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15 multiple-choice questions and 6 flashcards on Doubles and Triples, about 5% of the CDL Class A bank. Every one carries a written rationale.
Doubles and Triples is one of 12 chapters in CoStudy's CDL Class A (General Knowledge) bank, and it holds 15 of the bank's 300 multiple-choice questions — roughly 5% 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.
Trailer 'crack-the-whip' effect refers to:
Answer: A — Rear trailers swinging out wider than the front during sudden lane changes — most dangerous in triples
B) Misconception. A) Correct — sudden steering inputs amplify down the chain; rear trailer can swing into adjacent lane or roll over. C/D) Unrelated.
While inspecting a converter dolly a driver finds the pintle hook latch is not closed. What is the correct action?
Answer: A — Correct the latch and secure it before the combination is moved at all
A) Correct — an unlatched pintle hook can release the dolly, and the coupling must be secure before movement. B) Chains are a backup for a failure, not a substitute for a working coupling. C) Deferring the fix leaves the connection able to separate at any moment. D) A separation on private property is just as dangerous and the rule does not depend on location.
A driver inspecting a set of doubles should pay particular attention to which item that a single trailer does not have?
Answer: D — The dolly's air lines, pintle connection and its own tires and brakes
D) Correct — the converter dolly is the additional assembly in a doubles set and it has couplings, lines, tires and brakes of its own. B) The tractor fifth wheel is inspected on any combination, so it is not unique to doubles. C) Lamps and reflectors are checked on every trailer regardless of configuration. A) Landing gear is present on single trailers as well and is not what distinguishes the set.
Before backing a converter dolly under the second trailer, what should the driver check FIRST?
Answer: D — That the trailer height is right so the kingpin will enter above the fifth wheel
D) Correct — the trailer must be low enough that it is raised slightly by the dolly, so the fifth wheel goes under the apron and not over it or into the nose. B) Air lines are connected after the coupling is made, not before backing under. C) The dolly brakes are typically locked while backing under so the dolly does not roll away. A) Raising the landing gear before the coupling is locked drops the trailer on the ground.
A doubles driver must make a sudden lane change to avoid debris. What is the BEST technique?
Answer: B — Make a gradual steering change and avoid an abrupt return to the original lane
B) Correct — gradual inputs limit the amplification that a sharp return would multiply at the rear trailer. A) Quick reversals are exactly the input that produces the whip effect and rear rollover. C) Hard braking during a steering maneuver adds a skid risk to an already unstable input. D) Committing to the shoulder and holding removes your ability to adjust as conditions change.
In a set of triples, where must the air shut-off valve be closed?
Answer: D — At the rear of the last trailer in the combination
D) Correct — every valve must be open to pass air down the set except the one at the very back of the last trailer, which is closed to hold pressure in. B) Closing at the first trailer would starve the two trailers behind it of air. C) Closing at the middle trailer leaves the last trailer without service air. A) The front of the last trailer is where air must enter it, so closing there defeats the purpose.
A driver couples a converter dolly to the rear of the first trailer. What must be done with the dolly's safety chains?
Answer: C — They must be secured to the first trailer so the dolly cannot break free
C) Correct — the chains connect the dolly to the trailer ahead of it so a coupling failure does not release the dolly and the trailer behind it. B) Slack is needed for turning, but chains left unattached serve no purpose at all. A) Landing gear legs are not rated attachment points and would tear away under load. D) Hooking chains back to the dolly itself creates no connection to the towing vehicle.
Why do doubles and triples require more space when changing lanes than a single trailer combination?
Answer: D — Because the added length and off-tracking need more clear room front and rear
D) Correct — a longer set covers more roadway and its rear units track differently, so more clear space is needed on both ends. B) More axles add weight and slow acceleration rather than increasing it. C) Mirror coverage does change, but the space requirement comes from length and tracking. A) Couplings do not delay steering response at the tractor, though they do amplify it at the rear.
Doubles and triples require:
Answer: A — Special endorsement T
A) Correct — T endorsement required for doubles and triples. B) Tank vehicle. C) Wrong. D) Hazmat.
When coupling a second trailer to a set of doubles, which trailer should be positioned first in the combination?
Answer: D — The heavier trailer should be placed in the position closest to the tractor
D) Correct — the heavier trailer goes first because a heavy rear trailer amplifies whip and raises rollover risk at the back of the set. B) Putting the light trailer up front leaves the heavy one where rearward amplification is worst. C) Tire condition matters for safety but does not determine trailer order. A) Delivery sequence is an operational convenience that cannot override the weight rule.
3 cards from the 6 in this chapter.
Where should the shut-off valves at the rear of a trailer be positioned on the last trailer of a set?
Closed on the rear of the last trailer to seal the air system, and open on the rear of every trailer that has another trailer coupled behind it.
When coupling a second trailer, in what order should trailers be positioned by weight?
The more heavily loaded trailer goes directly behind the tractor in the first position; the lighter trailer goes to the rear to reduce crack-the-whip instability.
Before backing a tractor under a converter dolly, what brake and chock precautions apply?
Secure the second trailer with its brakes or wheel chocks, release the dolly brakes and lower its support, and make sure the dolly will not roll — never back under a dolly whose wheels are free to move on a grade.
These are a sample. The full Doubles and Triples chapter runs 21 items with per-chapter progress tracking, on the web and in the iOS app.