Covers

7.1 – Pulling Double/Triple Trailers

7.2 – Coupling and Uncoupling

7.3 – Inspecting Doubles and Triples

7.4 – Doubles/Triples Air Break Check


This section has information you need to pass the CDL knowledge test for driving safely with double and triple trailers. It tells about how important it is to be very careful when driving with more than 1 trailer, how to couple and uncouple correctly, and about inspecting doubles and triples carefully. (You should also study Sections 2, 5, and 6.)

Note: Triple combinations are not legal in California. Triples are discussed in this section because they are legal in many other states.

The endorsement is given by knowledge test only. Do not bring in a set of doubles for a Class A skills test. Drivers must demonstrate the ability to back up the combination during the skills test, and backing doubles is dangerous.

7.1 – Pulling Double/Triple Trailers

Take special care when pulling 2 and 3 trailers. There are more things that can go wrong, and doubles/triples are less stable than other commercial vehicles. Some areas of concern are discussed below.

7.1.1 – Prevent the Trailer From Rolling Over

To prevent trailers from rolling over, you must steer gently and go slowly around corners, on ramps, off ramps, and curves. A safe speed on a curve for a straight truck or a single trailer combination vehicle may be too fast for a set of doubles or triples.

7.1.2 – Beware of the Crack-the-Whip Effect

Doubles and triples are more likely to turn over than other combination vehicles because of the “crack-the-whip” effect. You must steer gently when pulling trailers. The last trailer in a combination is most likely to turn over. If you do not understand the crack-the-whip effect, study Subsection 6.1.2 of this handbook.

7.1.3 – Inspect Completely

There are more critical parts to check when you have 2 or 3 trailers. Check them all. Follow the procedures described later in this section.

7.1.4 – Look Far Ahead

Doubles and triples must be driven very smoothly to avoid rollover or jackknife. Therefore, look far ahead so you can slow down or change lanes gradually when necessary.

7.1.5 – Manage Space

Doubles and triples take up more space than other commercial vehicles. They are not only longer, but also need more space because they cannot be turned or stopped suddenly. Allow more following distance. Make sure you have large enough gaps before entering or crossing traffic. Be certain you are clear at the sides before changing lanes.

7.1.6 – Adverse Conditions

Be more careful in adverse conditions. In bad weather, slippery conditions, and mountain driving, you must be especially careful if you drive double and triple bottoms. You will have greater length and more dead axles to pull with your drive axles than other drivers. There is more chance for skids and loss of traction.

7.1.7 – Parking the Vehicle

Make sure you do not get in a spot you cannot pull straight through. You need to be aware of how parking lots are arranged in order to avoid a long and difficult escape.

7.1.8 – Antilock Braking Systems on Converter Dollies

Converter dollies built on or after March 1, 1998, are required to have antilock brakes. These dollies have a yellow lamp on the left side of the dolly.

7.2 – Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to safe operation of doubles and triples. Wrong coupling and uncoupling can be very dangerous. Coupling and uncoupling steps for doubles and triples are listed below.

7.2.1 – Coupling Double Trailers

Secure the Second (Rear) Trailer

If the second trailer does not have spring brakes, drive the tractor close to the trailer, connect the emergency line, charge the trailer air tank, and disconnect the emergency line. This will set the trailer emergency brakes (if the slack adjusters are correctly adjusted). Chock the wheels if you have any doubt about the brakes.

For the safest handling on the road, the more heavily loaded semitrailer should be in first position behind the tractor. The lighter trailer should be in the rear.

A converter gear on a dolly is a coupling device of 1 or 2 axles and a fifth-wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig. See Figure 7.1.

