CDL doubles and triples practice test
The Doubles and Triples (T) test is for you if you pull more than one trailer. It follows federal rules, so these 132 questions apply in every state.
Doubles and Triples practice test
Answer at your own pace. After each question you see the correct answer and why, and at the end a review of what you missed.
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Doubles and Triples questions with answers
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Why is a set of doubles or triples more likely to roll over than a tractor pulling a single semitrailer?
Show answer
Answer B: Rearward amplification produces a crack the whip effect through the couplings
Steering movement is passed back through each coupling and grows as it travels, so the trailers swing much harder than the tractor does.
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Commercial driver manuals give triples a rearward amplification figure of 3.5. What does that figure describe?
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Answer D: The last trailer rolls over about 3.5 times as easily as a five axle tractor semitrailer
Rearward amplification measures the crack the whip effect, so the figure describes how much more easily the last trailer rolls over, not length, braking or axle loads.
Source: oregon.gov
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You have taken a particular curve many times at the posted advisory speed in a single trailer rig. Now you are pulling triples. What should you assume?
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Answer A: That speed may be too fast for this set and you should slow down further
A speed that is safe for a straight truck or a single semitrailer can still tip the last trailer of a double or triple.
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Moving from the tractor toward the back of a set of triples, what happens to a sudden steering movement?
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Answer A: It grows larger at every coupling
Every articulation point adds to the swing, which is why the rear trailer moves far more than the tractor and reaches its tipping point first.
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Which driving habit does the most to protect a set of doubles or triples from the crack the whip effect?
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Answer B: Making every steering input and lane change gradual
The effect is triggered by abrupt steering, so smooth and unhurried inputs keep the rear trailer from being whipped sideways.
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Why does looking far down the road matter even more when pulling triples than when driving a straight truck?
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Answer C: It lets you slow down or change lanes gradually instead of suddenly
Early information gives you time for the gentle movements the set needs, because a sudden correction is exactly what rolls the rear trailer.
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An animal steps into your lane while you are pulling doubles at highway speed. What is the main danger of a violent swerve?
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Answer D: Rolling the rear trailer over
A violent swerve is the input that rearward amplification magnifies, and the rear trailer can be tipped even while the tractor stays upright.
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How does the height of the freight inside the trailers affect a set of doubles?
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Answer D: A higher center of gravity makes a rollover easier to trigger
Loading heavy freight high raises the center of gravity, and that combines with the amplified swing at the rear to make a rollover more likely.
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On a slippery road, why is a set of doubles harder to manage than a single semitrailer?
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Answer C: The amplified swing at the rear can break traction and push a trailer sideways
Reduced grip lets the whipped rear unit slide instead of following the tractor, so a movement that only feels sharp on dry pavement can start a skid.
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You are given one loaded trailer and one nearly empty trailer to build a set of doubles. How should the set be put together?
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Answer A: Loaded trailer first, empty trailer at the rear
The rear position takes the amplified swing, so the lighter unit belongs there and the heavier, steadier trailer goes directly behind the tractor.
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Where should the heaviest items be placed inside each trailer of a set of doubles?
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Answer D: Low on the floor and spread out
Heavy cargo kept low and evenly distributed holds the center of gravity down, which is the main defense against the rollover risk the set already carries.
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What is a converter dolly?
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Answer D: An axle assembly with a tow bar and a fifth wheel used to tow a semitrailer behind another trailer
The dolly supplies the front axle and the coupling that a second or third semitrailer needs in order to be towed as a full trailer.
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The rear trailer of your set has no spring brakes. How do you secure it before backing the dolly underneath?
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Answer A: Connect the emergency line, charge the trailer air tank, then disconnect the line so the emergency brakes set
Charging the tank and then pulling the emergency line applies the trailer emergency brakes, and the wheels should still be chocked if there is any doubt.
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How are the brakes on a converter dolly released so the dolly can be moved into position?
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Answer C: By opening the air tank petcock, or by using the dolly parking brake control if it has spring brakes
Letting the air out through the petcock, or releasing the spring brake control, frees the dolly wheels so it can be moved into place.
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When you wheel a converter dolly into place ahead of the rear trailer, how should it be aligned?
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Answer A: In line with the trailer kingpin
The dolly fifth wheel has to meet the kingpin squarely, so the dolly is lined up with the kingpin before it is backed under.
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A converter dolly is too heavy to be moved safely by hand. What is the alternative?
