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25 multiple-choice questions and 9 flashcards on Combination Vehicles, about 8% of the CDL Class A bank. Every one carries a written rationale.
Combination Vehicles is one of 12 chapters in CoStudy's CDL Class A (General Knowledge) bank, and it holds 25 of the bank's 300 multiple-choice questions — roughly 8% of the total. That proportion is not arbitrary: chapters follow the certifying body's published exam outline, and the number of questions in each is set by that domain's published weight, so the share of your practice time this chapter takes matches the share of the real exam it accounts for.
Studying by chapter is worth doing once you have a diagnostic score. A single overall percentage tells you whether you are close; it does not tell you which domain is dragging. Working a weak chapter in isolation, and re-testing it in isolation, is the fastest way to move a score that has stalled — and it is why the mock exams in CoStudy report by domain rather than as one number.
10 questions drawn from this chapter, with the full rationale shown — the controlling principle behind the right answer, and why each wrong option tempts and fails.
During uncoupling, when should the driver lower the landing gear?
Answer: B — Before easing forward out from under it
B) Right — lower the landing gear to support the trailer and take some weight before the tractor pulls out. A) Pulling the release first leaves the trailer unsupported. C) Lowering gear after the tractor leaves means the trailer has already dropped. D) That describes a step in coupling, not uncoupling.
A driver turns right at a tight urban corner in a long combination. What should the driver do?
Answer: B — Turn wide as the turn is completed to the right
B) Right — because trailer wheels off-track inside the tractor's path, the driver turns wide as the turn is completed, keeping the rear near the curb through the corner. A) Swinging left first invites another vehicle into the gap and can cause a squeeze. C) Riding the rear over the curb strikes fixed objects and pedestrians. D) Speed does not change the geometry of off-tracking.
A driver is dropping a loaded trailer on soft gravel. What is the BEST precaution?
Answer: A — Use blocks under the landing gear feet
A) Right — soft ground calls for blocking under the landing gear so the feet do not sink and drop the trailer. B) Released trailer brakes allow a rollaway. C) A shortened gear setting does not spread the load and may not reach. D) Nose-down parking encourages movement and does nothing for support.
A driver enters an exit ramp posted for cars at a given speed with a loaded van trailer. What is the BEST practice?
Answer: D — Slow well below the posted figure before entering
D) Right — posted ramp speeds are set for cars, and a high-profile loaded truck must be slower before entering the curve. A) Matching the sign is the classic ramp rollover. B) Braking inside the curve unsettles the vehicle when it is least stable. C) Acceleration in the curve raises lateral force rather than settling anything.
The supply line vs. service line in a combination vehicle differ how?
Answer: D — Supply (emergency) line charges trailer reservoirs and triggers spring brakes if pressure lost; service line applies trailer service brakes proportional to pedal
A) Confuses functions. D) Correct — supply/emergency line carries constant air to charge reservoir; service line modulates braking based on driver pedal pressure. C) Wrong line. B) Wrong.
An empty trailer in combination vehicle behavior:
Answer: D — Has poor braking traction because it bounces; can lock up or skid more easily
A) Direction reversal trap — empty trailers actually bounce and lose traction. D) Correct — empty trailer wheels can skid easily; use gentle braking. C) False. B) Drivable, just requires care.
What does the tractor protection valve accomplish?
Answer: A — It keeps tractor air when the trailer separates
A) Right — the valve closes to preserve tractor air if trailer air is lost, so the tractor keeps braking. B) It does not boost pressure anywhere. C) It has no mechanical role at the fifth wheel. D) Traction control on slick roads is a different system.
After coupling, you must perform a tug test by:
Answer: D — Pulling forward against locked trailer brakes to verify the kingpin is locked
A) Tug means specifically pulling against locked brakes. D) Correct — gently pull forward against the trailer's parking brake; if the jaws are locked, the tractor will not move forward. C) Doesn't test coupling. B) Not a coupling test.
What is the function of the emergency air line in a combination vehicle?
Answer: C — It supplies the trailer air tank
C) Right — the emergency line supplies air to the trailer tank and, if it loses pressure, the trailer emergency brakes apply. A) That describes the service line. B) Governor venting happens at the compressor, not through a trailer line. D) Air lines do not equalize axle pressures.
A driver is coupling and has just backed under the trailer. What should be done NEXT?
Answer: C — Test the connection with a gentle tug forward
C) Right — after backing under, lock the tractor brakes and pull gently forward against the locked trailer brakes to confirm the coupling holds. A) Landing gear comes up only after the connection is proven. B) Air lines are connected earlier so the trailer brakes can hold. D) The visual under-check follows the tug test, not precedes it.
4 cards from the 9 in this chapter.
What activates the trailer's emergency brakes?
Loss of air pressure in the emergency line (e.g., trailer break-away).
What is the trailer air supply valve and what does it do when pushed in versus pulled out?
Pushed in it supplies air to the trailer and releases the trailer spring brakes for driving. Pulled out it cuts the supply and sets the trailer emergency and parking brakes.
What happens if the service and emergency glad hands are crossed?
Air will not reach the trailer emergency system properly, so the trailer spring brakes may not release or the service brakes may not work when needed — the driver could pull the trailer with no working brakes.
Why do most truck rollovers happen, and what two loading and driving practices prevent them?
Rollovers occur when the driver takes a curve or ramp too fast for a high center of gravity. Keep the cargo mass as low as possible and centered, and slow to well below the posted ramp speed before entering the curve.
These are a sample. The full Combination Vehicles chapter runs 34 items with per-chapter progress tracking, on the web and in the iOS app.