20 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. When pulling a set of doubles or triples, which unit is most likely to turn over, and why?
- A.The tractor, because it carries the greatest share of the weight
- B.The first trailer, because it absorbs the full force of the turn
- C.The last trailer, because of the "crack-the-whip" effect caused by rearward amplification✓ Answer
- D.All units equally, since they are coupled together and share the same load
Each coupling point multiplies the sideways motion of the unit ahead of it. This rearward amplification produces the crack-the-whip effect, so a steering input the driver barely feels in the cab becomes a violent sideways swing at the tail of the rig. That is why the last trailer is the one most likely to roll, and why steering must be gentle and gradual.
Source: AAMVA Model CDL Manual 7.1.1 (Prevent Trailer from Flipping Over); 6.1.2 (rearward amplification)Report a problem with this question
2. The CDL manual states that triples have a rearward amplification of 3.5. What does that figure mean?
- A.The last trailer of a triple can turn over 3.5 times more easily than a five-axle tractor-semitrailer✓ Answer
- B.A triple needs 3.5 times the stopping distance of a single trailer
- C.A triple can safely take a curve 3.5 mph faster than a set of doubles
- D.The tractor is 3.5 times more stable than the trailers behind it
Rearward amplification is a ratio comparing the sideways motion of the rear unit to that of the tractor. A value of 3.5 means the whipping motion at the tail of a triple is three and a half times what the tractor experiences, so the last trailer rolls that much more easily than a conventional five-axle tractor-semitrailer. It describes rollover tendency, not speed or stopping distance.
Source: AAMVA Model CDL Manual 6.1.2 / 7.1.1 (rearward amplification of 3.5 for triples)Report a problem with this question
3. You are pulling a set of doubles in which one trailer is heavily loaded and the other is nearly empty. Where should the heavily loaded trailer go?
- A.In the rear position, to keep weight off the tractor drive axles
- B.In either position, as long as the total gross weight is legal
- C.In the rear position, so its weight sits over the converter dolly
- D.In the first position, directly behind the tractor✓ Answer
Because rearward amplification makes the rear trailer the one most likely to roll, the light trailer belongs at the tail where the whipping is worst, and the heavy trailer goes first, directly behind the tractor. A heavy trailer in the rear has a high load and more momentum exactly where the sideways motion is multiplied, which greatly increases rollover risk.
Source: AAMVA Model CDL Manual 7.2.1; 49 CFR 383.115(b) (proper placement of the heaviest trailer)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 air lines be set before driving?
- A.Open at the rear of the first trailer; closed at the rear of the last trailer✓ Answer
- B.All shut-off valves closed until the system reaches governor cut-out pressure
- C.Closed at the rear of the first trailer; open at the rear of the last trailer
- D.All shut-off valves open, including those at the rear of the last trailer
The rule is that every shut-off valve is open except the ones at the rear of the last trailer. The open valves let service and supply air pass back through the combination so every set of brakes works; the closed valves at the very rear seal the lines so the air is not simply dumped to the atmosphere.
Source: AAMVA Model CDL Manual 7.3.1 (Additional Things to Check on Doubles/Triples); 6.2.7Report a problem with this question
5. On a set of triples, which shut-off valves must be OPEN?
- A.Those at the rear of the first and middle trailers, and on the converter dollies — but not those at the rear of the last trailer✓ Answer
- B.Only the ones at the rear of the first trailer
- C.Only the ones at the rear of the last trailer
- D.None; on triples every shut-off valve is closed and air is fed through the dolly tanks
The same rule applies no matter how many trailers you pull: all shut-off valves are open except those at the rear of the last trailer. On a triple that means the first and the middle trailer both stay open (plus any valves on the dollies), because air has to travel all the way to the rearmost unit for its brakes to work.
Source: AAMVA Model CDL Manual 7.3.1 (shut-off valves on first, middle and last trailers)Report a problem with this question
6. During a walk-around inspection of a converter dolly, how should the dolly's air tank drain valve (petcock) and its shut-off valves be found?
