Missouri Combination Vehicles CDL Practice Test

This is a free 20-question practice test for the Combination Vehicles portion of the Missouri Commercial Driver's License knowledge exam. Questions are pulled from a pool of 55 drawn from the AAMVA CDL Manual, which is the source document the Missouri Department of Revenue Driver License Bureau uses to write its actual exam.

How it works: Click an answer. The correct choice highlights in green, and you'll see a short explanation. Aim for 85% or better before you sit for the real test in Missouri.

Question 1 of 20

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 2 of 20

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 3 of 20

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 4 of 20

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 5 of 20

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 6 of 20

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 7 of 20

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 8 of 20

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 9 of 20

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 10 of 20

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 11 of 20

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 12 of 20

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 13 of 20

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 14 of 20

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 15 of 20

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 16 of 20

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 17 of 20

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 18 of 20

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 19 of 20

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 20 of 20

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.

About the Missouri 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.

The Missouri Department of Revenue Driver License Bureau follows the federal CDL standards established by FMCSA. To earn the Combination Vehicles credential, you must answer at least 80% of the questions correctly. Many candidates score lower the first time because the test pulls from a large pool — refreshing this page will give you a different mix of questions, drawn from the same authoritative source.

Want more practice? Try the full Combination Vehicles question bank or browse all Missouri CDL practice tests.