Combination Vehicles CDL Practice Test

The Combination Vehicles test is required to obtain a Class A CDL — the license needed to legally drive a tractor-trailer or any combination where the towed unit weighs over 10,000 lbs. It covers coupling and uncoupling, off-tracking, trailer rollover, braking with combination vehicles, and inspecting the fifth wheel and kingpin.

Start Practice Test ↓ Study Guide

What's on the Combination Vehicles exam

Most states administer 20 Combination Vehicles questions and require 80% to pass. The exam emphasizes the unique handling of articulated vehicles: how trailers track behind the tractor, how to prevent rollover, how to manage rearward amplification with multi-trailer combinations, and the correct sequence to couple and uncouple.

Topics covered

Tractor-trailer coupling and uncoupling
Fifth wheel inspection
Kingpin and locking jaws
Trailer hand valve and tractor protection valve
Off-tracking and turning
Trailer skids: jackknife and trailer swing
Trailer rollover and high center of gravity
Backing a trailer
Brake systems on combinations: longer charge time
Rearward amplification on doubles and triples (overview)

Who needs this endorsement

Anyone pursuing a Class A CDL. If you stay with single straight trucks (Class B), you do not need this endorsement.

All 55 Combination Vehicles Practice Questions

Click any answer to reveal the correct one and the explanation. Take a state-specific 20-question randomized round on any of the state pages.

Question 1 of 55 · #169

Combination vehicles are usually:

Easier to drive than single trucks
Harder to drive than single trucks because of length, weight, and the unique handling of articulated vehicles
The same as single trucks
Faster than single trucks

Correct. Combination vehicles (tractor-trailer rigs) are heavier, longer, and articulate at the fifth wheel — all making them more demanding than single-unit trucks.

Question 2 of 55 · #170

When making a right turn at an intersection in a tractor-trailer:

Swing to the left first to make room for the right turn
Make the turn from the right lane and swing wide as needed at the end of the turn
Make the turn at high speed
Stay in the left lane through the turn

Correct. Make the turn from the right lane. If you must swing wide, do so as you complete the turn, never before — swinging left first invites a vehicle to pass on your right.

Question 3 of 55 · #171

The trailer hand valve should be used:

Only to test the trailer brakes; never to slow the rig in normal driving
To slow the rig in all conditions
To brake on slick roads
Continuously on long downgrades

Correct. Use the trailer hand valve only for testing trailer brakes. Using it to slow the rig can cause the trailer to skid or jackknife.

Question 4 of 55 · #172

A jackknife happens when:

The trailer wheels skid while the tractor continues forward
The tractor pulls forward in a straight line
The trailer slides sideways while the tractor pivots
The driver brakes too lightly

Correct. Jackknife is when the drive wheels skid and the trailer continues forward, causing the tractor to pivot — the rig folds at the fifth wheel like a closing knife.

Question 5 of 55 · #173

The most likely cause of a tractor jackknife is:

Light cargo
Drive wheels lock during braking
Slow speed
Loose load

Correct. When drive wheels lose traction (locked, oversteer, ice), the tractor pivots ahead of the trailer — a jackknife.

Question 6 of 55 · #174

Trailer rollover is more likely when:

Cargo is loaded low and centered
Cargo is loaded high or to one side, raising the center of gravity
Cargo is light
Trailer is empty

Correct. High cargo and uneven side-loading raise the center of gravity, increasing rollover risk on curves and ramps.

Question 7 of 55 · #175

On a tight curve or exit ramp, a tractor-trailer is most likely to roll over because:

The trailer brakes lock
The high center of gravity creates a tipping force greater than the tires can resist
The tractor accelerates
The road is dry

Correct. Centripetal forces in a curve push the cargo outward; a high center of gravity multiplies the tipping moment. Slow before the curve.

Question 8 of 55 · #176

When backing a tractor-trailer in a straight line, you should:

Let the trailer drift wherever it goes
Make small steering corrections to keep the trailer in line
Apply heavy brake pressure
Use the trailer hand valve

Correct. When backing straight, watch the trailer in your mirrors and make small steering corrections to keep it tracking straight. Big corrections lead to jackknifing.