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Answer D: Use the tractor and first semitrailer to pick it up and set it in position
The combination can hook the dolly, carry it to the nose of the rear trailer and set it down, which avoids the strain and the risk of pushing it by hand.
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You have backed the first semitrailer onto the dolly tow bar. What two things must be done next?
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Answer B: Lock the pintle hook and secure the dolly support in the raised position
The pintle hook has to be latched around the dolly eye and the support raised so the dolly can be towed without dragging on the ground.
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Before the dolly is backed under the rear trailer, how should the trailer height be set?
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Answer B: Slightly lower than the center of the fifth wheel, so the trailer is raised a little as the dolly goes under
A trailer set slightly low is lifted as the dolly slides under, which keeps the plates in contact and stops the fifth wheel passing beneath the apron.
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Just after backing the dolly under the rear trailer, why is the landing gear raised only slightly off the ground?
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Answer C: So the legs are not damaged if the trailer moves before the coupling has been proven
A small clearance protects the landing gear while leaving support close by until the coupling has been tested and inspected.
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How do you prove that the dolly is really locked to the rear trailer?
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Answer D: Pull gently against the trailer kingpin, then look at the coupling
A pull test loads the jaws and the visual check confirms they are closed around the kingpin with no gap between the fifth wheel plates.
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Where should the rig be standing before you begin uncoupling a set of doubles?
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Answer C: Straight, on firm level ground, with the parking brakes applied
A straight rig on firm level ground keeps the couplings from binding and stops anything rolling while the units are separated.
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During uncoupling, why is the landing gear of the rear trailer lowered before anything is disconnected?
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Answer A: To take part of the trailer weight off the dolly
Lowering the gear transfers load off the dolly so it can be pulled clear without the trailer dropping.
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Why are the tractor, first trailer and dolly pulled forward before the pintle hook is released?
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Answer C: To draw the dolly out from under the rear trailer while it is still hooked
The dolly must come out from beneath the trailer before it is unhooked, so the tow bar stays under control the whole time.
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What can happen if the pintle hook is unlocked while the dolly is still under the rear trailer?
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Answer B: The dolly tow bar can fly up and cause injury
The tow bar is under load while the dolly carries the trailer, so releasing the hook lets it spring upward, and recoupling then becomes very difficult.
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The dolly is clear of the rear trailer and ready to be dropped. What has to be done to it?
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Answer D: Lower its landing gear, unhook the chains, and chock the wheels or apply its spring brakes
Supporting the dolly and holding its wheels keeps it stable and stationary once the pintle hook is released.
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What are shut off valves, also called cut out cocks, used for?
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Answer C: Closing the service and supply air lines at the back of a trailer used to tow another trailer
They allow the lines at the rear of a towing trailer to be shut when nothing is coupled behind it.
Source: oregon.gov
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In a correctly coupled set of triples, how are the shut off valves at the rear of the first and second trailers set?
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Answer B: Both open
Any trailer with another unit behind it has to pass air along, so its rear valves are left open.
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What happens if the shut off valves at the rear of the last trailer are left open?
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Answer D: Air escapes and the system cannot build and hold pressure
An open valve at the end of the line is an open leak, so pressure cannot be maintained and the brakes will not work as they should.
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How must the converter dolly air tank drain valve be set for the trip?
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Answer A: Closed
The petcock is opened only to release the dolly brakes during coupling, and it has to be closed again so the dolly tank will hold air.
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A driver couples a second trailer but forgets to open the valves at the rear of the front trailer. What is the result?
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Answer D: The dolly and rear trailer get no air, so their brakes will not work
Closed valves block the service and supply lines at that point, leaving everything behind them without air.
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You drop the rear trailer of a set of doubles and continue with a single trailer. What must be done to the valves at the back of the remaining trailer?
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Answer C: Close them
With nothing coupled behind it, that trailer is now the last one, and its rear valves must be closed so air is not lost.
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When uncoupling, which valves are closed before the dolly air and electrical lines are disconnected?
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Answer B: The shut off valves at the rear of the first semitrailer, and those on the dolly if it has them
Closing the valves that feed the dolly stops the system from dumping air when the lines are pulled apart.
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How do you confirm that air is actually reaching the very back of a set of doubles?
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Answer D: With the system charged, open the emergency line shut off at the rear of the last trailer and listen for escaping air
Hearing air at the far end proves the supply line is open all the way through, and silence means something ahead of it is blocking the flow.