Image of Converter Gear on a Dolly

Figure 7.1

 

Position the Converter Dolly in Front of the Second (Rear) Trailer

  • Release the dolly brakes by opening the air tank petcock. (Or, if the dolly has spring brakes, use the dolly parking brake control.)
  • If the distance is not too great, wheel the dolly into position by hand so it is in line with the kingpin.
  • Or, use the tractor and first semitrailer to pick up the converter dolly:
    — Position the combination as close as possible to the converter dolly.
    — Move the dolly to the rear of the first semitrailer and couple it to the trailer.
    — Lock the pintle hook.
    — Secure the dolly support in the raised position.
    — Pull the dolly into position as close as possible to the nose of the second semitrailer.
    — Lower the dolly support.
    — Unhook the dolly from the first trailer.
    — Wheel the dolly into position in front of the second trailer in line with the kingpin.

Connect the Converter Dolly to the Front Trailer

  • Back the first semitrailer into position in front of the dolly tongue.
  • Hook the dolly to the front trailer.
    — Lock the pintle hook.
    — Secure the converter gear support in the raised position.

Connect the Converter Dolly to the Rear Trailer

  • Make sure the trailer brakes are locked and/or the wheels chocked.
  • Make sure the trailer height is correct. (It must be slightly lower than the center of the fifth-wheel, so the trailer is raised slightly when the dolly is pushed under.)
  • Back the converter dolly under the rear trailer.
  • Raise the landing gear slightly off the ground to prevent damage if the trailer moves.
  • Test the coupling by pulling against the pin of the second semitrailer.
  • Make visual check of the coupling. (No space between the upper and lower fifth-wheel and the locking jaws are closed on the kingpin.)
  • Connect the safety chains, air hoses, and light cords.
  • Close the converter dolly air tank petcock and shut-off the valves at the rear of the second trailer (service and emergency shut-offs).
  • Open the shut-off valves at the rear of the first trailer (and on the dolly, if so equipped).
  • Raise the landing gear completely.
  • Charge the trailer brakes (push the “air supply” knob in), and check for air at the rear of the second trailer by opening the emergency line shut-off. If the air pressure is not there, something is wrong and the brakes will not work.

7.2.2 – Uncoupling the Double Trailers

Uncouple the Rear Trailer

  • Park the rig in a straight line, on firm level ground.
  • Apply the parking brakes so the rig will not move.
  • Chock the wheels of the second trailer if it does not have spring brakes.
  • Lower the landing gear of the second semitrailer enough to remove some weight from the dolly.
  • Close the air shut-offs at the rear of the first semitrailer (and on the dolly, if so equipped).
  • Disconnect and secure all dolly air and electric lines.
  • Release the dolly brakes.
  • Release the converter dolly fifth-wheel latch.
  • Slowly pull the tractor, first semitrailer, and dolly forward to pull the dolly out from under the rear semitrailer.

Uncouple the Converter Dolly

  • Lower the dolly landing gear.
  • Disconnect the safety chains.
  • Apply the converter gear spring brakes or chock the wheels.
  • Release the pintle hook on the first semi-trailer.
  • Slowly pull clear of the dolly.

Never unlock the pintle hook with the dolly still under the rear trailer. The dolly tow bar may fly up, possibly causing injury, and making it very difficult to re-couple.

7.2.3 – Coupling and Uncoupling Triple Trailers

Couple the Tractor/First Semitrailer to the Second/ Third Trailers

  • Couple the tractor to the first trailer. Use the method already described for coupling tractor-semitrailers.
  • Move the converter dolly into position and couple the first trailer to the second trailer using the method for coupling doubles. The triples rig is now complete.

Uncouple the Triple-Trailer Rig

  • Uncouple the third trailer by pulling the dolly out, then unhitching the dolly using the method for uncoupling doubles.
  • Uncouple the remainder of the rig as you would any double-bottom rig using the method already described.

REMEMBER: Operating triples is not allowed in California.

7.2.4 – Coupling and Uncoupling Other Combinations

The methods described so far apply to the more common tractor-trailer combinations. However, there are other ways of coupling and uncoupling the many types of truck-trailer and tractor-trailer combinations that are in use. There are too many to cover in this handbook. You will need to learn the correct way to couple and uncouple the vehicle(s). Drive according to the manufacturer and/or owner specifications.

