U.S. CDL · COMBINATION · QUESTIONS & ANSWERS

U.S. CDL Combination Vehicles 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 coupling, inspection and combination-driving practice based on the combination vehicles section of state CDL manuals.

Every question, with its answer

Question 1

What is the crack-the-whip effect?

A noise from the air lines
A type of tire failure
A steering move at the tractor is amplified toward the rear, so the last trailer can swing or roll
The trailer brakes releasing suddenly

Correct answer: A steering move at the tractor is amplified toward the rear, so the last trailer can swing or roll

Quick steering input grows as it travels back through the combination; the rear trailer reacts last and hardest.

Question 2

Which part of a combination is most likely to roll over in a sudden lane change?

The last trailer
The middle of the frame
The tractor
The front axle

Correct answer: The last trailer

Rearward amplification means the last trailer feels the greatest sideways force and rolls first.

Question 3

How can you reduce the chance of a trailer rollover?

Keep the centre of gravity low, centre the load, and drive slowly around turns
Keep the cargo high and light
Brake hard in the middle of the curve
Take turns at the posted car speed

Correct answer: Keep the centre of gravity low, centre the load, and drive slowly around turns

A low, centred load and slow, smooth turns are the main defences against combination rollovers.

Question 4

After coupling, what must you check between the trailer apron (upper plate) and the fifth wheel?

The gap does not matter if the jaws are closed
There must be no space; the apron should sit flat on the fifth wheel
Only the colour should match
There should be a two-inch gap

Correct answer: There must be no space; the apron should sit flat on the fifth wheel

A visible gap means a high hook or a kingpin caught wrong, and the trailer can come loose.

Question 5

What is a tug test?

Gently pulling forward with the trailer brakes locked to confirm the kingpin is secure
Tugging the air lines to check for leaks
Pulling the release handle twice
Pulling the trailer with the parking brake off at highway speed

Correct answer: Gently pulling forward with the trailer brakes locked to confirm the kingpin is secure

A gentle pull against locked trailer brakes right after coupling proves the jaws are holding.

Question 6

When you back the tractor under a trailer, the trailer should be positioned so that:

The landing gear carries all the weight throughout
The tractor raises the trailer slightly as the fifth wheel slides under the apron
The trailer drops six inches onto the plate
The fifth wheel barely touches the trailer

Correct answer: The tractor raises the trailer slightly as the fifth wheel slides under the apron

If the trailer is slightly low, the fifth wheel lifts it a little as you back under, seating the kingpin correctly.

Question 7

What can happen if a trailer is too high when you back under it?

The brakes will lock permanently
The air lines will not reach
The kingpin can pass over the jaws or couple falsely, and the trailer may fall or come loose
Nothing; height does not matter

Correct answer: The kingpin can pass over the jaws or couple falsely, and the trailer may fall or come loose

A high trailer can miss the locking jaws or hook on top of the kingpin, a coupling that can fail on the road.

Question 8

What does the emergency (supply) air line to a trailer do?

It only powers the trailer lights
It drains the trailer tanks
It charges the trailer air tanks and triggers the trailer emergency brakes if pressure is lost
It works the trailer horn

Correct answer: It charges the trailer air tanks and triggers the trailer emergency brakes if pressure is lost

The red supply line fills the trailer system; losing its pressure applies the trailer emergency brakes.

Question 9

What does the service air line (blue) do?

It carries the air that applies the trailer brakes when you press the pedal or hand valve
It connects the glad hands to the cab
It is a spare line that is never used
It fills the trailer tanks overnight

Correct answer: It carries the air that applies the trailer brakes when you press the pedal or hand valve

The service line delivers controlled braking pressure to the trailer brake chambers.

Question 10

If the trailer supply line loses pressure while driving, what happens?

Nothing changes
The service brakes gently release
The tractor protection valve closes and the trailer emergency brakes come on
The trailer tanks refill from the tractor

Correct answer: The tractor protection valve closes and the trailer emergency brakes come on

The protection valve pops out to save tractor air, and the trailer brakes apply from their own stored air and springs.

Question 11

What should you do with the rear shut-off valves on the tractor when bobtailing or towing no trailer?

Use them to drain the tanks
Close them
Leave them open
Remove them

Correct answer: Close them

Shut-off valves at the rear of the tractor are closed when there is no trailer to supply.

Question 12

Why are dummy couplers (dead-end connectors) fitted to unused glad hands?

To make coupling faster
To increase air pressure
To keep dirt, water and insects out of the air lines
To test the brakes

Correct answer: To keep dirt, water and insects out of the air lines

Sealing unused couplers protects the air system from contamination.

Question 13

If a trailer’s wheels lock up during hard braking, the trailer will tend to:

Stop straighter
Swing out sideways in a trailer jackknife
Speed up
Lose its load

Correct answer: Swing out sideways in a trailer jackknife

A locked-wheel trailer loses sideways grip and can swing around; ease off the brakes so the wheels can roll and grip again.

Question 14

Why is an empty trailer more likely to have its wheels lock under braking?

The fifth wheel is unlocked
Empty trailers have no brakes
Light weight on the tires means less traction, while the brakes are still strong
The air pressure doubles

Correct answer: Light weight on the tires means less traction, while the brakes are still strong

Strong brakes and little weight make empty and lightly loaded trailers easy to skid.

Question 15

How does a bobtail tractor (no trailer) compare with a loaded combination when stopping?

It stops in the same distance
It can take longer to stop and is easier to lose control of
It stops much shorter
It cannot be stopped without a trailer

Correct answer: It can take longer to stop and is easier to lose control of

Without trailer weight pressing the drive tires down, a bobtail has less traction and can be harder to stop.

