22 Doubles & Triples (T) Practice Questions & Answers
Every Doubles & Triples (T) practice question from the CDL Practice Test, with the correct answer and a short explanation.
Start practice test →1. In a set of doubles or triples, which unit is the most likely to turn over, and why?
- A.The tractor, because its drive axles take the hardest side force in a turn
- B.The middle trailer, because its two couplings work against each other
- C.The last trailer, because rearward amplification multiplies your steering✓ Answer
- D.The first trailer, because it carries the greatest share of the load
The crack-the-whip effect is caused by rearward amplification: a steering movement made at the tractor is magnified at each coupling behind it, so the rear-most trailer swings hardest and rolls first.
Source: AAMVA Model CDL Manual, Doubles and Triples (crack-the-whip effect); 49 CFR 383.115Report a problem with this question
2. A chart rates a set of triples at a rearward amplification of 3.5, against 1.0 for a five-axle tractor-semitrailer. In a quick lane change, what does the 3.5 tell you?
- A.Its rear trailer rolls over about 3.5 times as easily as the tractor-semi✓ Answer
- B.Its stopping distance grows about 3.5 times once all three are loaded
- C.Its rear trailer swings about 3.5 feet wider than the path of the tractor
- D.It needs about 3.5 seconds more following distance than the tractor-semi
Rearward amplification is a rollover ratio, not a distance or a time: a value of 3.5 means the last trailer can be rolled 3.5 times as easily as the tractor unit in the same quick lane change.
Source: AAMVA Model CDL Manual, Combination Vehicles (rearward amplification) and Doubles and TriplesReport a problem with this question
3. You are making up a set of doubles with one heavily loaded trailer and one light trailer. For the safest handling on the road, where does the heavy trailer belong?
- A.Directly behind the tractor, with the lighter trailer in the rear position✓ Answer
- B.In either position, as long as the total gross weight stays within limits
- C.In the rear, but only when the converter dolly carries two axles, not one
- D.In the rear position, so that its weight steadies the lighter trailer ahead
The more heavily loaded semitrailer goes in first position, right behind the tractor, and the lighter one goes last, because a heavy trailer at the tail end of the set feeds the crack-the-whip effect and is far easier to roll.
Source: AAMVA Model CDL Manual, Doubles and Triples (coupling twin trailers)Report a problem with this question
4. On a set of doubles, how must the shut-off (cut-out) cock valves in the service and supply lines be set?
- A.Open at the rear of both trailers, so that air circulates through the set
- B.Open at the rear of the front trailer and closed at the rear of the last one✓ Answer
- C.Closed at the rear of the front trailer and open at the rear of the last one
- D.Closed at both trailers, because the dolly tank already feeds the brakes
Every shut-off valve ahead of the last trailer must be open so air can pass rearward, and the pair at the very back must be closed so the system can hold pressure instead of venting to atmosphere.
Source: AAMVA Model CDL Manual, Combination Vehicles (shut-off valves) and Doubles and TriplesReport a problem with this question
5. On a set of triples, how should the shut-off valves at the rear of the middle trailer be set?
- A.Open on the service line only, with the supply line closed at that point
- B.Closed, because the middle trailer already has its own charged air tank
- C.Closed on the service line only, with the supply line left open there
- D.Open, because air must still pass rearward to the third trailer's brakes✓ Answer
The middle trailer of a set of triples is not the last trailer, so both of its rear valves stay open; only the valves at the rear of the third and final trailer are closed.
Source: AAMVA Model CDL Manual, Doubles and Triples (inspecting doubles and triples)Report a problem with this question
6. During the walk-around of a set of doubles, how must the converter dolly's air tank drain valve (petcock) be found?
- A.Open a half turn, so the compressor cannot overcharge the dolly tank
- B.Closed, so that the dolly tank can hold the air that its brakes need✓ Answer
- C.Closed in freezing weather only, and left open the rest of the year
- D.Open, so that water drains out of the dolly tank while you are rolling
The dolly's brakes work off the air stored in its own tank, so the drain valve is opened only to bleed off water or to release the dolly brakes while positioning it, and must be closed for the road.
Source: AAMVA Model CDL Manual, Doubles and Triples (converter dolly air tank drain valve: closed)Report a problem with this question
7. How can you tell during your inspection that a converter dolly is equipped with antilock brakes?
- A.An amber decal is fixed to the dolly's tow bar near the pintle eye
- B.A blue malfunction lamp is mounted on the right side of the dolly
- C.A red reflective triangle is stamped on the dolly's air tank shell
- D.A yellow malfunction lamp is mounted on the left side of the dolly✓ Answer
Federal rules require antilock brakes on converter dollies built on or after March 1, 1998, and require those dollies to carry an external yellow malfunction lamp on the left side so a driver can see it from outside.