Question 9 of 55 · #177

When you are backing a trailer to the right, you should turn the steering wheel:

Left to start the trailer right
Right to start the trailer right
In the same direction every time
Never use the steering wheel; just push

Correct. When backing, the trailer goes opposite to the way you initially turn the steering wheel. To make the trailer go right (passenger side), start by turning the wheel left.

Question 10 of 55 · #178

Off-tracking refers to:

Trailer brakes overheating
Trailer wheels following a tighter path than the tractor wheels in turns
The trailer pulling left
Tires losing tread

Correct. Off-tracking is the tendency for trailer wheels to follow a smaller-radius path than the tractor wheels through a turn, more pronounced with longer wheelbases.

Question 11 of 55 · #179

The air lines connecting the tractor and trailer are:

Red (emergency) and blue (service)
Red and yellow
Blue and yellow
Black and red

Correct. The red glad-hand carries emergency/supply air; the blue glad-hand carries service air. They cross to keep matched colors when coupling.

Question 12 of 55 · #180

When coupling, after backing under the trailer, you should:

Pull the trailer immediately
Perform a tug test (gently pulling forward in low gear with trailer brakes set) to confirm the kingpin is locked
Disconnect the air lines
Lower the landing gear

Correct. A tug test confirms the kingpin is fully engaged. Skipping it is the leading cause of trailer drops shortly after coupling.

Question 13 of 55 · #181

Before backing under a trailer to couple, you should:

Check that the trailer height is just below the fifth wheel so the fifth wheel will lift the trailer slightly when backed under
Set the trailer at any height
Lock the trailer brakes off
Disconnect the air lines

Correct. Trailer should be just low enough that the fifth wheel will lift it slightly. Too low: the fifth wheel hits the trailer body. Too high: the fifth wheel slides under without engaging the kingpin.

Question 14 of 55 · #182

A high-mounted trailer kingpin is dangerous because:

It will not engage the fifth wheel and the trailer can drop
It is illegal
It improves coupling
It changes color

Correct. If the trailer is too high, the fifth wheel slides under without the kingpin engaging the locking jaws. The trailer is not coupled and can drop when you pull away.

Question 15 of 55 · #183

Before uncoupling, you should:

Park on level ground, set the trailer parking brake, lower the landing gear, disconnect air and electrical lines, then release the fifth-wheel jaws
Just pull away
Disconnect electrical only
Drive forward fast

Correct. Proper sequence: park level, lower landing gear to support the trailer, disconnect lines, release jaws, then pull forward slowly. Skipping any step risks dropping the trailer.

Question 16 of 55 · #184

When uncoupling, you should lower the landing gear:

After releasing the jaws
Before releasing the jaws so the trailer is supported when you pull away
Halfway
Not at all

Correct. Lower the landing gear and snug it against the ground before releasing the fifth-wheel jaws so the trailer doesn't drop when you pull away.

Question 17 of 55 · #185

When you check the fifth wheel during pre-trip, you look for:

Mounting to the frame is secure, no missing or damaged parts, no visible space between the upper and lower fifth-wheel plates, locking jaws around the kingpin shank, release arm in the locked position
Only the color
Only the bolts
Only the grease

Correct. Check mount, condition, gap, jaw engagement around kingpin shank (not the head), and release-arm lock position. Lubrication should be present but not excessive.

Question 18 of 55 · #186

A combination vehicle requires more time to stop than a single vehicle because:

It is much heavier and air must travel further to reach the trailer brakes
It accelerates faster
It is shorter
Drivers wait longer

Correct. Greater weight + brake-lag in long air lines = longer stopping distance. Plan ahead.

Question 19 of 55 · #187

When approaching a curve in a tractor-trailer, you should:

Brake during the curve
Slow down before entering the curve
Speed up
Maintain steady speed

Correct. Slow before entering the curve. Once in the curve, accelerate gently to maintain stability. Braking in a curve invites trailer skid or rollover.