Question 16

Using the following-distance rule (1 second per 10 feet of length, plus 1 second above 40 mph), a 60-foot combination at 50 mph needs at least:

12 seconds
5 seconds
3 seconds
7 seconds

Correct answer: 7 seconds

Six seconds for 60 feet of vehicle, plus one extra second because the speed is above 40 mph.

Question 17

Why must you pull farther forward before turning a combination vehicle?

The brakes respond slowly
The steering wheel is larger
The trailer off-tracks, following a shorter path than the tractor
The mirrors are wider

Correct answer: The trailer off-tracks, following a shorter path than the tractor

Rear wheels track inside the front wheels, so you swing later and wider to keep the trailer clear.

Question 18

Which statement about railroad crossings in a combination vehicle is correct?

Never stop on the tracks, and know the combination needs more time to clear
Stop on the tracks if traffic is heavy
You may shift gears on the tracks to move faster
Combinations never need to slow at crossings

Correct answer: Never stop on the tracks, and know the combination needs more time to clear

Long, heavy combinations clear crossings slowly; stopping or shifting on the tracks can leave you stranded.

Question 19

Trailers built on or after which date are required to have antilock brakes?

March 1, 1998
They are not required
January 1, 1980
June 30, 2010

Correct answer: March 1, 1998

Trailers manufactured on or after March 1, 1998 must be equipped with ABS (tractors from March 1, 1997).

Question 20

What does the yellow ABS malfunction lamp on the trailer itself tell you?

The trailer is coupled correctly
The trailer ABS is not working properly
The air tanks are full
The trailer is overloaded

Correct answer: The trailer ABS is not working properly

The lamp, mounted at the left rear (or front) corner of the trailer, signals an ABS fault; you still have regular brakes.

Question 21

Before coupling, the fifth wheel should be checked to confirm:

The release handle is tied shut
The plate is tilted toward the rear, the jaws are open and the safety latch works
It is painted the same colour as the tractor
It is completely dry of grease forever

Correct answer: The plate is tilted toward the rear, the jaws are open and the safety latch works

A tilted-back plate, open jaws, free movement and proper lubrication make a clean coupling possible.

Question 22

After the tug test, what must be done with the trailer landing gear?

Raise it fully, secure the crank handle and put the crank in low gear
Remove it
Leave it half down
Lower it onto blocks

Correct answer: Raise it fully, secure the crank handle and put the crank in low gear

Fully raised landing gear with the handle secured prevents it dragging or catching on crossings and turns.

Question 23

To make pulling the fifth-wheel release handle easier when uncoupling, you should:

Raise the trailer as high as possible
Pull forward hard against the kingpin
Back the tractor slightly to relieve pressure on the locking jaws, with the trailer brakes set or wheels chocked
Release the trailer air supply first

Correct answer: Back the tractor slightly to relieve pressure on the locking jaws, with the trailer brakes set or wheels chocked

Easing the pressure off the jaws (after securing the trailer) lets the release handle move freely.

Question 24

If the emergency and service glad hands are cross-connected, what problem can result?

The engine will stall
Nothing at all
The trailer lights will be brighter
The trailer brakes may not apply or release correctly, and the emergency system can behave wrongly

Correct answer: The trailer brakes may not apply or release correctly, and the emergency system can behave wrongly

Swapped lines defeat the intended braking control; always match the colour-coded couplers.

Question 25

When you first supply air to a coupled trailer, what should you listen and check for?

Only the exhaust sound
The radio volume
The fuel gauge
Leaks, and that the trailer brakes apply and release when controlled

Correct answer: Leaks, and that the trailer brakes apply and release when controlled

Charging the trailer system is the time to find leaks and confirm the trailer responds to its controls.

Question 26

Which driving habit most helps prevent combination skids?

Steering sharply to settle the trailer
Waiting until the last moment to brake
Looking far ahead and managing speed and space gently and early
Braking only with the trailer hand valve

Correct answer: Looking far ahead and managing speed and space gently and early

Early, gentle control inputs keep both tractor and trailer within their grip limits.

Question 27

After passing another vehicle in a combination, when is it safe to move back over?

As soon as your cab passes theirs
After one second regardless of length
When they flash their lights
When you can see their vehicle fully in your mirrors, leaving extra room for the trailer

Correct answer: When you can see their vehicle fully in your mirrors, leaving extra room for the trailer

The whole combination must clear the passed vehicle with space to spare before you change lanes.

Question 28

Why should cargo in a trailer be loaded low and centred?

A high or off-centre load raises rollover risk, especially in turns and sudden steering
To make unloading faster
So the fifth wheel carries more weight
To reduce tire pressure

Correct answer: A high or off-centre load raises rollover risk, especially in turns and sudden steering

Weight carried high or to one side makes the crack-the-whip and rollover effects worse.

Question 29

On a downgrade in a combination vehicle, the safest plan is to:

Ride the trailer hand valve down the hill
Let speed build and brake hard at the bottom
Coast in neutral to save the brakes
Select a safe speed and a lower gear before starting down, using the engine to help hold speed

Correct answer: Select a safe speed and a lower gear before starting down, using the engine to help hold speed

Grade control starts at the top; gearing down protects the brakes from fade on long descents.

Question 30

What should you check about the air and electrical lines after coupling?

That the electrical cord is disconnected
That they are supported, not pinched or rubbing, with enough slack for turns
That they are dragging slightly for slack
That they are as tight as possible

Correct answer: That they are supported, not pinched or rubbing, with enough slack for turns

Lines need support and turning slack without hanging where they can snag, chafe or pull loose.

Ready to try them for real?