- A.Drain valve open and shut-off valves closed
- B.Drain valve closed and shut-off valves open✓ Answer
- C.Drain valve closed and shut-off valves closed
- D.Drain valve open and shut-off valves open
The drain valve is opened only to bleed water out of the tank or to release the dolly brakes while positioning it; left open in service it prevents the tank from holding pressure, so it must be closed before driving. The dolly's shut-off valves sit in the through-lines feeding the trailer behind it, so they must be open for air to reach the rear of the combination.
Source: AAMVA Model CDL Manual 7.3.1 (converter dolly inspection items)Report a problem with this question
7. Why must you never unlock the pintle hook while the converter dolly is still under the rear trailer?
- A.Air pressure will be dumped from the entire combination and the tractor brakes will lock
- B.The dolly tow bar may fly up, which can injure you and makes recoupling very difficult✓ Answer
- C.Unlocking the hook releases the dolly's spring brakes and it can roll away
- D.The rear trailer's landing gear will collapse under the shifted weight
While the dolly is still coupled under the trailer, the trailer's weight loads the fifth wheel and puts the tow bar under tension. Release the pintle hook and that stored force snaps the tow bar upward — a serious injury hazard, and once it is up you cannot easily line it back up to recouple. Always pull the dolly out from under the trailer first, then unhook it.
Source: AAMVA Model CDL Manual 7.2.2 (Uncouple the Converter Dolly)Report a problem with this question
8. What is the correct way to check that air actually reaches the rearmost trailer of a set of doubles or triples?
- A.Open the drain valve on the converter dolly air tank and listen for escaping air
- B.Watch the dash air gauge for a drop while pressing the brake pedal in the cab
- C.Pull the trailer air supply knob out and count how long the trailer brakes take to apply
- D.With the system charged, go to the rear of the LAST trailer and open the emergency line shut-off — you should hear air escaping✓ Answer
Only air that actually arrives at the last unit proves the whole system is charged. After charging the supply line with the trailer air supply knob pushed in, opening the emergency line shut-off at the rear of the last trailer should produce an audible rush of air; repeating it on the service line with the brakes applied proves control air gets there too. A cab gauge cannot tell you whether a valve back in the string is closed.
Source: AAMVA Model CDL Manual 7.4.1 (Check Air Flow to All Trailers)Report a problem with this question
9. How can you tell that a converter dolly is equipped with antilock brakes (ABS)?
- A.By the yellow ABS malfunction lamp on the left side of the dolly✓ Answer
- B.By a blue lamp next to the pintle eye on the tow bar
- C.By a red triangular decal mounted on the rear of the dolly
- D.You cannot — converter dollies are never equipped with ABS
Converter dollies and trailers manufactured on or after March 1, 1998 are required to have antilock brakes, and the way the standard makes that visible to the driver is a yellow malfunction lamp mounted on the left side of the unit. Checking the lamp tells you both that ABS is present and, if it stays lit, that it is not working.
Source: AAMVA Model CDL Manual 7.1.8 / 6.3.1 (ABS on converter dollies built on or after March 1, 1998)Report a problem with this question
10. Converter dollies and trailers built before 1975 are not required to have spring brakes. What does that mean for the driver?
- A.Their brakes lock automatically whenever air pressure falls below 20 psi and stay locked
- B.Their parking brakes must be set with the trailer hand valve before leaving the cab
- C.Those units may not legally be used in a double or triple combination
- D.They have only emergency brakes fed by the trailer air tank and no parking brake, so the wheels must be chocked✓ Answer
Without spring brakes there is nothing mechanical to hold the unit; the emergency brakes hold only as long as pressure remains in the trailer air tank, and that pressure can bleed off over time. Because the unit can roll once the air leaks down, wheel chocks are the only dependable way to secure it while coupling, uncoupling or parking.
Source: AAMVA Model CDL Manual 7.1.8 (dollies/trailers built before 1975 and spring brakes)Report a problem with this question
11. Before backing a converter dolly under the second trailer, what should the height of that trailer be?
- A.At whatever height the landing gear happens to be, since the fifth wheel self-adjusts
- B.Slightly higher than the fifth wheel so the dolly slides under without contact
- C.Slightly lower than the center of the fifth wheel, so the trailer is raised slightly as the dolly is pushed under✓ Answer
- D.Exactly level with the top of the fifth wheel
Setting the trailer slightly below the center of the fifth wheel means the dolly lifts the trailer a little as it slides under, which loads the fifth wheel and lets the locking jaws close fully around the kingpin. If the trailer is too high the dolly can pass underneath without engaging or make a high hitch, leaving the jaws unlocked.
Source: AAMVA Model CDL Manual 7.2.1 (Connect Converter Dolly to Rear Trailer)Report a problem with this question
12. Which of the following is an accepted way to secure the second trailer before backing the converter dolly under it?
- A.Rely on the weight of the loaded trailer to keep it from moving
- B.Apply the trailer hand valve (trolley valve) in the cab and leave it applied
- C.Lower the landing gear all the way down and let it hold the trailer in place
- D.Drive up, connect the emergency line, charge the trailer air tank, then disconnect the emergency line to set the trailer emergency brakes✓ Answer
The manual gives three ways to secure the second trailer: its own spring brakes, wheel chocks, or charging the trailer tank through the emergency line and then disconnecting that line so loss of supply pressure applies the emergency brakes. The third method works only if the slack adjusters are properly set. The hand valve is a test device that can bleed off, and landing gear or cargo weight will not stop a trailer from rolling.
Source: AAMVA Model CDL Manual 7.2.1 (Secure Second Trailer — three methods)Report a problem with this question
13. When uncoupling the rear trailer of a set of doubles, what must be done before you pull the converter dolly out from under it?
- A.Release the pintle hook and unhook the safety chains so the dolly comes free easily
- B.Raise the dolly's landing gear all the way up and drain its air tank completely
- C.Set the trailer hand valve and back up firmly against the dolly to break the coupling loose
- D.Lower the rear trailer's landing gear enough to take some weight off the dolly, close the air shut-offs, disconnect and secure the dolly's air and electric lines, release the dolly brakes and unlatch its fifth wheel✓ Answer
The rig must be straight on firm level ground with the parking brakes on. Lowering the landing gear takes weight off the dolly fifth wheel so it will slide out, closing the shut-offs keeps the system from dumping air, and the lines must be disconnected and hung up so they are not torn off. Only after the dolly has been pulled clear of the trailer is the pintle hook released.
Source: AAMVA Model CDL Manual 7.2.2 (Uncouple Rear Trailer)Report a problem with this question
14. What is the first step in uncoupling a set of triples?
- A.Release both pintle hooks at the same time so the dollies drop free together
- B.Pull the dolly out from under the third (rear) trailer and unhitch that dolly, then uncouple the rest as an ordinary double✓ Answer
- C.Remove the middle trailer first, because it carries the least weight
- D.Uncouple the tractor from the first trailer, then work backward
A triple is taken apart from the back forward. The rearmost unit is removed first so the remaining rig is simply a double-bottom, which is then uncoupled by the normal doubles procedure. Working from the front would leave unbraked, unsupported trailers coupled behind the point you disconnected.
Source: AAMVA Model CDL Manual 7.2.3 (Coupling and Uncoupling Triple Trailers)Report a problem with this question
15. After coupling the converter dolly to the rear trailer, what should the visual inspection of the coupling show?
- A.About one inch of space between the upper and lower fifth wheel to allow for flexing
- B.The release arm pulled out and the safety latch left open so the coupling can flex
- C.No space between the upper and lower fifth wheel, with the locking jaws closed around the shank of the kingpin✓ Answer
- D.The locking jaws closed around the head of the kingpin
Any gap between the upper and lower fifth wheel means the coupling is not fully seated and the trailer can come loose. The jaws must close around the shank — the narrow part of the kingpin below the head — because jaws sitting on the head only appear locked and can pop free under load. Confirm this visually after pulling against the pin to test the lock.
Source: AAMVA Model CDL Manual 7.2.1 (Connect Converter Dolly to Rear Trailer — visual check); 6.5.1Report a problem with this question
16. What is the trailer hand valve (trolley valve) intended for?
- A.Parking the combination once the tractor has been shut down
- B.Straightening out a trailer skid at highway speed
- C.Testing the trailer brakes only — it should not be used while driving or for parking✓ Answer
- D.Slowing the rig on long downgrades to save the service brakes
The hand valve applies only the trailer brakes. Using it while moving can put the trailer wheels into a skid — exactly what causes a jackknife — and using it to park is unsafe because air can leak out of the system and release the brakes on trailers that have no spring brakes. It is a test tool, and on the road the foot brake, which applies all brakes, is what you use.
Source: AAMVA Model CDL Manual 6.2.2 (Trailer Hand Valve)Report a problem with this question
17. With the system charged, you open the emergency line shut-off at the rear of the last trailer and hear no air escaping. What is the most likely cause?
- A.A shut-off valve on one of the trailers or on a dolly is not open, so air is not reaching the rear✓ Answer
- B.The tractor protection valve has failed and is venting in the cab
- C.The trailer spring brakes are applied, which blocks the emergency line
- D.Nothing is wrong — silence means the system is fully charged and sealed
If the supply line were charged all the way back, opening that valve would vent air loudly. Silence means the path is blocked somewhere ahead of it — almost always a shut-off valve left closed at the rear of a trailer or on a dolly. Until air reaches the last unit its brakes cannot work, so you must find and open the closed valve before moving.
Source: AAMVA Model CDL Manual 7.4.1 / 7.3.1 (shut-off valve positions and air flow check)Report a problem with this question
18. How should a driver handle curves, ramps and highway off-ramps with a set of doubles or triples?
- A.Hold the posted ramp speed, since posted speeds are set to be safe for every vehicle
- B.Accelerate through the curve to keep the trailers tracking straight behind the tractor
- C.Slow down before entering the curve and steer smoothly through it, because a speed that is safe for a straight truck or a single trailer can be too fast for doubles or triples✓ Answer
- D.Brake hard in the middle of the curve so the trailers settle behind the tractor
Posted ramp speeds are advisory and are not calculated for the rollover threshold of a multi-trailer rig. Slowing before the curve lets you take it at a steady, gentle rate; braking or steering sharply once you are in it shifts weight and feeds the crack-the-whip effect, which is what puts the rear trailer on its side.
Source: AAMVA Model CDL Manual 7.1.2 (Take Curves, Ramps and Off-Ramps Slowly)Report a problem with this question
19. Compared with a single-trailer rig, what do doubles and triples require in traffic?
- A.The same space as any other truck, since length does not affect stopping
- B.No lane-change check at all, as long as the mirrors are properly adjusted
- C.Less following distance, because the extra axles give them shorter stopping distances
- D.More following distance and larger gaps before entering or crossing traffic, plus a full check of both sides before any lane change✓ Answer
A multi-trailer combination is longer and cannot be turned or stopped suddenly, so it needs more room in every direction: a bigger following gap (about one second for every 10 feet of length below 40 mph, plus one more second above that), a longer clear gap to enter or cross traffic, and a deliberate check of the full length of both sides before changing lanes. Looking far ahead lets you make speed and lane changes gradual instead of abrupt.
Source: AAMVA Model CDL Manual 7.1.3–7.1.5 (More Space, Look Far Ahead); 49 CFR 383.115(d)Report a problem with this question
20. What is a converter gear, commonly called a converter dolly?
- A.A jack used to raise and lower the trailer landing gear
- B.A coupling device of one or two axles and a fifth wheel, by which a semitrailer is coupled to the rear of a tractor-trailer to form a double-bottom rig✓ Answer
- C.The sliding fifth wheel assembly mounted on the tractor frame
- D.The air valve that switches supply air from one trailer to the next
A semitrailer has no front axle of its own, so it cannot be towed behind another trailer directly. The converter dolly supplies that missing front axle set plus a fifth wheel, and it connects to the trailer ahead through its tow bar and pintle eye — which is why the dolly is inspected as a separate unit with its own tires, brakes, air tank and lights.
Source: AAMVA Model CDL Manual 7.1.8 / Section 7 Test Your Knowledge (definition of converter gear)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 →