Question 20 of 55 · #188

Cargo securement: the heaviest part of the load should be:

Loaded over the front axle area, with weight evenly distributed side to side
Loaded at the rear
Loaded on one side
Stacked on top

Correct. Loading heavy cargo to the front of the trailer over the drive axles helps with traction; loading to one side or top creates dangerous handling.

Question 21 of 55 · #189

A "trailer skid" happens when:

Trailer wheels lose traction and the trailer slides outward
Tractor wheels lock
Brakes overheat
Cargo shifts

Correct. When trailer wheels lose traction (often from over-application of trailer brakes alone), the trailer can slide sideways — a trailer skid or trailer swing.

Question 22 of 55 · #190

To recover from a trailer skid, you should:

Apply more brakes
Release the brakes to let the trailer wheels regain traction
Steer hard left
Honk

Correct. Release the brakes so the trailer wheels can rotate again and re-establish traction. Continued braking will worsen the skid.

Question 23 of 55 · #191

When you connect the glad-hands, you should:

Cross-connect the red and blue lines so they cannot be confused
Connect both to the same color glad-hand
Pinch the seal first
Push them in straight without rotating

Correct. Push the glad-hands together so the rubber seals match, then rotate to lock. Be sure red goes to red and blue goes to blue (the lines themselves often cross to keep colors matched).

Question 24 of 55 · #192

When you supply air to the trailer for the first time after coupling, you should:

Wait until air pressure builds in the trailer reservoir, then test the trailer brakes by pulling out the trailer-supply knob
Drive away immediately
Disconnect the lines
Skip the test

Correct. Charge the trailer system. Then test the trailer brake function by tugging gently or pulling the trailer-supply knob to confirm trailer brakes apply.

Question 25 of 55 · #193

Front-trailer (lead) wheels lock and the trailer behind continues forward — this is:

Trailer jackknife
Tractor jackknife
Tire blowout
Off-tracking

Correct. When the trailer wheels lose traction and the front-of-trailer pivots while the tractor continues forward, the result is a trailer jackknife.

Question 26 of 55 · #194

When making a turn, you should signal:

Just before the turn
Well in advance — at least 100 feet for a turn at low speed
Only if other vehicles are present
At the start of the turn

Correct. Signal early enough that other drivers see and process the signal — generally at least 100 feet before in town, longer at highway speeds.

Question 27 of 55 · #195

A combination vehicle on slippery roads should be driven:

Faster to maintain control
Slower with greater following distance
At normal speed
In a higher gear

Correct. Reduce speed by at least one third on wet roads and one half on snow; double following distance on slick surfaces.

Question 28 of 55 · #196

Your trailer ABS warning lamp is on. The trailer service brakes:

Will not work at all
Still work normally — only ABS function is unavailable
Lock up
Apply automatically

Correct. Trailer service brakes still work normally without ABS. Have the system repaired; ABS gives extra control during emergency braking.

Question 29 of 55 · #197

After connecting the trailer, before pulling away, you should always:

Visually inspect the fifth-wheel/kingpin connection from outside the cab
Just trust the gauge
Honk the horn
Set the trailer brake

Correct. Always visually confirm: no gap between fifth wheel and trailer plate, jaws closed around the kingpin shank, locking lever fully in place.

Question 30 of 55 · #198

The "rearward amplification" effect:

Is greatest in the lead trailer
Is greatest in the rear-most trailer of doubles or triples
Does not exist
Is the same in every trailer

Correct. Rearward amplification — the crack-the-whip effect — is most severe in the rear trailer of a multi-trailer combination.

Question 31 of 55 · #199

Most jackknifes happen because:

Drive wheels skid during braking on slippery surfaces
Brakes work too well
The road is dry
Cargo is too light

Correct. Loss of traction on the tractor drive axles during braking — typically on slick surfaces or with overly aggressive braking — causes a jackknife.