Source: 49 CFR 393.55 (antilock brake systems, external malfunction lamp); AAMVA Model CDL Manual, Doubles and TriplesReport a problem with this question
8. Why must you never unlock the pintle hook while the converter dolly is still under the rear trailer?
- A.The dolly air tank can drain, setting the rear trailer's spring brakes
- B.The dolly fifth wheel can jam shut, locking the kingpin in place
- C.The dolly tow bar can fly up, risking injury and making re-coupling hard✓ Answer
- D.The dolly wheels can lock up, dragging the rear trailer out of line
While the dolly is still under the trailer its tow bar is loaded, and releasing the pintle hook lets that stored force snap the bar upward, which can injure you and leaves the dolly badly placed for re-coupling.
Source: AAMVA Model CDL Manual, Doubles and Triples (uncoupling twin trailers)Report a problem with this question
9. After coupling a set of doubles and charging the system, how do you check that air actually reaches the rearmost trailer?
- A.Crack a glad hand at the front of the dolly and feel for escaping air
- B.Pull the trailer air supply knob out and count how long the warning buzzer sounds
- C.Watch the dash gauge for a pressure drop while a helper sets the brakes
- D.Open the emergency line shut-off at the rear of the last trailer and listen✓ Answer
With the trailer air supply knob pushed in, opening the emergency line valve at the very back and hearing air escape proves the supply line is charged all the way through every trailer and dolly in the set.
Source: AAMVA Model CDL Manual, Doubles and Triples (check that air flows to all trailers)Report a problem with this question
10. You open the emergency line shut-off valve at the rear of the last trailer and hear no air escaping. What does that tell you?
- A.Air is not reaching the back of the set, so those brakes will not work✓ Answer
- B.The set is fully charged, because a sealed line is what you want to hear
- C.The dolly relay valve is holding pressure, so the check has passed
- D.The tractor protection valve has closed, which is normal for this test
Silence at the rear valve means the supply line is not charged all the way back, usually because a shut-off valve on a trailer or dolly was left closed, and brakes that get no air cannot be applied.
Source: AAMVA Model CDL Manual, Doubles and Triples (doubles/triples air brake check)Report a problem with this question
11. Converter dollies and trailers built before 1975 are not required to have spring brakes. What does that mean when you park one of them?
- A.Chock the wheels, because its emergency brakes hold only while air lasts✓ Answer
- B.Leave the trailer air supply knob pushed in, because that keeps air on
- C.Set the trailer hand valve, because it will hold the brakes on overnight
- D.Close the emergency line shut-off valve, because it traps air in the drums
A trailer without spring brakes has no true parking brake: its emergency brakes are held on by air stored in the trailer tank, and once that air leaks away the trailer is free to roll unless it is chocked.
Source: AAMVA Model CDL Manual, Combination Vehicles (trailer service, parking and emergency brakes); 49 CFR 393.43Report a problem with this question
12. Before you back a converter dolly under the second trailer, what must that trailer's height be?
- A.Slightly lower than the center of the fifth wheel, so it is raised a little✓ Answer
- B.As high as the landing gear will crank, so the jaws clear the apron plate
- C.Slightly higher than the fifth wheel, so the skid plate slides under easily
- D.Exactly level with the fifth wheel, so the kingpin drops straight in
The trailer must sit slightly below the center of the fifth wheel so that backing under it lifts the trailer a little, which is what seats the apron plate flat and lets the jaws close on the kingpin.
Source: AAMVA Model CDL Manual, Doubles and Triples (connect converter dolly to rear trailer)Report a problem with this question
13. Which of these is an accepted way to secure the second trailer before you back the dolly under it?
- A.Lower the second trailer's landing gear fully and trust it not to roll
- B.Turn the dolly wheels against the trailer tires so they cannot roll away
- C.Set the tractor parking brake and let it hold the second trailer in place
- D.Charge its air tank through the emergency line, then disconnect that line✓ Answer
Charging the trailer air tank and then pulling the emergency line sets that trailer's emergency brakes, which is one of the three accepted methods along with its own spring brakes and chocking the wheels.
Source: AAMVA Model CDL Manual, Doubles and Triples (secure second trailer)Report a problem with this question
14. What must the visual check of the finished dolly-to-trailer coupling show?
- A.The locking jaws closed around the head of the kingpin, not on the shank
- B.A thin gap between the apron plate and the skid plate to allow for flex
- C.No space between the apron plate and skid plate, jaws shut on the kingpin✓ Answer
- D.The release arm standing clear so the safety latch can be reached by hand
A correct coupling shows the apron plate flat against the fifth wheel skid plate with no daylight between them, and the locking jaws closed around the shank of the kingpin rather than over its head.
Source: AAMVA Model CDL Manual, Doubles and Triples (make visual check of coupling)Report a problem with this question
15. When uncoupling a set of doubles, why do you pull the dolly out from under the rear trailer before unhitching it from the trailer in front?
- A.So the dolly air tank drains completely before the lines are taken apart
- B.So the tow bar stays held down and cannot fly up and injure someone✓ Answer
- C.So the dolly brakes stay charged while the rear trailer is still coupled
- D.So the rear trailer's landing gear can take the weight of the dolly axles
Pulling the dolly clear first unloads the tow bar, so that when the pintle hook is finally released nothing is under spring tension to whip the bar upward at the person standing beside it.
