U.S. CDL · DOUBLES & TRIPLES · QUESTIONS & ANSWERS

U.S. CDL Doubles and Triples Practice — questions and answers

All 30 original Rig Toolbox practice questions for this test, printed in full with the correct answer and an explanation for each one. Read them here, then put them to work in Practice or the Exam Simulator — no sign-up, ever.

Start Practice Exam Simulator Quick Review

Independent practice: these are Rig Toolbox's own study questions, not official exam questions. Always confirm rules in the official handbook for the licence you are taking.

About this question set

How many questions are here? 30 — the full bank for this test. The interactive test draws 20 of them at a time.

What is this test? Original doubles and triples practice covering converter dollies, coupling order and safe handling, based on state CDL manuals.

Every question, with its answer

Question 1

When building a set of doubles, the HEAVIER semitrailer should be placed:

  1. At the very back
  2. On the dolly
  3. It makes no difference
  4. In front, directly behind the tractor

Correct answer: In front, directly behind the tractor

The heavier trailer in the lead position keeps the set stable and reduces the crack-the-whip effect on the rear trailer.
Question 2

In a set of triples, how should the trailers be ordered by weight?

  1. Heaviest nearest the tractor, then lighter trailers behind it
  2. Lightest first
  3. Random order
  4. All weight in the last trailer

Correct answer: Heaviest nearest the tractor, then lighter trailers behind it

Weight steps down toward the rear so the most whip-prone last trailer is the lightest.
Question 3

What is the first step when coupling a set of doubles?

  1. Couple the tractor to the rear trailer
  2. Raise all landing gear
  3. Secure the rear (second) trailer against movement with its spring brakes or wheel chocks
  4. Connect the dolly air lines

Correct answer: Secure the rear (second) trailer against movement with its spring brakes or wheel chocks

The rear trailer must be held still before anything is backed toward it.
Question 4

A converter dolly connects to the lead trailer using:

  1. The trailer’s landing gear
  2. Safety chains alone
  3. A drawbar ring (eye) locked into a pintle hook, with a safety latch
  4. A fifth wheel only

Correct answer: A drawbar ring (eye) locked into a pintle hook, with a safety latch

The dolly’s fifth wheel carries the rear trailer while its drawbar eye locks into the pintle hook on the trailer ahead.
Question 5

What must be checked on the pintle hook connection after coupling the dolly?

  1. That it is painted
  2. That the chains are removed
  3. That the hook is latched around the drawbar eye and the safety latch is secured
  4. That the dolly tank is empty

Correct answer: That the hook is latched around the drawbar eye and the safety latch is secured

An unlatched pintle hook can let the dolly and rear trailer separate on the road.
Question 6

Using the recommended coupling method, the converter dolly is positioned in front of the rear trailer by:

  1. A forklift only
  2. Using the tractor and lead trailer to spot it there
  3. Rolling it downhill
  4. Pushing it by hand the whole way

Correct answer: Using the tractor and lead trailer to spot it there

The manual method couples the tractor to the lead trailer first, then uses that unit to place the dolly.
Question 7

After backing the dolly under the rear trailer, you verify the coupling by checking:

  1. The mirror adjustment
  2. The fuel level
  3. That the fifth wheel jaws are locked on the kingpin with no gap, then giving a gentle tug test
  4. The trailer colour

Correct answer: That the fifth wheel jaws are locked on the kingpin with no gap, then giving a gentle tug test

Dolly couplings get the same visual check and tug test as a tractor fifth wheel.
Question 8

For air to reach the rear trailer’s brakes, the shut-off valves at the back of the lead (front) trailer must be:

  1. Half open
  2. Closed
  3. Removed
  4. Open

Correct answer: Open

Air flows to the dolly and rear trailer through those valves; closed valves mean no rear brakes.
Question 9

The shut-off valves at the very back of the LAST trailer in a set should be:

  1. Closed, because no trailer behind needs air
  2. Left as found
  3. Removed
  4. Open

Correct answer: Closed, because no trailer behind needs air

Open rear valves on the last trailer would just vent the system; they open only when another trailer is added behind.
Question 10

How do you test that air is being supplied to the rear trailer?