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Before walking back to check for air at the rear of the set, what must be done in the cab?
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Answer A: Push the trailer air supply knob in so the trailer lines are charged
Pushing the red knob in opens the tractor protection valve and sends air back to the trailers, which is what the check at the rear is meant to prove.
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You open the emergency line shut off at the rear of the last trailer and hear nothing at all. What does that tell you?
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Answer C: Air is not getting through, so the brakes back there will not work and the fault must be found
Silence means the supply is interrupted somewhere ahead, and the rig must not be driven until the cause is found and corrected.
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What must be done with the valve immediately after the air check at the rear of the last trailer?
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Answer B: Close it again
The valve was opened only to test for air, and it has to go back to the closed position or the system will leak while you drive.
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Why is the air supply to every trailer checked before the trip rather than at the first stop?
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Answer A: A trailer with no air has no working service brakes, and you would discover it only when you needed them
A blocked supply cannot be seen from the cab, so the check has to be made while the rig is still parked.
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Which part of the converter dolly is checked to make sure it is properly seated in the pintle hook?
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Answer C: The pintle eye at the end of the tow bar
The eye on the dolly tow bar has to sit inside the pintle hook with the hook latched before the set is safe to move.
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A spare tire is carried on the converter dolly. What does the inspection have to confirm?
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Answer B: That it is securely fastened in its carrier
A spare that works loose can fall into traffic, so its mounting is part of the doubles and triples inspection.
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What are you looking for when you inspect the air lines running back to the dolly and the rear trailer?
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Answer D: That they are supported, correctly connected at the glad hands, and free of leaks
Lines must be held clear so they do not drag or chafe, sealed at the glad hands, and left with enough slack to allow for turns.
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How can you tell that a converter dolly was built after the federal antilock brake requirement took effect?
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Answer A: It has a yellow lamp on the left side of the dolly
Converter dollies built on or after March 1, 1998 must have antilock brakes, and the yellow malfunction lamp on the left side is the visible sign.
Source: oregon.gov
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What condition must the landing gear be in before the set is driven away?
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Answer B: Fully raised and undamaged, with the crank handle in place and secured
Gear left partly down can catch on the road surface, and a loose crank handle can swing out into passing traffic.
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Inspecting the dolly fifth wheel connection, what shows that the coupling is correct?
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Answer A: The locking jaws are closed around the kingpin shank and there is no gap between the plates
Closed jaws on the shank and flush fifth wheel plates are the two visual signs of a sound connection.
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A set of triples about 100 feet long is running at 55 mph. Using the standard length based rule, what is the minimum following distance?
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Answer C: 11 seconds
The rule allows one second for every 10 feet of vehicle length, which gives 10 seconds, plus one extra second because the speed is above 40 mph.
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Why does a set of doubles need a bigger gap than a straight truck before pulling out across traffic?
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Answer B: It is far longer, so it occupies the lanes you are crossing for much more time
Length decides how long the rig blocks the road you are crossing, and a double or triple blocks it far longer than a single unit does.
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What should a driver assume about stopping a loaded set of triples?
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Answer A: It needs more room and cannot be stopped suddenly
Doubles and triples cannot be turned or stopped suddenly, so the room needed to stop has to be planned well in advance.
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Turning a set of doubles at an intersection, how do the trailer wheels track compared with the tractor wheels?
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Answer D: They cut inside the tractor path, so extra room is needed on the inside of the turn
Off tracking means each unit follows a tighter line than the one ahead, so the rear trailer can climb a curb that the tractor cleared easily.
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You need to change lanes in traffic while pulling triples. What does the extra length require?
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Answer A: A gap big enough for the whole set, entered with a gradual movement
The set needs a far larger opening than a single vehicle, and the change must be gentle because a fast one whips the rear trailer.
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You are pulling doubles across open country in a strong crosswind and the rear trailer is empty. What is the main danger?
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Answer C: A gust can shove the light rear trailer sideways or tip it over
An empty trailer is light and presents a large flat side, so a gust moves it far more easily than a loaded one and the rear position is the least stable in the set.
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What should you assume about backing up a coupled set of doubles or triples?
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Answer A: It cannot be done safely, so the set has to be uncoupled or driven forward instead
Each extra articulation point folds on its own, so a set jackknifes almost at once in reverse and drivers plan routes that never require backing.
