Section 7: Doubles and Triples

This section is for drivers who tow doubles or triples

This section has information you will need to pass the CDL knowledge test for driving safely with double/triple trailers. 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 for doubles is given by written test only. Do not bring in a set of doubles for the driving test. Drivers must demonstrate the ability to back up the combination during the skills test, and backing doubles is dangerous.

A Doubles/Triples Endorsement is needed.


Towing Double/Triple Trailers

Take special care when towing two or three trailers. There are more things that can go wrong and doubles/triples are less stable than other commercial vehicles.

Prevent Trailer Rollover

To prevent trailers from rolling over, you must steer gently and go slowly around corners, onramps, offramps, 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.

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.

Inspect Completely

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

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

Manage Space

Doubles and triples take up more space than other commercial vehicles. They are not only longer, but also need more space on the road because they can't 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. Look far ahead so you can slow down or change lanes gradually when necessary.

Adverse Conditions

Be more careful in bad weather conditions. In bad weather, slippery conditions, and mountain driving you must be especially careful if you drive double or 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. You should never disable the steering axle brakes.

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.

Antilock Braking Systems on Converter Dollies

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


Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to safe operation of doubles and triples. Incorrect coupling and uncoupling can be very dangerous. Couple the tractor and first semitrailer as described in Section 6.

Coupling Twin Trailers

Secure the second or 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.

Note: For safe handling on the road, the more heavily loaded semitrailer must always be in the first position behind the tractor. The lighter trailer should be in the rear.

A converter gear or dolly is a coupling device of one or two axles and a fifth-wheel by which a semitrailer can be coupled to the rear of a tractortrailer combination forming a double bottom rig.

Position the converter dolly in front of the second or 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 possible, 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 combination as close as possible to converter dolly
    • move dolly to rear of first semitrailer and couple it to the trailer
    • lock pintle hook
    • secure dolly support in raised position
    • pull dolly into position as close as possible to nose of the second semitrailer
    • lower dolly support
    • unhook dolly from first trailer
    • wheel dolly into position in front of second trailer in line with the kingpin

Connect the converter dolly to the front trailer:

  • Back first semitrailer into position in front of the dolly tongue.
  • Hook dolly to front trailer:
    • lock pintle hook
    • secure converter gear support in raised position

Connect the converter dolly to the rear trailer:

  • Lock trailer brakes and/or chock wheels.
  • Make sure 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 converter dolly under rear trailer.
  • Raise landing gear slightly off ground.
  • Test coupling by pulling against pin of rear trailer.
  • Check coupling and locking jaws.
  • Connect safety chains, air hoses, and electrical cords.
  • Close converter dolly air tank petcock, and shut-off valves at rear of second trailer.
  • Open shut-off valves at rear of first trailer and on the dolly, if so equipped.
  • Raise the landing gear completely.
  • Charge trailers and check for air at the rear of the second trailer by opening the emergency line shut-off.
Uncoupling Double Trailers

Uncouple rear trailer:

  • Park rig in a straight line.
  • Apply parking brakes.
  • Chock wheels of the second trailer.
  • Lower the landing gear of the second semitrailer enough to remove some weight from the dolly.
  • Close air shut-offs at rear of the first semitrailer and on the dolly, if so equipped.
  • Disconnect all dolly air and electric lines and secure them.
  • Release dolly brakes.
  • Release converter dolly fifth-wheel latch.
  • Slowly pull tractor, first semitrailer and dolly forward to pull dolly out from under rear semitrailer.
Uncouple Converter Dolly:
  • Lower dolly landing gear.
  • Disconnect safety chains.
  • Apply converter gear spring brakes or chock wheels.
  • Release pintle hook on first semitrailer.
  • Slowly pull clear of dolly.

CAUTION: 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.

Coupling and Uncoupling Triple Trailers

Couple second and third trailers:

  • Couple second and third trailers using the method for coupling doubles.
  • Uncouple tractor and pull away from the second and third trailers.

Couple tractor/first semitrailer to second/ third trailers:

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

Uncouple triple trailer rig:

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

REMEMBER: Operating triples is not allowed in California.

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. Learn the right way to couple the vehicle(s) you will drive according to the manufacturer and/or vehicle owner.


Inspecting Doubles and Triples

There are more items 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. However, there are also some new items to check. These are discussed below.

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 glad hands, 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.

Double and triple trailers:

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

Doubles/Triples Air Brake Check

Check the brakes on a double or triple trailer as you would any combination vehicle. Refer to the information in Section 5 to learn how to check air brakes on combination vehicles.

Additional Air Brake Checks
  • Check that air flows to all trailers (double and triple trailers). Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for 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 tractor protection valve. Charge the trailer air brake system. (That is, 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 "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 doesn't 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 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 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).


Section 6: Combination Vehicles | Table of Contents | Section 8: Tank Vehicles