5.0 Combination Vehicles CDL-A Practice Quiz
45 exam-style questions covering 15% of the CDL-A exam. Instant feedback on every answer, progress tracking, no signup required.
This domain is part of the CDL Class A practice test. Each question is tagged by exam objective and difficulty so you can drill exactly the areas you need.
Sample Questions
Rollover is the leading cause of truck-driver deaths in crashes involving heavy combinations. According to the CDL manual, what two practices most directly help prevent rollovers in a combination vehicle?
- A. Keep the cargo as close to the ground as possible, and drive slowly around turns
- B. Use the trailer hand valve aggressively to keep the trailer in line
- C. Lock the inter-axle differential in every left-hand curve
- D. Reduce tire pressure by 20 percent in cold weather
The CDL manual states the two things that most help prevent rollover are keeping the cargo as close to the ground as possible and driving slowly around turns.
An empty or lightly loaded tractor-trailer has different braking behavior than a fully loaded one. According to the CDL manual, what specific issue arises when the rig is empty?
- A. Stiff suspension springs and strong brakes give poor traction, and it is very easy to lock up the wheels
- B. The empty trailer always stops in the shortest possible distance
- C. The tractor's automatic transmission shifts up two gears to compensate
- D. The trailer ABS deactivates because there is no weight on the wheels
The CDL manual states that when lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels.
The CDL manual identifies a phenomenon where a quick steering input at the tractor causes a much larger, more violent motion at the rear trailer of a combination. What is this phenomenon called, and what does it cause?
- A. Off-tracking, which causes the trailer to follow a wider arc through turns
- B. Rearward amplification, which produces the crack-the-whip effect that can flip the rear trailer in a quick lane change
- C. Brake fade, which results from heat building up in the lining
- D. Hydroplaning, which causes loss of traction at speeds as low as 30 mph
The CDL manual describes rearward amplification as the cause of the crack-the-whip effect; in a quick lane change it can flip the rear trailer, with triples having a published rearward amplification of about 3.5.
On a routine straight-line stop on dry pavement, the CDL manual notes a counterintuitive comparison between empty and loaded large combination vehicles. Which statement matches the manual?
- A. Empty combinations stop in roughly half the distance of fully loaded ones
- B. Large combination vehicles take longer to stop when they are empty than when they are fully loaded
- C. Empty and loaded combinations always stop in identical distances
- D. Federal regulation prohibits stopping-distance comparisons of empty rigs
The CDL manual states that large combination vehicles take longer to stop when they are empty than when they are fully loaded.
For a triple-trailer combination, the CDL manual makes a specific point about which trailer's wheels off-track the most through a corner. Which statement matches the manual?
- A. The first trailer's wheels off-track the most
- B. The rear wheels of the last trailer off-track the most
- C. All trailers off-track equally because they share the same kingpin geometry
- D. None of the trailer wheels off-track because the dolly absorbs all of the path difference
The CDL manual states that if there is more than one trailer, the rear wheels of the last trailer off-track the most.
A driver bobtails a tractor (no trailer) back to the yard. The CDL manual gives a specific warning about how a bobtail tractor stops compared with a fully loaded combination. Which warning matches the manual?
- A. Bobtails always stop in less than half the distance of a loaded combination
- B. Bobtails behave identically to a loaded combination under braking
- C. Tests have shown bobtails can be very hard to stop smoothly, and they may take longer to stop than a tractor-semitrailer loaded to maximum gross weight
- D. Bobtails may not be operated above 35 mph under any federal regulation
The CDL manual states tests have shown bobtails can be very hard to stop smoothly and may take longer to stop than a tractor-semitrailer loaded to maximum gross weight.
The CDL manual cites a striking comparison between fully loaded and empty rigs in rollover crashes. About how much more likely are fully loaded rigs to roll over in a crash than empty ones?
- A. About 2 times more likely
- B. About 5 times more likely
- C. About 10 times more likely to roll over in a crash than empty rigs
- D. About 50 times more likely
The CDL manual states that fully loaded rigs are 10 times more likely to roll over in a crash than empty rigs.
You see in the mirror that the trailer of your tractor-semitrailer is starting to swing out into a jackknife while braking on a slippery road. According to the CDL manual, what is the correct response?
- A. Apply the trailer hand valve harder to bring the trailer back in line
- B. Apply the parking brakes immediately to stop both units
- C. Steer sharply away from the trailer's swing direction while continuing to brake hard
- D. Release the brakes to get traction back; do not use the trailer hand valve to straighten the rig, since brakes on the trailer wheels caused the skid
The CDL manual instructs drivers to release the brakes to regain traction; do not use the trailer hand valve to straighten the rig, because brakes on the trailer wheels caused the skid in the first place.
Key Terms in This Domain
- Tractor-semitrailer: Class A combination: a tractor pulling one semitrailer connected via a fifth wheel and kingpin
- Trailer skid (jackknife): Trailer swings out of line with tractor; release the brakes and countersteer; do not use the trailer hand valve
- Fifth wheel: Coupling device on the rear of the tractor that engages the kingpin on the trailer
- Kingpin: Steel pin under the front of the trailer that locks into the tractor's fifth wheel
- Coupling sequence: Inspect, supply air to trailer, lock trailer brakes, position tractor, back slowly, test with low gear and tug, supply lines, raise landing gear, secure
- Tug test: After coupling, gently pull forward in low gear to verify the kingpin is locked in the fifth wheel before raising the landing gear
- Uncoupling sequence: Position tractor, unload weight by easing tractor forward, lower landing gear, disconnect lines and electrical, release fifth wheel, ease tractor out from under
- Endorsement T (doubles/triples): Required to operate longer combination vehicles with two or three trailers; Class A only
- Empty truck stopping distance: An empty truck takes longer to stop than a fully loaded one because there is less weight to push the tires onto the road for traction
- GCWR: Gross Combination Weight Rating: manufacturer-set maximum total weight for a tractor and trailer combination
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Other CDL-A Domains
- 1.0 Driver Qualifications and CDL Fundamentals
- 2.0 Driving Safely and On-Road Operation
- 3.0 Transporting Cargo Safely
- 4.0 Air Brakes
- 6.0 Pre-Trip Vehicle Inspection
- 7.0 Basic Vehicle Control and Maneuvering
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