Question 32 of 55 · #200

When inspecting the kingpin during pre-trip, look for:

Excessive wear, bending, cracks, or improper lock-jaw engagement
Only the color
Only the size
Only the grease

Correct. A worn, cracked, or bent kingpin can fail under load. The locking jaws must close completely around the shank, not just the head.

Question 33 of 55 · #201

A safe practice when arriving at a destination is to:

Pull straight in fast
Pull straight ahead, slowly, watching mirrors and clearance
Back in without help
Stop on the road

Correct. Slow approach with mirror checks gives time to identify clearance issues and pedestrians.

Question 34 of 55 · #202

When you "G.O.A.L." in trucking, you:

Get Off And Look — visually verify the area before backing
Go Over A Limit
Grab On And Lift
Greet Other Approaching Loaders

Correct. G.O.A.L. — Get Out And Look — is the universal driver-school rule before any tight backing maneuver.

Question 35 of 55 · #203

To make a trailer go where you want when backing, the steering wheel should:

Be turned in the direction you want the trailer to go
Be turned opposite to the direction you want the trailer to go (then corrected)
Stay still
Be held lightly

Correct. Backing is opposite-direction steering: to swing the trailer right, turn the wheel left first, then correct as the trailer follows.

Question 36 of 55 · #204

Length and weight increase what about a combination vehicle?

Stopping distance, off-tracking, blind spots, and time required for lane changes
Visibility
Speed
Acceleration

Correct. Longer, heavier rigs need more stopping distance, swing wider in turns, have larger blind spots, and require more time and space to change lanes.

Question 37 of 55 · #205

When you couple, the locking jaws of the fifth wheel must close around:

The kingpin shank, not just the head
Just the head
The trailer skirt
The wheel

Correct. The locking jaws must engage the kingpin shank below the head. If the jaws close above the head, the connection is unsafe.

Question 38 of 55 · #206

If the trailer is too low for coupling, you should:

Crank the trailer up with the landing gear before backing under
Skip the inspection
Drive faster
Use the trailer brake

Correct. Use the landing gear hand crank to raise the trailer to the proper height (just below fifth-wheel level) before backing under.

Question 39 of 55 · #207

A combination-vehicle pre-trip should include:

All single-vehicle items plus fifth wheel, kingpin, sliding fifth wheel/sliding tandem locks, mud flaps, trailer brakes, lights, and trailer tires
Only the tractor
Only the trailer
Only the cargo

Correct. You inspect the entire rig — tractor + trailer + connections + cargo securement — not just the tractor.

Question 40 of 55 · #208

Maximum allowable trailer-brake leakage rate (engine off, brakes released, combination vehicle) is:

1 psi/min
3 psi/min
5 psi/min
7 psi/min

Correct. Combination vehicles: no more than 3 psi/min with brakes released, 4 psi/min with brakes applied.

Question 41 of 55 · #209

Red and amber lights on the rear of the trailer must be:

Working and visible — never broken or covered
Optional
Required only at night
Decorative only

Correct. Brake lights, turn signals, marker lights, and reflectors at the rear must be operational and clean. Damaged or non-working lights are an out-of-service item.

Question 42 of 55 · #210

A loaded trailer "bows" (sways) at speed because:

Air pressure changes
Crosswinds, road bumps, or heavy rear loading can produce trailer sway
Tires wear
Brakes are applied

Correct. Crosswinds and uneven loading can produce trailer sway. Slow down and allow the sway to dampen; do not over-correct with steering.

Question 43 of 55 · #211

Auxiliary equipment (refrigeration, hydraulic lift gates) on the trailer:

Has no effect on the rig
Adds weight and may change rear-axle loading; verify securement and operation
Replaces the brakes
Cannot be operated while loaded

Correct. Reefer units, lift gates, and other auxiliary equipment add weight and may shift CG. Inspect securement and check operation in pre-trip.

