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CDL Doubles/Triples study materials
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CDL Doubles/Triples practice questions

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Question 1Pulling Double/Triple Trailers

What is the main job of a converter dolly in a double or triple trailer combination?

  1. It adds a second compressor so the extra trailers get enough air pressure.
  2. It provides a fifth wheel and a pintle hook for pulling another trailer. Correct answer.
  3. It lowers the rear trailer so the whole combination rides level on the highway.
  4. It ties the trailer light circuits together so all the lamps flash together.

Explanation

A converter dolly is a coupling device: the rear trailer's kingpin rides on the dolly's fifth wheel, and the dolly itself attaches to the trailer ahead through a pintle hook and drawbar eye. Without it a tractor could not pull a second or third trailer.

Question 2Pulling Double/Triple Trailers

Which color glad hand is used for the emergency air line on a double or triple combination?

  1. Red Correct answer.
  2. Blue
  3. Green
  4. Black

Explanation

The emergency line uses red glad hands and the service line uses blue ones. Couplers are matched color to color, red to red and blue to blue.

Question 3Pulling Double/Triple Trailers

Compared with pulling a single trailer, turns with doubles or triples must be:

  1. Wider, so the rear wheels clear the curb. Correct answer.
  2. Started earlier and finished faster so the trailers stay in line.
  3. Made from the inside edge of the lane so the tractor clears traffic.
  4. Taken a little faster so the trailers do not bunch up behind you.

Explanation

The wheels of the last trailer track inside the path of the tractor, so a turn that clears a curb with a single trailer can put the rear trailer over it. Swinging wider keeps all the trailer wheels in the clear.

Question 4Pulling Double/Triple Trailers

How should your following distance change when you move from a single trailer to doubles or triples?

  1. It can stay the same, because the combination has more brakes.
  2. It can be shorter, because you sit higher and see farther ahead.
  3. It should be based only on the posted speed limit for trucks.
  4. It should increase with the longer combination. Correct answer.

Explanation

A common guide is at least one second of gap for every ten feet of vehicle length, and a longer combination is a longer vehicle. Extra weight and brake lag also stretch stopping distance, so the gap must grow.

Question 5Pulling Double/Triple Trailers

You are hooking up to an unhooked trailer in a set of doubles. What is the best way to keep it from rolling?

  1. Set the trailer spring brakes and rely on them to hold the trailer.
  2. Lower the landing gear fully and let it drag on the pavement.
  3. Have a helper stand behind the trailer and watch the wheels.
  4. Chock the trailer wheels before backing under it. Correct answer.

Explanation

A trailer that is not connected to an air supply may not have fully charged, reliable parking brakes. Wheel chocks hold it positively while you back under and couple.

Question 6Pulling Double/Triple Trailers

A strong crosswind is hitting your triple-trailer combination. What should you do?

  1. Speed up a little so the wind passes over the trailers more quickly.
  2. Move one lane to the left until you are out of the wind.
  3. Slow down and keep a firm grip on the wheel. Correct answer.
  4. Apply the trailer hand valve to steady the rear trailer in the gusts.

Explanation

Long, light combinations are pushed sideways easily, and the last trailer is pushed the hardest. Reducing speed and steering smoothly keeps the units from being blown out of the lane.

Question 7Pulling Double/Triple Trailers

Where should the shut-off valves on the rear of a trailer in a double or triple combination be set?

  1. Closed on every trailer so air cannot escape from the system.
  2. Half open on all trailers so the pressure equalizes.
  3. Open on every trailer except the last one. Correct answer.
  4. Open on the last trailer only, so the system can breathe.

Explanation

Air must pass through each trailer to reach the units behind it, so those valves stay open. The last trailer has nothing behind it, so its valves are kept closed to seal the system.

Question 8Pulling Double/Triple Trailers

What does off-tracking mean for a double or triple combination?

  1. The trailers drift sideways whenever the brakes are applied hard.
  2. The steer wheels wander because of the extra weight up front.
  3. The rear trailer's wheels roll faster than the tractor's do.
  4. The last trailer's wheels follow a shorter path. Correct answer.

