CDL Doubles Triples practice questions and answers 2026. Tap an option to test yourself — you'll see the correct answer and a plain-English explanation for every question. Free, no login.
Q21You connect the service gladhand from the tractor to the converter dolly and then attempt to build system air pressure. The gauge climbs normally but the third trailer's brakes stay applied. What should you inspect FIRST?
✓ Correct answer: A. The shut-off valve on the rear of the second trailer feeding the third trailer's emergency line
If supply air cannot reach the third trailer, its emergency (spring) brakes remain applied; the first thing to check is whether the shut-off valve on the rear of the second trailer is open to pass air to the third unit.
Q22During inspection of a doubles set, you notice the converter dolly's pintle hook latch is not fully latched around the first trailer's drawbar eye. What must you do before moving the vehicle?
✓ Correct answer: B. Fully latch and secure the pintle hook latch, then verify it is locked
An unlatched pintle hook can release the rear unit at any time; the CDL manual requires all coupling devices to be completely engaged and locked before any movement.
Q23A driver completes the emergency-line shut-off valve test during a pre-trip on a doubles set. After closing the emergency shut-off valve on the first trailer's rear and waiting 30 seconds, the second trailer's brake application light extinguishes. What does this indicate?
✓ Correct answer: B. The second trailer's emergency brakes failed to apply, indicating a leak or malfunction in that trailer's emergency circuit
Closing the emergency line should trigger the spring brakes on the second trailer; if the brake light goes out instead of illuminating (or spring brakes don't hold), it signals a failure in the emergency brake circuit that must be fixed before departure.
Q24When visually inspecting the landing gear on a converter dolly that has just been pre-positioned for coupling, which finding requires immediate correction before you attempt to couple the second trailer?
✓ Correct answer: B. One of the landing gear legs is lower than the other, causing the fifth wheel plate to tilt
Both dolly landing gear legs must be at equal height so the fifth wheel plate is level; an uneven fifth wheel can cause misalignment that prevents the kingpin from seating correctly, creating an unsafe coupling.
Q25How does a driver confirm that the converter dolly's fifth wheel jaws have properly locked around the second trailer's kingpin after backing under?
✓ Correct answer: C. Tug the first trailer forward in low gear while watching for movement between the dolly and second trailer
The CDL manual instructs drivers to tug forward gently to test the coupling; if the trailer separates from the dolly's fifth wheel, the kingpin was not properly locked.
Q26While inspecting a triple combination after re-fueling, you notice that the electrical cable between the second and third trailer is coiled and hanging low enough to contact the trailer's rear axle tire. What hazard does this present and what is the required correction?
✓ Correct answer: A. The cable can be cut by the tire, disabling rear lights on the third trailer and creating a road hazard; re-route and secure the cable above tire contact level
An electrical cable dragging on or against a tire will be abraded and severed, disabling running lights and brake lights on the rearmost trailer — a serious safety violation; the cable must be secured clear of all moving parts.
Q27What color coding convention does the CDL manual describe for gladhands to help drivers connect air lines correctly between trailers?
✓ Correct answer: D. Blue for the service line and red for the emergency (supply) line
The CDL manual uses the standard color code of BLUE for the service (control) line and RED for the emergency (supply) line gladhands, helping prevent cross-connection errors during doubles/triples coupling.
Q28During a roadside walk-around of a triple combination, an officer points out that the dolly between the second and third trailer has no reflective markings on its side. Under FMCSA lighting and marking rules, which statement is correct?
✓ Correct answer: A. The converter dolly is a separate vehicle subject to its own rear and side marking requirements, including retroreflective tape or conspicuity material
Under FMCSA 49 CFR Part 393, a converter dolly is classified as a trailer and must carry the same retroreflective conspicuity markings required of other trailers in the combination.
Q29When a vehicle combination makes a sudden lane change at highway speed, which trailer in a triple combination experiences the greatest lateral amplification of movement?
✓ Correct answer: D. The third (rear) trailer, because each connection point multiplies the swing
The CDL manual explains that the crack-the-whip effect causes each successive trailer to swing farther than the one ahead of it, so the rearmost trailer in a triple experiences the greatest amplified lateral movement.
Q30What driving behavior is most likely to trigger a crack-the-whip rollover in a double or triple combination?
✓ Correct answer: B. Sudden steering inputs such as abrupt lane changes or sharp swerves
The CDL manual specifically warns that sudden steering movements are the primary cause of the crack-the-whip effect, which can roll over the rear trailer even when the tractor remains upright.
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