Question 44 of 55 · #212

On a slippery surface, drive in:

Higher gears at higher RPM
A gear that minimizes torque to the drive wheels and brake earlier and gentler
The lowest gear
Whatever is fastest

Correct. Reduce engine power to drive wheels (higher gear, gentler accelerator) and brake earlier and gentler to avoid wheel spin or skid.

Question 45 of 55 · #213

A sliding fifth wheel allows you to:

Adjust the weight distribution between drive axles and trailer axles
Speed up
Slow down
Disable the trailer brakes

Correct. Sliding fifth wheels move forward to put more weight on drive axles or aft to reduce drive-axle weight, helping comply with axle-weight limits.

Question 46 of 55 · #214

A sliding-tandem trailer allows you to:

Adjust the trailer-axle position to redistribute weight between trailer axles and tractor drive axles
Speed up
Cool the load
Steer the trailer

Correct. Sliding tandems shift trailer axles forward (more weight on tractor) or aft (more weight on trailer axles), helping meet axle-weight limits.

Question 47 of 55 · #215

When you cross railroad tracks in a tractor-trailer, you should:

Shift gears to maintain speed
Cross in a single gear without shifting, and never start across without room to clear the rails
Stop on the tracks if the light turns red
Honk repeatedly

Correct. Crossing in one gear (without shifting) prevents stalling on the tracks. Never enter a crossing unless you can clear it without stopping.

Question 48 of 55 · #216

A sliding fifth wheel that is unlocked or improperly secured can:

Slide unexpectedly during braking or acceleration, dangerously changing handling
Improve fuel economy
Save time
Have no effect

Correct. An unsecured sliding fifth wheel can shift during braking, changing the rig's handling unpredictably. Always verify the lock pin is engaged.

Question 49 of 55 · #217

Trailer wheels off-track:

Outside the path of the tractor wheels
Inside (tighter turn radius) than the tractor wheels
At the same path as the tractor wheels
Cannot be predicted

Correct. Trailer wheels follow a tighter path than tractor wheels through a turn — off-tracking. Plan turns to keep trailer tires on the pavement and clear of curbs.

Question 50 of 55 · #218

Which is the proper following distance for a 60-foot rig at 50 mph?

3 seconds
6 seconds
7 seconds
12 seconds

Correct. 1 second per 10 feet at speeds below 40 mph, plus 1 extra second above 40. 60 feet = 6 seconds + 1 = 7 seconds at highway speed.

Question 51 of 55 · #219

A trailer that has been sitting may have:

Brakes that need careful adjustment, rust on the kingpin, or other issues that should be inspected
No issues — trailers are static
Built-in safety
Better brakes

Correct. Trailers idle for long periods can develop brake corrosion, kingpin rust, low tires, and other issues. Inspect carefully before use.

Question 52 of 55 · #220

During a pre-trip inspection, you should also check that the cargo:

Is loaded inside the vehicle
Is secured, balanced, within weight limits, and within size limits
Is brightly colored
Is all the same shape

Correct. Cargo securement, weight balance, and size compliance are all driver responsibilities — check before each trip.

Question 53 of 55 · #221

When you turn on a tight radius (e.g., into a tight loading dock):

Off-tracking is exaggerated; the trailer will swing far inside the line of the tractor wheels
Off-tracking disappears
Trailer follows tractor exactly
Trailer goes wide

Correct. On a tight radius, off-tracking is more pronounced. Sweep wide enough that the trailer wheels clear curbs and obstacles.

Question 54 of 55 · #222

When making a U-turn, a tractor-trailer needs:

Less space than a car
Substantially more space than a car — often more than a single intersection allows
The same space as a car
No turning space

Correct. Tractor-trailer turning radius is much larger than a car. A standard intersection rarely allows a U-turn — find a wider turning area.

Question 55 of 55 · #223

Trailer cargo can shift if:

Securement is inadequate, the load was packed unevenly, or the rig undergoes hard braking or sharp turns
Cargo never shifts
Roads are smooth
Trailer is empty

Correct. Inadequate securement, uneven packing, hard braking, sharp turns, or rough roads can all shift cargo. Inspect cargo periodically en route.