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You are connecting the air lines to a converter dolly. Which color coding identifies the emergency, or supply, glad hand?
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Answer D: Red, while the service line glad hand is blue
Red marks the emergency or supply line and blue marks the service line, and keeping them straight stops you from crossing the connections at the dolly.
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What should be done with the spare glad hands at the rear of the last trailer of a set?
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Answer B: Coupled to dummy couplers or locked together so dirt and water stay out
Unused glad hands are parked on dummy couplers or joined to each other, which keeps road grit and moisture from entering the lines and reaching the brake system.
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You have just coupled a set of doubles and the air system is still filling. When may you move the set?
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Answer A: Once the gauge shows normal operating pressure, about 100 to 125 psi
The whole set is fed from the same system, so the trailers behind you have no usable brakes until pressure has built to the normal operating range.
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You have hooked the dolly to the first trailer and connected its air hoses and light cord. What comes next?
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Answer C: Open the shut off valves at the rear of the first trailer and on the dolly
Air can only pass into the dolly and the trailer behind it once the valves at that point are opened, so they are opened after the hoses are connected.
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How must the converter dolly safety chains be attached?
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Answer D: Secured to the frame of the vehicle ahead, with no more slack than turning needs
Chains fastened to solid frame structure ahead of the dolly can hold it if the pintle hook lets go, and excess slack would let the tow bar hit the ground first.
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Before backing the converter dolly under the second trailer, what must be true of that trailer?
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Answer A: Its brakes are applied or its wheels are chocked so it cannot roll
The trailer is pushed on as the dolly goes under, so it has to be held still or it will roll away from the coupling.
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You are inspecting the pintle hook joining the dolly to the trailer ahead. What confirms it is safe to drive?
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Answer C: The hook is closed around the eye and its latch is locked by the keeper or pin
A pintle hook is only secure when the latch is closed over the eye and held by its keeper, because vibration can shake an unsecured latch open.
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What must you look for on the dolly tow bar and its eye during the inspection?
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Answer D: Cracks, bends and worn metal at the eye and its welds
The tow bar carries the pull of everything behind it, so cracked welds, a bent bar or a badly worn eye can let the rear units break away.
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The yellow antilock lamp on a trailer in your set comes on and stays on while you are driving. What does that mean?
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Answer B: You still have normal brakes on that trailer, but the antilock system needs repair
A lit malfunction lamp means the antilock function is out on that unit, so the ordinary brakes still work and the fault has to be fixed.
Source: oregon.gov
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Your dolly and both trailers have antilock brakes. How should that change the way you drive the set?
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Answer A: It should not change anything, because the set still needs the same room to stop
Antilock brakes help you keep steering control during a hard stop, but they do not shorten the distance a long heavy set needs to come to rest.
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Which coupling points does a set of doubles add to the walk around inspection of a tractor semitrailer?
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Answer C: The pintle hook and tow bar, plus the fifth wheel on the converter dolly
The dolly brings two more connections, the pintle hook at its tow bar and the fifth wheel that carries the rear trailer, and both must be checked every trip.
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You are approaching a railroad crossing with a loaded set of triples. What does the length of the set require?
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Answer D: Making sure there is room on the far side for the whole set before you start across
A set this long needs far more clear roadway beyond the tracks, and stopping partway across because traffic ahead has stalled is what puts a rig in front of a train.
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You must stop a set of doubles quickly without antilock brakes. Why is controlled or stab braking used instead of holding the pedal down hard?
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Answer B: It keeps the wheels turning, because a locked trailer wheel lets that unit swing out
A wheel that is locked has lost its grip sideways, and on a set that already swings at the rear this is how a trailer comes around into another lane.
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You had to make a quick move to the left with a set of triples and you are now back in your lane. What is the mistake that most often follows?
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Answer A: Steering back to the right harder than you steered left, which whips the rear trailer
The countersteer is a second sharp input arriving while the rear trailer is still swinging from the first, and the two together are what rolls it.
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What must be confirmed about the light cords running back through a set of doubles?
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Answer C: That they are plugged in and secured, and that the rear lights work
A cord that is loose at a socket can drop out on the road and leave the last trailer of a very long rig unlit to traffic coming up behind.
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Why are the dolly wheels and tires part of the pre trip inspection for a set of doubles?
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Answer B: Because they carry the front of the rear trailer and cannot be seen from the cab
The dolly axle takes the front load of the trailer behind it, and a low tire or a loose lug nut back there gives no warning at the driver seat.