Source: AAMVA Model CDL Manual, Doubles and Triples (uncouple converter dolly)Report a problem with this question
16. What is the first step in uncoupling a set of triples?
- A.Disconnect the middle trailer's air lines before anything else is moved
- B.Release the pintle hook on the front dolly so the set breaks apart in the middle
- C.Uncouple the third trailer by pulling its dolly out, then unhitch the dolly✓ Answer
- D.Uncouple the tractor from the first trailer, then work your way rearward
A set of triples is taken apart from the back forward: the third trailer comes off first by pulling its dolly out and then unhitching that dolly, after which the rest is uncoupled like any double.
Source: AAMVA Model CDL Manual, Doubles and Triples (uncouple triple-trailer rig)Report a problem with this question
17. What is the trailer hand valve (trolley valve) intended for?
- A.Pulling a jackknifed trailer back into line behind the tractor unit
- B.Parking a set of doubles once the trailer air supply knob is pulled out
- C.Holding the rig still on an upgrade while the air pressure rebuilds
- D.Testing that the trailer brakes work, and for nothing else on the road✓ Answer
The hand valve applies only the trailer brakes, so using it to park lets the trailer roll free once air bleeds off, and using it on a skid drags the trailer and makes a jackknife worse instead of better.
Source: AAMVA Model CDL Manual, Combination Vehicles (trailer hand valve)Report a problem with this question
18. An off-ramp can be taken safely at 35 mph in a single-trailer rig. What should you do on that same ramp with a set of doubles?
- A.Enter it slower than that, because a set rolls over more easily than one✓ Answer
- B.Take it at the same speed, since the ramp rating covers all combinations
- C.Brake hard inside the curve, so the trailers stay stretched out behind
- D.Take it faster, so the rear trailer is pulled straight through the curve
A speed that is safe on a curve or ramp for a straight truck or a single-trailer combination can still be too fast for a set of doubles, so you slow down before the ramp and steer gently through it.
Source: AAMVA Model CDL Manual, Doubles and Triples (prevent trailer from rolling over)Report a problem with this question
19. Compared with a single-trailer rig, what do doubles and triples require of you in traffic?
- A.A tighter merge, so the last trailer is never left hanging in a lane
- B.More following distance and bigger gaps before entering or crossing traffic✓ Answer
- C.The same following distance, since the added trailers add brake drums too
- D.Less following distance, because the extra axles shorten the stopping path
Doubles and triples are longer and cannot be turned or stopped suddenly, so the manual calls for extra following distance, larger gaps before entering or crossing traffic, and careful side checks before any lane change.
Source: AAMVA Model CDL Manual, Doubles and Triples (manage space)Report a problem with this question
20. What is a converter gear, or converter dolly?
- A.A spare axle set carried under a trailer to spread weight over a bridge
- B.A hitch plate bolted to a trailer frame so a pintle hook can be mounted
- C.A coupling device with one or two axles and a fifth wheel for a trailer✓ Answer
- D.A gearbox that splits engine torque between the tractor's two drive axles
A converter dolly is a coupling device of one or two axles carrying a fifth wheel, by which a second semitrailer can be hitched behind a tractor-trailer to make up a double-bottom rig.
Source: AAMVA Model CDL Manual, Doubles and Triples (converter gear definition)Report a problem with this question
21. You are going to wheel a converter dolly by hand into position in front of the second trailer. How do you release the dolly's brakes first?
- A.Pull the tractor's trailer air supply knob out to vent the dolly air tank
- B.Open the air tank petcock, or use the parking brake control if it has one✓ Answer
- C.Back the tractor against the tow bar to compress the dolly brake springs
- D.Crack both glad hands on the dolly so that the service line bleeds down
A dolly without spring brakes is held by air trapped in its own tank, so opening the petcock bleeds that air and frees the wheels; a dolly that does have spring brakes is released with its parking brake control.
Source: AAMVA Model CDL Manual, Doubles and Triples (position converter dolly)Report a problem with this question
22. Why must you be extra careful with doubles and triples in bad weather or on mountain grades?
- A.The extra couplings cut engine braking, so the retarder must be shut off
- B.More dead axles are dragged by your drive axles, so traction is lost sooner✓ Answer
- C.The added trailer brakes make the drive wheels lock before the trailers
- D.The longer wheelbase keeps the rear trailer out of your mirrors on the grades
A set has greater length and many more dead (unpowered) axles for the drive axles to pull, which reduces the grip available at the drive wheels and makes skids and loss of traction more likely.
Source: AAMVA Model CDL Manual, Doubles and Triples (adverse conditions)Report a problem with this question
Practice questions based on the AAMVA/FMCSA model CDL manual. Requirements vary by state — study your state's official CDL manual before your exam. Official CDL info →