7.3 – Inspecting Doubles and Triples

Use the 7-step inspection procedure described in Section 2 to inspect your combination vehicle. There are more things to inspect on a combination vehicle than on a single vehicle. Many of these items are simply more of what you would find on a single vehicle. (For example, tires, wheels, lights, reflectors, etc.) There are also some new things to check. These are discussed below.

7.3.1 – Additional Checks

Do these checks in addition to those already listed in Section 2, Step 5: Do Walk Around Inspection.

Coupling System Areas

  • Check the fifth-wheel (lower).
    — Is securely mounted to the frame.
    — Has no missing or damaged parts.
    — Has enough grease.
    — Has no visible space between the upper and lower fifth-wheel.
    — Locking jaws are around the shank, not the head of the kingpin.
    — Release arm is properly seated and safety latch/lock is engaged.
  • Check the fifth-wheel (upper).
    — Glide plate is securely mounted to the trailer frame.
    — Kingpin is not damaged.
  • Air and electric lines to the trailer.
    — The electrical cord firmly plugged in and secured.
    — The air lines properly connected to the glad hands, no air leaks, and are properly secured with enough slack for turns.
    — All lines are free from damage.
  • Sliding the fifth-wheel.
    — Slide is not damaged or parts missing.
    — Is properly greased.
    — All locking pins are present and locked in place.
    — If air powered, there are no air leaks.
    — Check that the fifth-wheel is not so far forward that the tractor frame will hit the landing gear, or the cab will hit the trailer, during turns.

Landing Gear

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

Double and Triple Trailers

  • Shut-off the valves (at the rear of the trailers, in service and emergency lines).
    — The rear of the front trailers: OPEN.
    — The rear of the  last trailer: CLOSED.
    — The converter dolly air tank drain valve: CLOSED.
  • Be sure the air lines are supported and the glad hands are properly connected.
  • If the spare tire is carried on the converter gear (dolly), make sure it is secured.
  • Be sure the pintle-eye of the dolly is in place in the pintle hook of the trailer(s).
  • Make sure the pintle hook is latched.
  • The safety chains should be secured to the trailer(s).
  • Be sure the light cords are firmly connected in the sockets on the trailers.

7.3.2 – Additional Things to Check During a Walk Around Inspection

Do these checks in addition to Section 5, Inspecting Air Brake Systems.

7.4 – Doubles/Triples Air Brake Check

Check the brakes on a double or triple trailer as you would any combination vehicle. Section 6 explains how to check air brakes on combination vehicles. You must also make the following checks on your double or triple trailers.

7.4.1 – Additional Air Brake Checks

  • Check That Air Flows to all the Trailers (Double and Triple Trailers). Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for the air pressure to reach normal, and then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), and then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly(ies) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.
  • Test the Tractor Protection Valve. Charge the trailer air brake system (build up normal air pressure and push the “air supply” knob in). Shut the engine off. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from the “normal” to“emergency”position)when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).
  • If the tractor protection valve does not work properly, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.
  • Test the Trailer Emergency Brakes. Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called tractor protection valve control or trailer emergency valve) or place it in the “emergency” position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.
  • Test the Trailer Service Brakes. Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve, but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)


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Section 7


Test Your Knowledge


  1. What is a converter dolly?
  2. Do converter dollies have spring brakes?
  3. What 3 methods can you use to secure a second trailer before coupling?
  4. How do you check to make sure trailer height is correct before coupling?
  5. What do you check when making a visual check of coupling?
  6. Why should you pull a dolly out from under a trailer before you disconnect it from the trailer in front?
  7. What should you check for when inspecting the converter dolly? The pintle hook?
  8. Should the shut-off valves on the rear of the last trailer be open or closed? On the first trailer in a set of doubles? On the middle trailer of a set of triples?
  9. How can you test that air flows to all trailers?
  10. How do you know if your converter dolly is equipped with antilock brakes?

These questions may be on your test. If you cannot answer them all, re-read Section 7.

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