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You have to break down a set of triples at the terminal. Which trailer is taken off first?
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Answer D: The rear trailer, working forward one unit at a time
Each unit is only reachable once everything behind it is gone, so a set is taken apart from the back and put together from the front.
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You have moved a set of doubles into the left lane to pass and you now want to return to the right. When is it safe to come back over?
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Answer C: Once the whole set, including the rear trailer, is clear in your right hand mirror
The rear trailer is many car lengths behind the cab, so the mirror check has to show the end of the set clear before you begin moving back.
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How should you use your turn signals when driving a set of triples in traffic?
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Answer D: Signal earlier than usual and leave it on until the whole set has finished the move
The maneuver takes far longer to complete than in a shorter vehicle, and drivers behind the last trailer need warning well before the rig starts to move.
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You are pulling a loaded set of triples up a long grade on a two lane highway with a slow vehicle lane. What should you do?
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Answer A: Keep right and let faster traffic go by, because the set accelerates very slowly
A heavy set loses speed on a climb, and staying right keeps a very long slow vehicle out of the path of traffic that is still moving at highway speed.
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Why does loose cargo that can slide about inside the rear trailer matter more in a set of doubles than in a straight truck?
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Answer D: The load slides just as the trailer is already swinging, which can complete a rollover
The rear trailer is already thrown sideways by the amplified swing, and cargo sliding the same way at that moment adds the push that tips it over.
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Your tractor has a speed retarder and you are pulling doubles onto a road covered with packed snow. What should you do with the retarder?
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Answer A: Turn it off, because it brakes the drive wheels and can start a skid
A retarder slows the drive wheels alone, and on a slippery surface that is enough to break their traction and let the units behind push out of line.
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You are approaching a tight exit ramp with a loaded set of doubles. Where should the braking be done?
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Answer C: Before entering the curve, so no braking is needed once the set is turning
Braking and turning at the same time asks the tires for both jobs at once, so the speed is taken off while the rig is still straight.
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A supply line to the rear of your set of doubles is torn away while you are driving. What happens?
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Answer B: The pressure in that line drops and the trailer emergency brakes come on
The emergency brakes on a trailer are held off by air in the supply line, so losing that pressure is what makes them apply.
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How does the length of a set of triples affect the areas you cannot see?
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Answer C: The blind areas down each side run much further back, so a car can hide alongside
The no zones along both sides stretch the whole length of the rig, so a vehicle can travel unseen beside the trailers for a long time.
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How does off tracking change as each trailer and dolly is added behind the tractor of a set of triples?
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Answer D: It builds up, because every added unit cuts the corner a little more than the last
Each unit follows a tighter path than the one ahead of it, so the total offset grows with every trailer and dolly you add behind the tractor.
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You are behind a slower vehicle on a two lane road with a set of doubles. What should you expect about passing it?
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Answer B: That you need much more clear road ahead, because the pass will take far longer
You have to pull out, travel past and get the entire length of the set back into the lane, so a pass that a car finishes quickly takes a very long stretch of road.
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Why does the back of a set of triples whip harder than the back of a set of doubles?
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Answer B: It has one more coupling point, and every coupling adds to the swing
Rearward amplification grows at each articulation point, so the extra dolly and trailer of a triple pass along a larger movement than a double does.
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A strip of tire tread is lying in your lane and you are pulling doubles at highway speed with traffic beside you. What is the safer choice?
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Answer D: Hold the wheel steady and ride over it rather than making a violent swerve
A sharp swerve is the one input that rearward amplification turns into a rollover at the back, so a small object is the lesser risk.
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Can the rear trailer of a set turn over while the driver still has the tractor upright and under control?
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Answer A: Yes, and you may not feel it happen, so the mirrors matter
The rear unit is thrown far harder than the tractor and can go over on its own, and the cab gives little warning that it has happened.
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How does road speed change the crack the whip effect in a set of doubles?
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Answer C: The faster you go, the further a sudden steering movement throws the rear trailer
The same quick movement of the wheel produces a much larger swing at the rear as speed rises, which is why sudden inputs are most dangerous on the highway.
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The right wheels of your lead trailer drop off the pavement onto a soft shoulder. Why is jerking the wheel back onto the road especially dangerous with doubles?
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Answer A: The sudden movement is multiplied at each coupling and can throw the rear trailer over
Climbing back onto the pavement should be done gently at reduced speed, because a hard correction is amplified all the way to the last trailer.
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You are entering a work zone where the lanes shift sharply to the left and you are pulling triples. What should you do first?
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Answer B: Slow down before the shift so the set can be steered through it gently
Losing speed first turns a sharp shift into a gradual one, and a gradual movement is the only kind a set of triples can take without whipping the rear trailer.
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You are changing lanes with a set of doubles. When is the maneuver actually finished?
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Answer D: When the last trailer has followed the tractor into the new lane
The set occupies both lanes until the rear trailer has come across, so a second steering input before that point arrives while the back is still moving.
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You are waiting to turn left across a busy two way road with a set of triples. What size gap do you need?
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Answer C: A gap big enough for the whole set to clear the oncoming lanes, which takes far longer
The rig blocks the lanes it is crossing until the last trailer is through, and that takes several times as long as it does for a shorter vehicle.
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You are making a right turn at a corner with a set of doubles. When may you begin to straighten the wheel?
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Answer B: Not until the last trailer has cleared the corner
Each unit tracks tighter than the one ahead of it, so straightening early pulls the rear trailer across the curb the tractor already cleared.
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Where should the driver of a coupled set of triples try to park at a truck stop?
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Answer A: In a pull through space that needs no backing at all
A coupled set folds at every joint the moment it is reversed, so the route and the parking space are chosen so that backing is never needed.
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Your set of doubles is about 70 feet long and you are driving through town at 30 mph. Using the standard length rule, what is the least following distance?
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Answer B: 7 seconds
The rule allows one second for every 10 feet of vehicle length, and no extra second is added because the speed is below 40 mph.
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You are given a loaded trailer, a half loaded trailer and an empty trailer to build a set of triples. In what order should they run behind the tractor?
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Answer D: Loaded first, half loaded second, empty last
Weight is placed in descending order behind the tractor because the amplified swing grows toward the back and the lightest unit tolerates it best.
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You must build a set of doubles but you cannot tell by looking which of the two trailers is heavier. What should you do before coupling?
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Answer B: Find the weight of each trailer from the paperwork so the heavier one can go first
The order of a set depends on knowing the weights, so the shipping documents or scale tickets are consulted before the trailers are hooked together.
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What does a converter dolly turn a semitrailer into?
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Answer A: A full trailer that can be towed behind another trailer
The dolly gives the semitrailer a front axle and a coupling of its own, which is exactly what makes it a full trailer.
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How many axles does a converter dolly have?
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Answer C: One or two
A dolly is a small axle assembly carrying a fifth wheel and a tow bar, and it is built with either one axle or two.
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What is the difference between a semitrailer and a full trailer?
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Answer D: A full trailer supports its own weight at both ends, while a semitrailer rests on the unit towing it
A semitrailer needs a fifth wheel under its nose, and adding a converter dolly supplies that support so the unit can be towed as a full trailer.
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How many converter dollies are used in a set of triples?
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Answer B: Two
The first semitrailer couples to the tractor fifth wheel, and each of the two trailers behind it needs a dolly of its own.
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How many fifth wheels are there in a properly coupled set of triples?
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Answer C: Three, one on the tractor and one on each converter dolly
Each dolly carries a fifth wheel for the trailer above it, so the inspection of a triple covers three of them rather than one.
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In what order is a set of doubles built up?
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Answer A: The tractor is coupled to the front trailer, then the dolly, then the rear trailer
A set is assembled from the front toward the back, because each new unit is coupled to the equipment that is already hooked to the tractor.
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Why is the converter dolly hooked to the first trailer before it is backed under the second one?
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Answer D: So the tractor can back it under the trailer instead of it being pushed there by hand
Once the dolly is on the pintle hook the driver reverses the tractor and first trailer to push it under the nose of the rear trailer under power.
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You connect the glad hands at the converter dolly but cross the service and emergency lines. What is the danger?
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Answer B: The trailers behind the dolly may have no working brakes when you need them
Crossed lines send supply air to the service side and leave the emergency side dead, so the units behind the dolly do not brake as the driver expects.
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You open the shut off valves at the rear of the first trailer and hear air rushing back through the lines. What does that tell you?
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Answer A: Air is now passing back into the units behind, which is what should happen
Opening those valves lets the charged system fill the dolly and the trailer behind it, so a rush of air at that moment is the expected sound.