  1. Listen to the engine
  2. Open the emergency-line shut-off valve at the rear of the last trailer and listen for air escaping
  3. Check the fuel gauge
  4. Tap the tires

Correct answer: Open the emergency-line shut-off valve at the rear of the last trailer and listen for air escaping

Air escaping at the last trailer proves the supply line charged the whole set.
Question 11

How do you check the service line to the rear trailer?

  1. By reading the dashboard only
  2. By pulling the pintle latch
  3. By watching the mirrors
  4. Have a helper press the brake pedal (or use the hand valve) while you listen at the rear service coupling for air escaping when it is opened

Correct answer: Have a helper press the brake pedal (or use the hand valve) while you listen at the rear service coupling for air escaping when it is opened

Service air reaching the back of the set confirms the brake signal travels the full length.
Question 12

When uncoupling doubles, which trailer is uncoupled and parked FIRST?

  1. The dolly stays attached to the tractor
  2. The front trailer
  3. Both at once
  4. The rear trailer

Correct answer: The rear trailer

The rear trailer is spotted and uncoupled first; then the dolly is removed from the lead trailer.
Question 13

Why must you never unlock the pintle hook while the dolly is still under the rear trailer?

  1. The dolly tongue can snap up or the dolly can roll and run away once freed, causing serious injury
  2. The air will drain slowly
  3. It makes coupling harder later
  4. The paint can chip

Correct answer: The dolly tongue can snap up or the dolly can roll and run away once freed, causing serious injury

With the trailer resting on it, a freed dolly is under load and can kick violently.
Question 14

Before uncoupling a converter dolly from the lead trailer, you must:

  1. Lower the dolly’s landing gear (support legs) so the dolly stays upright
  2. Empty the rear trailer
  3. Remove the dolly wheels
  4. Speed up to loosen it

Correct answer: Lower the dolly’s landing gear (support legs) so the dolly stays upright

The dolly needs its own support before its drawbar leaves the pintle hook.
Question 15

How are a dolly’s safety chains supposed to be rigged?

  1. Crossed under the drawbar and secured to the frame of the trailer ahead
  2. Attached to the air lines
  3. Hanging loose to the ground
  4. Only one chain is needed

Correct answer: Crossed under the drawbar and secured to the frame of the trailer ahead

Crossed chains cradle the drawbar if the pintle connection fails.
Question 16

Rearward amplification in a set of doubles is about:

  1. 0.5 — half the tractor’s movement
  2. 5.0
  3. 1.0 — no amplification
  4. 2.0 — the last trailer whips with twice the sideways movement

Correct answer: 2.0 — the last trailer whips with twice the sideways movement

That doubling is why the last trailer of a double is the most likely vehicle in the set to roll over.
Question 17

Why do doubles and triples need more following distance than a single trailer?

  1. They are longer, heavier and slower to stop, and need room for trailer sway
  2. Following distance is the same for everything
  3. They accelerate faster
  4. They are shorter

Correct answer: They are longer, heavier and slower to stop, and need room for trailer sway

Space is the main tool for managing a long articulated set in traffic.
Question 18

Lane changes in doubles or triples should be:

  1. Made from the far lane
  2. Slow and smooth, with long mirror checks, because sharp steering is amplified at the rear
  3. Quick, to limit time beside traffic
  4. Done without signalling if the road is empty

Correct answer: Slow and smooth, with long mirror checks, because sharp steering is amplified at the rear

Gentle steering keeps the crack-the-whip effect small at the last trailer.
Question 19

In strong crosswinds, a set of doubles or triples:

  1. Is unaffected
  2. Catches more wind and can sway or be pushed; if control becomes difficult, stop in a safe place
  3. Should be driven faster to punch through
  4. Is more stable than a single

Correct answer: Catches more wind and can sway or be pushed; if control becomes difficult, stop in a safe place

Multiple trailers present a large sail area and can wander; slowing or stopping is the safe answer.
Question 20

Off-tracking in a set of doubles is greatest at:

  1. The last trailer, which follows the tightest inside path
  2. There is no off-tracking
  3. The dolly only
  4. The tractor front wheels

Correct answer: The last trailer, which follows the tightest inside path

Each unit tracks inside the one ahead, so turns must be planned for the final trailer.
Question 21

When parking a set of doubles or triples for a break, choose:

  1. Any space you can angle into
  2. The fuel island
  3. A pull-through space so you do not have to back the set
  4. A space requiring a blind-side back

Correct answer: A pull-through space so you do not have to back the set

Backing doubles is extremely difficult; plan stops around pull-through parking.
Question 22

Converter dollies manufactured on or after which date must have antilock brakes?

