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Doubles and triples practice test

The doubles and triples test earns the T endorsement. It builds on the combination vehicles test with the extra risks of multiple trailers: the crack-the-whip effect, coupling a converter dolly, and checking the air system across every trailer. The sample questions below come from Section 7 of the FMCSA CDL manual.

Free doubles/triples practice questions

Ten questions from the PassRig bank. Pick an answer in your head, then show the answer to read why. The full bank, timed mock exams and AI explanations live in the app.

  1. The "crack-the-whip" effect is most dangerous for which vehicle?

    • A straight truck
    • A single semitrailer
    • The rear trailer of a set of triples
    • A bobtail tractor
    Show answer
    Answer: C. The rear trailer of a set of triples

    Rearward amplification grows with every added trailer. A quick steering move at the tractor can swing the last trailer of a triple far enough to roll it over. Steer gently and leave room.

    Source: FMCSA CDL Manual, Section 7.1 (Pulling Double/Triple Trailers: Prevent Trailer from Rolling Over)

  2. In a set of doubles, which trailer should be directly behind the tractor?

    • The lighter one
    • The heavier one
    • It makes no difference
    • The empty one
    Show answer
    Answer: B. The heavier one

    Putting the heavier trailer first and the lighter one at the rear keeps the combination stable. A light rear trailer is the one most likely to whip and roll, so do not load it heavy.

    Source: FMCSA CDL Manual, Section 7.2.1 (Coupling Twin Trailers: Position Converter Dolly)

  3. If you cross the air lines between a tractor and an old trailer with no spring brakes, what happens?

    • Nothing; the lines are interchangeable
    • You could drive away with no trailer brakes at all
    • The tractor brakes lock
    • The trailer brakes apply and will not release
    Show answer
    Answer: B. You could drive away with no trailer brakes at all

    Crossed lines send supply air to the service line, so the trailer tanks never charge. On a trailer with spring brakes the brakes stay locked and you notice. On an older trailer without spring brakes you could pull out with no trailer brakes. Always check the glad hands and do a brake test.

    Source: FMCSA CDL Manual, Section 7.2 and 6.2 (Air Line Connections)

  4. When the service line glad hands on the dolly are connected, you should also:

    • Disconnect the emergency line
    • Make sure the air and electrical lines will not catch or rub when the rig turns
    • Tie the lines to the drawbar with wire
    • Leave extra slack dragging on the ground
    Show answer
    Answer: B. Make sure the air and electrical lines will not catch or rub when the rig turns

    Lines that catch on the dolly or landing gear in a turn will tear. Route them so they have slack for turning but cannot snag or drag.

    Source: FMCSA CDL Manual, Section 7.2.1 (Coupling Twin Trailers: Connect Air and Electrical Lines)

  5. A sudden movement of the steering wheel in a set of doubles or triples can:

    • Affect only the tractor
    • Help settle a swaying trailer
    • Apply the trailer brakes
    • Tip over a trailer, so you must steer gently
    Show answer
    Answer: D. Tip over a trailer, so you must steer gently

    Quick steering starts the crack-the-whip effect. Steer smoothly and look far enough ahead that you never need a sudden move.

    Source: FMCSA CDL Manual, Section 7.1.1 (Prevent Trailer from Rolling Over)

  6. To move a converter dolly into position by hand, you first release its brakes by:

    • Pushing in the tractor's trailer air supply knob
    • Disconnecting the safety chains
    • Pulling the pintle hook release
    • Opening the air tank petcock, or caging the spring brakes if it has them
    Show answer
    Answer: D. Opening the air tank petcock, or caging the spring brakes if it has them

    The dolly's brakes hold it still. Draining the dolly air tank, or caging its spring brakes, lets it roll so you can wheel it in front of the second trailer.

    Source: FMCSA CDL Manual, Section 7.2.1 (Coupling Twin Trailers: Position Converter Dolly)

  7. After coupling the dolly to the second trailer, a visual check should show:

    • No space between the upper and lower fifth wheel, and the locking jaws closed around the kingpin
    • A one-inch gap between the fifth wheel plates
    • The kingpin resting on top of the jaws
    • The landing gear still touching the ground
    Show answer
    Answer: A. No space between the upper and lower fifth wheel, and the locking jaws closed around the kingpin

    A gap or open jaws mean the trailer is not locked on. Pulling against the pin tests the lock, but you must also look to confirm it.

    Source: FMCSA CDL Manual, Section 7.2.1 (Coupling Twin Trailers: Connect Converter Dolly to Rear Trailer)

  8. Never unlock the pintle hook while the converter dolly is still under the rear trailer because:

    • The trailer brakes will release
    • The air tank will drain
    • The tractor will stall
    • The dolly tow bar may fly up, injuring you and making it very hard to re-couple
    Show answer
    Answer: D. The dolly tow bar may fly up, injuring you and making it very hard to re-couple

    The trailer's weight loads the dolly tongue. Release the pintle hook only after the dolly has been pulled clear and its landing gear lowered.

    Source: FMCSA CDL Manual, Section 7.2.2 (Uncoupling Twin Trailers: Uncouple Converter Dolly)

  9. During the inspection of doubles or triples, you should check that the pintle hook is:

    • Latched, with the safety chains secured
    • Unlatched so it can swivel
    • Greased and left open
    • Replaced each trip
    Show answer
    Answer: A. Latched, with the safety chains secured

    The pintle hook carries the whole rear trailer. If it is not latched and chained, the trailer can break away.

    Source: FMCSA CDL Manual, Section 7.3 (Inspecting Doubles and Triples: Coupling System)

  10. To test the trailer service brakes, you:

    • Pull the trailer air supply knob at highway speed
    • Open the shut-off valves at the rear of the last trailer
    • Chock the wheels and listen for air
    • Release the parking brakes, move forward slowly, and apply the trailer brakes with the hand control; you should feel them come on
    Show answer
    Answer: D. Release the parking brakes, move forward slowly, and apply the trailer brakes with the hand control; you should feel them come on

    Feeling the trailers brake when you use the hand valve shows the service brakes are connected and working all the way back.

    Source: FMCSA CDL Manual, Section 7.4.4 (Test Trailer Service Brakes)

What the doubles/triples test covers

  • Pulling double and triple trailers: rollover and crack-the-whip
  • Coupling and uncoupling twin trailers and converter dollies
  • Inspecting doubles and triples
  • Checking the air brakes on multiple trailers
  • Managing space and following distance with long combinations

Who needs it

Drivers who want the T endorsement to pull two or three trailers. You need a Class A CDL first.

The test has 20 multiple-choice questions and the passing score is 80%, so you can miss 4. Your state DMV or DOT administers it; the federal rules are on the FMCSA CDL page. Study Section 7 of the CDL manual alongside these questions.

More CDL practice tests

Doubles/Triples test questions

How many questions are on the doubles and triples test?

20 questions. 16 correct answers reach the 80% pass mark.

Do I need the combination vehicles test before doubles and triples?

Yes. The T endorsement sits on top of a Class A CDL, and Class A already requires the combination vehicles knowledge test.

Are triples legal everywhere?

No. Several states do not allow triples at all, and routes are restricted. The endorsement test is the same, but check your state DMV or DOT and the carrier's route rules.