Explanation

Off-tracking is the tendency of the rear wheels of the last unit to cut inside the path traced by the tractor. It is why the driver must swing wider on turns and watch the mirrors through the whole turn.

Question 9Pulling Double/Triple Trailers

Before pulling the tractor out from under a trailer you are dropping, you should:

  1. Leave the electrical cord plugged in to keep the marker lights on.
  2. Raise the landing gear so the trailer rests on the fifth wheel.
  3. Lower the landing gear so the trailer is supported. Correct answer.
  4. Disconnect the air lines while the trailer brakes are still released.

Explanation

The landing gear must carry the trailer once the tractor leaves, so it is lowered to the ground and checked before the fifth wheel is released. Air lines and the electrical cord are disconnected first so they are not torn off.

Question 10Pulling Double/Triple Trailers

After backing under a trailer, how do you confirm the fifth wheel has locked onto the kingpin?

  1. Look at the release handle and see that it is pulled all the way out.
  2. Honk the horn and watch whether the trailer moves backward.
  3. Pull forward gently with the trailer brakes set. Correct answer.
  4. Check that the air lines are connected and fully charged.

Explanation

A gentle pull against set trailer brakes is the tug test; if the jaws are not locked, the trailer will slide off the kingpin instead of holding. The driver should also look underneath to confirm the jaws are closed and no gap shows.

Question 11Pulling Double/Triple Trailers

Why should a trailer be raised slightly before you back the tractor under it?

  1. So the kingpin will sit lower than the top of the fifth wheel plate.
  2. So the fifth wheel lifts the trailer as you back under. Correct answer.
  3. So the air lines reach the glad hands without stretching.
  4. So the trailer's spring brakes release when the air is connected.

Explanation

With the trailer at the right height, the fifth wheel rides up the approach ramps and picks the trailer up as the tractor backs in, and the jaws snap shut around the kingpin. Too low and the kingpin strikes the fifth wheel; too high and the tractor passes under without coupling.

Question 12Pulling Double/Triple Trailers

While pulling doubles at highway speed, the rear trailer begins to sway. What should you do?

  1. Accelerate to pull the rear trailer back in line quickly.
  2. Ease off the accelerator and steer straight. Correct answer.
  3. Steer sharply the other way to catch the swing.
  4. Apply the brakes hard to straighten the combination out.

Explanation

Backing off the throttle lets the combination settle and keeps the trailers from gaining momentum side to side. Hard braking or quick steering inputs feed the sway and can lead to a jackknife or rollover.

Question 13Pulling Double/Triple Trailers

When checking the pintle hook that connects the converter dolly to the trailer ahead, you must make sure that:

  1. The safety pin is removed so the latch can open in a turn.
  2. The hook is heavily greased so the ring can spin freely in the hook.
  3. The hook is loose enough that the ring rattles a little.
  4. The latch is closed and the safety pin is in place. Correct answer.

Explanation

The pintle hook carries the pulling force for every trailer behind it, so the latch must be closed and locked with the safety pin. A latch that can open in service will release the dolly and the trailers behind it.

Question 14Pulling Double/Triple Trailers

What is the correct check for the converter dolly's safety chains?

  1. They should be left off whenever the dolly is running empty.
  2. They should be hooked to the rear bumper of the trailer with slack.
  3. They should be wrapped around the air lines for support.
  4. They should be attached, with no kinks or dragging. Correct answer.

Explanation

Safety chains are a backup that keeps the dolly attached if the pintle hook fails, so they must be properly connected and free of kinks, twists, or dragging. Chains that drag wear through, and wrapped or tangled chains cannot do their job.

Question 15Pulling Double/Triple Trailers

How should the air lines and electrical cord be run between the trailers?

  1. Twisted tightly together to keep them away from the frame.
  2. Taped to the brake chambers to make a shorter path.
  3. Left loose so they can be inspected during the trip.
  4. Supported so they do not drag, kink, or chafe. Correct answer.