  1. July 4, 2005
  2. Dollies never need ABS
  3. January 1, 1990
  4. March 1, 1998

Correct answer: March 1, 1998

Dollies follow the same March 1, 1998 ABS requirement as trailers.
Question 23

During inspection of a converter dolly, you should also check:

  1. Only its colour
  2. Its air tank (drain moisture), tires, lights, spare tire and the condition of its fifth wheel and drawbar
  3. Nothing; dollies need no inspection
  4. The lead trailer’s cargo

Correct answer: Its air tank (drain moisture), tires, lights, spare tire and the condition of its fifth wheel and drawbar

The dolly is a full unit of the combination with its own air, brakes and coupling gear.
Question 24

After coupling a full set, how do you confirm all brakes work?

  1. Ask another driver
  2. Look at the dashboard only
  3. Charge the system, then apply the trailer hand valve and service brakes and check that brakes apply and release at every axle
  4. Drive at speed and brake hard

Correct answer: Charge the system, then apply the trailer hand valve and service brakes and check that brakes apply and release at every axle

A controlled brake check on the whole set catches dead axles before the road does.
Question 25

If the rear trailer has no air supply when you finish coupling, the likely cause is:

  1. A burned-out light
  2. The wrong radio channel
  3. Too much fuel
  4. A shut-off valve left closed at the back of the lead trailer or on the dolly

Correct answer: A shut-off valve left closed at the back of the lead trailer or on the dolly

Air reaches the rear only through open valves; trace the path from the tractor back.
Question 26

Steering corrections in a set of triples should be:

  1. Large and fast
  2. Made with the hand valve
  3. As small and gentle as possible, because each trailer amplifies the movement
  4. Unnecessary

Correct answer: As small and gentle as possible, because each trailer amplifies the movement

Small inputs at the wheel become large movements three trailers back.
Question 27

At railroad crossings, a driver of doubles or triples must remember:

  1. The set clears quickly
  2. To shift gears on the tracks
  3. Crossings do not apply to doubles
  4. The long set needs much more time to clear, so never stop on the tracks or enter unless the whole set fits beyond them

Correct answer: The long set needs much more time to clear, so never stop on the tracks or enter unless the whole set fits beyond them

Length is the hazard: be certain the entire combination can clear before entering the crossing.
Question 28

Before moving a freshly coupled set, the dolly’s spring or parking brake should be:

  1. Removed from the dolly
  2. Left applied
  3. Used as the main brake
  4. Released once the dolly is securely coupled into the set

Correct answer: Released once the dolly is securely coupled into the set

Dolly brakes hold it while parked alone; they must be off before the set rolls.
Question 29

Which inspection item is unique to doubles and triples compared with a single trailer?

  1. Checking the extra couplings, shut-off valves and dolly equipment between the trailers
  2. Checking the fuel
  3. Checking tire pressure
  4. Checking the mirrors

Correct answer: Checking the extra couplings, shut-off valves and dolly equipment between the trailers

Every connection between units is a potential failure point and gets its own check.
Question 30

In an emergency stop, a set of doubles or triples compared with a single needs:

  1. Locked wheels to stop fastest
  2. The hand valve only
  3. More room, and braking must stay as straight and controlled as possible to avoid whipping the rear trailer
  4. Less room

Correct answer: More room, and braking must stay as straight and controlled as possible to avoid whipping the rear trailer

Hard, crooked stops excite the whip effect; straight, early braking keeps the set behind you.

Ready to try them for real?

Start Practice Exam Simulator Quick Review

More questions & answers

Every Rig Toolbox practice bank, printed in full with answers: