CDL Combination Vehicles Practice Test
Combination Vehicles is the Class A section. It is about what changes when you put a trailer behind the tractor — how the unit tracks through a turn, how it skids, and the exact sequence for coupling and uncoupling without dropping a trailer on the ground.
Written and reviewed by Christopher Scott Sneed, 30+ year diesel technician, founder of Blue Collar Empire.
What this section covers
- Coupling and uncoupling step by step, including the tug test
- Fifth wheel, kingpin, and landing gear inspection
- Off-tracking and how to make turns without hitting the curb or the car beside you
- Trailer skids, jackknifes, and why you release the brakes to recover
- Rollover prevention: load height, center of gravity, and curve speed
- Air line hookups — service (blue) versus emergency (red)
Take this test now — free
Answer the 10 questions and get scored instantly. Free with your email, no card.
A trailer skid recovery starts with:
Free. No card, no password — just the email you signed in with.
10 practice questions with explanations
1. A trailer skid recovery starts with:
- A. Slamming the brakes
- B. Releasing the brakes and countersteering — correct
- C. Turning the wheel toward the skid direction
- D. Applying the trailer brake alone
Why: Release brakes to let wheels roll again, then steer where you want the tractor to go.
2. Off-tracking means:
- A. The tractor drifts left
- B. The trailer wheels follow a shorter path than the tractor through turns — correct
- C. The trailer sways side-to-side
- D. The load shifts forward
Why: Trailer wheels cut inside the tractor's path — you must swing wide correctly.
3. Before coupling, the fifth-wheel jaws must be:
- A. Closed
- B. Open — correct
- C. Partially closed
- D. Doesn't matter
Why: Jaws must be open so the kingpin can enter and lock.
4. After coupling, the trailer height should be:
- A. Slightly above the fifth wheel
- B. Slightly below the fifth wheel so it lifts as you back — correct
- C. Exactly level
- D. Two inches above
Why: Slightly low so the tractor lifts the trailer as it slides under, ensuring a solid lock.
5. The tug test verifies:
- A. Air line seal
- B. That the kingpin is locked in the fifth wheel — correct
- C. Tractor engine power
- D. Trailer lights
Why: Pulling forward gently in low gear with trailer brakes set confirms lock-in.
6. A trailer is most likely to roll over when:
- A. Loaded with heavy freight low
- B. Loaded high with light cargo, on a curve — correct
- C. Empty on straight road
- D. Reversing in a yard
Why: High center of gravity + lateral force in a curve dramatically increases rollover risk.
7. Trailer brakes should never be used to:
- A. Straighten a jackknifing tractor — correct
- B. Hold on a hill briefly
- C. Test the tractor-trailer connection
- D. Slow while parked
Why: Using the trailer hand valve during a jackknife makes it worse — release brakes.
8. Before uncoupling, you should:
- A. Pull the fifth-wheel release first
- B. Chock the trailer wheels and lower the landing gear — correct
- C. Disconnect air lines first
- D. Disconnect electrical first
Why: Secure the trailer first: chocks, landing gear, then air/electrical, then release.
9. Glad hands: which side supplies air to the trailer service brakes?
- A. The red (emergency) line
- B. The blue (service) line — correct
- C. Both lines
- D. Neither
Why: Blue is service (applies brakes), red is emergency (supply and spring release).
10. When making a right turn in a combination vehicle:
- A. Swing left first, then right
- B. Turn wide immediately
- C. Keep the rear of the trailer close to the curb — correct
- D. Signal only after turning
Why: Keep the trailer close to the curb so cars can't sneak up the right side.
How to study this section
- Learn the coupling sequence in order; the test asks about what comes before what.
- For any skid question, the answer usually starts with releasing the brakes so the wheels roll again.
- Rollover questions come down to center of gravity and speed in a curve.
- Keep the trailer close to the curb on right turns — swing wide left first and cars will pass you on the right.
Frequently asked questions
- Do I need the combination vehicles test for a Class A CDL?
- Yes. Class A applicants take General Knowledge, Combination Vehicles, and Air Brakes at minimum.
- Is combination vehicles the same as doubles and triples?
- No. Combination Vehicles covers a tractor with one trailer and is required for Class A. Doubles and Triples is a separate T endorsement test.
Take the timed version — free
The CDL Training Center runs this chapter as a scored quiz and tracks what you keep missing. CDL prep is free. Pro ($14.99 a month or $99.99 a year) covers the diagnostics, handbooks and business tools.