Explanation

Lines are routed with enough slack for turns but supported so they cannot drag on the road or rub against sharp edges. Kinked or chafed lines leak air or lose the light signal.

Question 16Pulling Double/Triple Trailers

You are pulling triples and approach an entrance ramp curve posted at 35 mph. What is the safest approach?

  1. Keep 50 mph and let the trailers follow the curve.
  2. Hold 45 mph, since the ramp is banked for trucks.
  3. Brake hard once the trailers are in the middle of the curve.
  4. Slow down to a safe speed before entering the curve. Correct answer.

Explanation

Speed must be set before the curve, because braking in the middle of a turn with multiple trailers invites a swing or a rollover. A combination this long needs to be slower than the posted advisory to keep every unit in its lane.

Question 17Pulling Double/Triple Trailers

The red emergency line separates between the tractor and the first trailer. What happens?

  1. The trailer's emergency spring brakes apply. Correct answer.
  2. The trailer lights flash and the engine loses power.
  3. Nothing happens until the driver presses the brake pedal.
  4. The service brakes apply and stay on until the line is fixed.

Explanation

The emergency line supplies the air that holds the trailer's spring brakes off. When that line separates, the air escapes, the tractor protection valve closes, and the spring brakes set on the affected trailer and on the units behind it.

Question 18Pulling Double/Triple Trailers

After coupling to two loaded trailers, you start the engine and the low-air warning stays on much longer than it did with one trailer. What is the best explanation?

  1. The low-air warning light is faulty and should be ignored.
  2. The system must charge the air tanks on the added trailers. Correct answer.
  3. The converter dolly blocks air flow to the rear trailer.
  4. The compressor is driven only by the tractor's rear wheels.

Explanation

Every trailer has its own air tank, and all of them must be brought up to operating pressure before the warning clears. The tractor's compressor does the work, so more trailers mean a longer build-up time.

Question 19Pulling Double/Triple Trailers

In a triple-trailer combination, which unit is most likely to sway first at highway speed?

  1. The converter dolly that sits in the middle.
  2. The last trailer in the combination. Correct answer.
  3. The first trailer, right behind the tractor.
  4. The tractor's drive axle group, because it carries the most weight.

Explanation

Side-to-side motion is amplified as it travels rearward, so the rearmost unit swings the widest and becomes unstable first. That is why speed, steering smoothness, and load placement behind the tractor matter so much with triples.

Question 20Pulling Double/Triple Trailers

Besides the extra weight, why does a triple combination need more stopping distance than a single trailer?

  1. The rear trailer's brakes apply first and push the combination.
  2. Air takes longer to reach the rear trailer brakes. Correct answer.
  3. The tractor's brakes are de-rated when trailers are added.
  4. Each axle in a triple carries less weight, so it grips less.

Explanation

This delay is called brake lag: the air has to travel through much longer lines before it reaches the brakes on the trailers at the back. The rear units therefore begin braking after the tractor, which adds to the stopping distance.

Question 21Coupling and Uncoupling

Before backing the tractor under a trailer, the fifth wheel locking jaws should be in which position?

  1. Closed and latched
  2. Open, with the release lever in the open position Correct answer.
  3. Greased and closed
  4. Removed from the plate

Explanation

The jaws must be open so the kingpin can enter the throat of the fifth wheel and force the jaws closed as the tractor backs under. The release lever should be in the open position and the plate should be greased.

Question 22Coupling and Uncoupling

On a tractor-trailer air system, the emergency (supply) line gladhand is normally which color?

  1. Blue
  2. Green
  3. Red Correct answer.
  4. Black

Explanation

The emergency or supply line is coded red and the service line is coded blue. Matching the colors keeps the two lines from being crossed when you connect them.

Question 23Coupling and Uncoupling

After the trailer is coupled and secured, what should be done with the landing gear?

  1. Removed and stored in the cab
  2. Left partly extended for support
  3. Raised fully and the crank secured Correct answer.
  4. Lowered until it touches the ground

Explanation

Landing gear must be cranked all the way up so it cannot drag, strike pavement seams, or catch on railroad crossings. The crank handle is then secured so it does not swing out.

Question 24Coupling and Uncoupling

What is the first step when uncoupling a trailer?

  1. Disconnect the electrical cord
  2. Pull the fifth wheel release lever
  3. Lower the landing gear to the ground
  4. Set the parking brakes Correct answer.

Explanation

The parking brakes must be set before any other step so the tractor and trailer cannot roll while you work around and under them. Only then do you lower the landing gear and separate the connections.

Question 25Coupling and Uncoupling

In a doubles or triples combination, a converter dolly is best described as:

  1. A rigid bar welded between two trailers
  2. A bracket that holds the spare tire and tools
  3. A hydraulic lift that raises the rear trailer
  4. A separate frame with its own fifth wheel and a drawbar Correct answer.

Explanation

A converter dolly is a separate towed unit with its own axle, fifth wheel, and drawbar or pintle eye. It carries the kingpin of the next trailer so that trailer can pivot behind the one in front of it.

Question 26Coupling and Uncoupling

The electrical cord between the tractor and trailer supplies power for:

  1. The trailer's parking brakes only
  2. The trailer's air compressor only
  3. The trailer lights and ABS Correct answer.
  4. The fifth wheel locking jaws

Explanation

The cord carries electricity for clearance, marker, stop, and turn lights as well as the trailer ABS. Air for the brakes comes through the red supply line, not the cord.

Question 27Coupling and Uncoupling

Where should you uncouple a trailer?

  1. On soft ground to cushion the landing gear
  2. On level, firm ground Correct answer.
  3. On a slope so the trailer can settle
  4. In a travel lane so it stays visible

Explanation

Level, firm ground keeps the trailer from rolling and keeps the landing gear from sinking or shifting. A dropped trailer must also never be left where it blocks traffic.

Question 28Coupling and Uncoupling

When inspecting the trailer's kingpin before coupling, you should check that it is:

  1. Wrapped in a protective sleeve
  2. Coated with grease
  3. Straight and undamaged Correct answer.
  4. Fitted with a locking pin

Explanation

A bent or cracked kingpin will not seat properly in the jaws and can let the trailer separate. Check that the kingpin is straight and that its mounting bolts are tight.

Question 29Coupling and Uncoupling

After coupling and connecting the air lines, how do you perform the pull test?

  1. Set the trailer brakes and gently pull forward Correct answer.
  2. Back up sharply against the kingpin
  3. Raise the landing gear and drive at highway speed
  4. Release the trailer brakes and accelerate hard

Explanation

With the trailer spring brakes applied, a gentle forward pull should not move the trailer. If the trailer holds, the kingpin is locked in the jaws and the trailer brakes are working.

Question 30Coupling and Uncoupling

After backing under a trailer, what visual check confirms the fifth wheel is locked?

  1. The trailer apron rests on the frame rails
  2. The air lines hang clear of the plate
  3. The jaws are closed around the kingpin Correct answer.
  4. The release lever points straight up

Explanation

Get under the trailer with a light and look at the jaws; they must be closed around the kingpin with no gap. The release lever should also be in the locked position with its safety latch engaged.

Question 31Coupling and Uncoupling

Why must the safety latch on the fifth wheel release lever be engaged?

  1. To hold the air lines in place
  2. To keep the lever from vibrating open in motion Correct answer.
  3. To lock the landing gear crank
  4. To keep the lever from being greased

Explanation

Road vibration can move an unlatched release lever, which can open the jaws and let the trailer separate from the tractor. The latch is a positive lock against that movement.

Question 32Coupling and Uncoupling

You connect the air lines and build pressure, but the trailer air gauge keeps dropping. What is the most likely cause?

  1. The landing gear is still down
  2. The electrical cord is plugged in backwards
  3. The fifth wheel plate needs grease
  4. A leak or a poorly seated gladhand Correct answer.

Explanation

Pressure that will not hold means air is escaping somewhere in the system, often at the gladhand seals or in a damaged line. Find and correct the leak before moving the vehicle.

Question 33Coupling and Uncoupling

What is a sign that the service and emergency air lines have been crossed?

  1. The fifth wheel will not lock
  2. The trailer brakes do not release Correct answer.
  3. The trailer lights do not work
  4. The landing gear will not crank

Explanation

Color-coded gladhands, red for emergency and blue for service, are meant to prevent crossing. If the lines are crossed, the trailer brakes will not release as they should.

Question 34Coupling and Uncoupling

If the trailer is too high when you back under it, what can happen?

  1. The trailer brakes will release by themselves
  2. The landing gear will retract on its own
  3. The air lines will disconnect automatically
  4. The kingpin may ride over the top of the fifth wheel Correct answer.

Explanation

The trailer must sit at the right height so the kingpin enters the jaws squarely. If it is too high the kingpin can pass over the plate, and if it is too low the tractor can strike and damage the trailer.

Question 35Coupling and Uncoupling

Why should you chock the trailer wheels when uncoupling on a grade?

  1. To keep the trailer from rolling Correct answer.
  2. To keep the landing gear from sinking
  3. To keep the air lines off the ground
  4. To make the fifth wheel easier to release

Explanation

On a grade the trailer can creep or roll even with the spring brakes applied, especially if they are weak or out of adjustment. Chocks give a positive block against movement.

Question 36Coupling and Uncoupling

After connecting the air lines on a double or triple, how should they be positioned?

  1. Supported so they do not drag, tangle or rub Correct answer.
  2. Wrapped around the dolly frame for support
  3. Tied to the exhaust stack
  4. Looped loosely so they touch the ground

Explanation

Lines that drag, tangle, or rub will wear through and cause a loss of trailer brakes. Support them and keep them clear of hot parts, moving parts, and the ground.

Question 37Coupling and Uncoupling

When hooking a converter dolly to the pintle hook of the lead trailer, what must you confirm?

  1. The pintle latch is closed and secured Correct answer.
  2. The safety chain is hooked to the air line
  3. The pintle latch is left open for flexibility
  4. The drawbar eye is resting on the ground

Explanation

The pintle hook is the towing connection for the dolly, so its latch must be closed and secured with the provided pin or latch. An open latch lets the drawbar eye jump out under load or on bumps.

Question 38Coupling and Uncoupling

After coupling, you see a gap between the trailer apron and the top of the fifth wheel. What does this mean?

  1. The fifth wheel is over-greased
  2. The trailer is not fully coupled Correct answer.
  3. The trailer is at the correct height
  4. The air lines are crossed

Explanation

A complete coupling leaves the apron resting solidly on the fifth wheel with no gap. A gap means the kingpin is not seated in the jaws, so the connection must be redone before moving.

Question 39Coupling and Uncoupling

Before pulling out with a newly coupled trailer, you should check that:

  1. The landing gear is touching the ground
  2. The air lines are disconnected
  3. The release lever is unlatched
  4. The trailer lights and brakes work Correct answer.

Explanation

A final check of the lights and trailer brakes confirms the electrical and air connections are correct. The landing gear must be up and the release lever latched before you move.

Question 40Coupling and Uncoupling

What should you check on the gladhand couplers before connecting them?

  1. That they are painted the same color
  2. That the springs are removed
  3. That the rubber seals are undamaged Correct answer.
  4. That they are packed with grease

Explanation

The gladhand seals make the airtight joint, so a missing, cracked, or worn seal will leak. Replace bad seals and keep couplers clean before joining them.

Question 41Coupling and Uncoupling

Why are the trailer parking brakes set before the air lines are disconnected?

  1. So the electrical cord can be pulled free
  2. So the fifth wheel jaws stay open
  3. So the trailer cannot roll once the lines are off Correct answer.
  4. So the gladhands release more easily

Explanation

Once the lines are off, no air is available to control the trailer brakes through the coupling, so the parking brakes are the only thing holding the trailer. They must be set first to prevent rolling.

Question 42Coupling and Uncoupling

When backing the tractor under a trailer, which practice keeps the units aligned?

  1. Use the mirrors and back slowly Correct answer.
  2. Keep the trailer brakes released
  3. Have someone stand between the units
  4. Back quickly to keep momentum

Explanation

Backing slowly while watching the mirrors keeps the tractor centered so the kingpin enters the jaws squarely. No one should ever stand between the tractor and trailer during coupling.

Question 43Coupling and Uncoupling

When breaking down a set of doubles, which unit is uncoupled first?

  1. The converter dolly from the lead trailer
  2. All units at the same time
  3. The rear trailer from the converter dolly Correct answer.
  4. The tractor from the lead trailer

Explanation

Work from the rear forward: the last trailer comes off the dolly first, then the dolly comes off the lead trailer, then the lead trailer comes off the tractor. This keeps supporting units in place while you separate them.

Question 44Coupling and Uncoupling

You find a crack in the fifth wheel mounting during your pre-trip inspection. What should you do?

  1. Fill the crack with grease
  2. Take the vehicle out of service Correct answer.
  3. Couple and drive at reduced speed
  4. Report it after the trip

Explanation

A cracked fifth wheel mounting can fail completely and let the trailer separate from the tractor. Any crack in the mounting or plate makes the vehicle unsafe to operate until it is repaired.

Question 45Coupling and Uncoupling

A trailer you have uncoupled will be left standing in a lot. What should you do with its shut-off valves at the rear?

  1. Leave them half open
  2. Remove them
  3. Open them fully
  4. Close them Correct answer.

Explanation

The shut-off valves at the rear of a trailer seal the air supply lines when no vehicle is connected. Closing them keeps air from escaping through the open couplers and keeps dirt and moisture out of the lines.

Question 46Coupling and Uncoupling

Why should the fifth wheel be greased before coupling to a trailer?

  1. To make the fifth wheel jaws close tighter
  2. To ground the trailer's electrical system
  3. To reduce wear and let the trailer pivot Correct answer.
  4. To help the trailer brakes release faster

Explanation

The fifth wheel must be lubricated so the trailer can turn without binding. Grease also reduces wear between the fifth wheel plate and the trailer apron. The locking jaws and the brake system are separate from the greased surface.

Question 47Coupling and Uncoupling

After coupling a trailer, how high should the landing gear be raised?

  1. Just high enough to clear the ground
  2. About halfway up
  3. Fully up, until the crank stops Correct answer.
  4. Only if the trailer is loaded

Explanation

Landing gear must be fully raised after coupling so it does not catch on railroad tracks, curbs, or rough pavement. The crank should stop when the gear is all the way up. Partial raising leaves the gear low enough to strike obstacles.

Question 48Coupling and Uncoupling

The emergency (supply) air line on a trailer is usually what color?

  1. Black
  2. Green
  3. Blue
  4. Red Correct answer.

Explanation

By convention, the emergency or supply line is red and the service line is blue. The red line supplies air that keeps the trailer spring brakes released. Matching colors helps prevent crossing the lines.

Question 49Coupling and Uncoupling

What happens to a trailer's spring brakes when its air lines are disconnected?

  1. They apply automatically Correct answer.
  2. They must be applied by hand with a lever
  3. They release and the wheels roll freely
  4. They stay in the same position they were in

Explanation

Spring brakes are held off by air pressure. When the air lines are disconnected and pressure is lost, the springs apply the brakes automatically. That is why a dropped trailer stays braked.

Question 50Coupling and Uncoupling

When uncoupling a trailer, what should you do before releasing the fifth wheel?

  1. Disconnect the electrical cord first
  2. Raise the landing gear fully
  3. Lower the landing gear to the ground Correct answer.
  4. Release the tractor parking brake

Explanation

The landing gear must support the trailer before the fifth wheel is released. Lower it to firm ground contact first, then release the jaws. Otherwise the trailer can drop when the tractor pulls away.