Combination Vehicles (national) practice test
25 questions · 80% to pass · Required for Class A combination drivers.
How to use this practice test
Read each question, click an answer, and the correct choice is highlighted with a short explanation referencing the underlying CDL Manual concept. Your live score appears at the top of the page. Refresh to reset.
This test runs entirely in your browser. Nothing is uploaded; no account is required. Pages refresh to a new randomized cut from the bank.
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1. Off-tracking happens because:
Explanation. Trailer wheels cut inside the tractor wheels in a turn — the longer the trailer, the more pronounced the effect.
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2. When operating a CMV in interstate commerce, crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
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3. After coupling, you should test the connection by:
Explanation. A "tug test" with trailer brakes set verifies the kingpin is locked in the fifth wheel.
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4. Empty trailers:
Explanation. Empty trailers bounce, can lose traction, and may lock up faster than loaded trailers.
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5. During a pre-trip inspection, crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
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6. When coupling, you should back under the trailer slowly and:
Explanation. Stop just before contact, then back firmly to lock the kingpin.
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7. In real-world commercial driving, trailers are most likely to swing out (jackknife) when:
Explanation. Locked trailer wheels with a turning tractor lead to a trailer swing or jackknife. (Reinforcement variant — same underlying CDL Manual concept.)
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8. In real-world commercial driving, when coupling, you should back under the trailer slowly and:
Explanation. Stop just before contact, then back firmly to lock the kingpin. (Reinforcement variant — same underlying CDL Manual concept.)
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9. In real-world commercial driving, when backing a trailer, the trailer turns:
Explanation. When you back, turning the steering wheel one way causes the trailer to go the opposite way at first. (Reinforcement variant — same underlying CDL Manual concept.)
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10. Trailer hand valve (trolley valve) should NOT be used:
Explanation. Never use the trolley valve for parking, and avoid using it for service braking; it may cause skids.
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11. Combination vehicles are more likely to roll over because:
Explanation. Loaded combination vehicles have a high center of gravity and roll easily in sharp turns.
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12. When operating a CMV in interstate commerce, when uncoupling, you should lower the landing gear until:
Explanation. Land gear should support some weight before pulling out, but not lift the trailer off the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
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13. Trailers are most likely to swing out (jackknife) when:
Explanation. Locked trailer wheels with a turning tractor lead to a trailer swing or jackknife.
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14. Per federal regulations, when backing a trailer, the trailer turns:
Explanation. When you back, turning the steering wheel one way causes the trailer to go the opposite way at first. (Reinforcement variant — same underlying CDL Manual concept.)
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15. Most state DMV CDL handbooks state that the emergency air line is usually:
Explanation. Red is the emergency line; blue is the service (control) line. (Reinforcement variant — same underlying CDL Manual concept.)
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16. Per federal regulations, glad hands are used to connect:
Explanation. Glad hands are the air-line couplers between tractor and trailer. (Reinforcement variant — same underlying CDL Manual concept.)
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17. Experienced commercial drivers know that glad hands are used to connect:
Explanation. Glad hands are the air-line couplers between tractor and trailer. (Reinforcement variant — same underlying CDL Manual concept.)
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18. During a pre-trip inspection, the emergency air line is usually:
Explanation. Red is the emergency line; blue is the service (control) line. (Reinforcement variant — same underlying CDL Manual concept.)
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19. Experienced commercial drivers know that empty trailers:
Explanation. Empty trailers bounce, can lose traction, and may lock up faster than loaded trailers. (Reinforcement variant — same underlying CDL Manual concept.)
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20. According to the FMCSA CDL Manual, when uncoupling, you should lower the landing gear until:
Explanation. Land gear should support some weight before pulling out, but not lift the trailer off the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
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21. Experienced commercial drivers know that crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
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22. According to the FMCSA CDL Manual, off-tracking happens because:
Explanation. Trailer wheels cut inside the tractor wheels in a turn — the longer the trailer, the more pronounced the effect. (Reinforcement variant — same underlying CDL Manual concept.)
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23. When taking the CDL knowledge exam, when uncoupling, you should lower the landing gear until:
Explanation. Land gear should support some weight before pulling out, but not lift the trailer off the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
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24. In real-world commercial driving, after coupling, you should test the connection by:
Explanation. A "tug test" with trailer brakes set verifies the kingpin is locked in the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
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25. During a pre-trip inspection, trailer hand valve (trolley valve) should NOT be used:
Explanation. Never use the trolley valve for parking, and avoid using it for service braking; it may cause skids. (Reinforcement variant — same underlying CDL Manual concept.)
About the Combination Vehicles exam
Combination Vehicles knowledge is required for any driver pulling a trailer with a Class A vehicle. It covers off-tracking, rollover prevention, fifth-wheel coupling and uncoupling, trailer brake operation, antilock braking on trailers, and managing space and stopping distance with a loaded trailer.
This is the national edition of the Combination Vehicles practice test. Visit your state page for a state-framed version of the same exam. The questions are identical because the federal source material is identical — only the wording around the test changes from state to state.
Tips that actually work
- Read every answer choice before clicking. CDL questions are famous for "best answer" wording where two choices look right.
- Don’t memorize question text — learn the underlying rule. The DMV reshuffles wording constantly.
- Keep a list of the questions you miss. Re-drill them in isolation until you can’t miss them.
- Practice in short, frequent sessions. Two 20-minute sessions per day beats one two-hour cram.
What happens after I pass?
Passing the knowledge test earns you a Commercial Learner’s Permit (CLP). You must hold the CLP for at least 14 days before you can take the road skills test, and you must drive with a CDL-holding instructor in the cab during that period. Once you pass the road skills test, your CDL is issued.