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A trailer you have been asked to run as the lead unit of a set has no shut off valves at its rear. What does that mean?
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Answer C: It cannot be used to tow another trailer, because air could not be passed back
Only a trailer fitted with rear service and supply connections and their shut off valves can feed air to a dolly and another trailer behind it.
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You are checking the service line at the rear of the last trailer of your set. What must be happening for that check to prove anything?
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Answer B: The brakes must be applied, because the service line carries pressure only then
The service line is charged only while the brakes are being applied, so nothing escapes at the rear valve unless the pedal or the hand valve is held down.
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You open the emergency line shut off valve at the rear of the last trailer of a set of triples and no air escapes. What should you check first?
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Answer D: That the shut off valves on the trailers ahead and on the dollies are open
A valve left closed at any point in the set blocks both lines from there back, and that is the most common reason no air reaches the last trailer.
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How should the rig be secured before you walk back to check for air at the rear of the last trailer?
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Answer A: Set the tractor parking brake, and chock the wheels if there is any doubt
You are about to work behind a charged and unattended rig, so the parking brake and, if needed, chocks keep the set from rolling while you are back there.
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You have closed the valves and are about to separate the dolly during uncoupling. What should be done with the dolly air lines and light cord?
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Answer B: Disconnected and secured where they cannot drag on the ground or be damaged
Loose lines and cords are pulled apart or chafed through when the units move, so they are unhooked and hung or tied clear before anything is separated.
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Why are the safety chains left hooked while the dolly is being pulled out from under the rear trailer?
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Answer C: Because they still hold the dolly if the pintle hook should let go as it is pulled clear
The chains are the backup for the pintle hook, so they stay connected for as long as the dolly is being towed and are removed only once it is standing on its support.
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The landing gear crank handle is missing from the second trailer of the set you are about to build. Why does that matter?
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Answer D: That trailer could not be supported when the dolly is taken out from under it
The landing gear sets the coupling height during the build and holds the nose of the trailer up when the dolly is removed, and neither can be done without the crank.
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What are you checking at the converter dolly air tank during the walk around?
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Answer C: That it is mounted securely, drained of water and free of leaks
The dolly carries its own reservoir for the brakes on its axle, so it is checked for secure mounting, for water in the tank and for leaks like any other tank.
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What are you looking for when you inspect the converter dolly safety chains themselves?
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Answer A: Cracked or stretched links, bent hooks, and any chain that is missing
Chains only serve as a backup if every link and hook is sound, so damaged or absent chains put the set out of service until they are replaced.
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What must the inspection of the converter dolly fifth wheel confirm?
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Answer C: That it is mounted securely and that the jaws and release handle are sound
This fifth wheel carries the front of the trailer behind it, so its mounting, its locking jaws and its release mechanism are all checked before the trip.
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Which parts of a modern set of doubles are required to have antilock brakes?
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Answer D: The tractor, both trailers and the converter dolly
The requirement covers trailers and converter dollies as well as the power unit, so every braked unit in a recent set has a system of its own.
Source: oregon.gov
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Where would you look for the antilock malfunction lamp on a trailer in your set?
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Answer A: On the left side of the trailer, at the front or the rear corner
Trailer antilock lamps are placed on the roadside of the unit, at the front or rear corner, so they can be seen during the walk around and in the mirrors.
Source: oregon.gov
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You inspect an older converter dolly and find no yellow lamp anywhere on it. What is the most likely explanation?
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Answer B: The dolly was built before antilock brakes were required on this equipment
Only dollies and trailers built from March 1, 1998 onward had to have antilock brakes, and older equipment simply has no system and no lamp.
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How does working antilock braking help a set of doubles during a hard stop?
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Answer C: It keeps the wheels rolling, so the trailers are less likely to swing out of line
A rolling wheel still grips sideways, and that is what stops a trailer in a long set from coming around into another lane during a panic stop.
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You are driving an empty set of doubles across an exposed bridge in a strong crosswind. What is the best action?
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Answer A: Slow down and be ready for the gust that hits as you come out of the shelter
Lower speed gives the light rear trailer less energy to be pushed with and gives you time to correct gently when the wind first catches the set.
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You are pulling triples on a steeply crowned rural road. What effect does the crown have on the set?
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Answer B: It leans the trailers toward the edge of the road and adds to the rollover risk
A high crown tilts each trailer downhill toward the shoulder, and that tilt is added to whatever lean a curve or a gust is already producing at the rear.
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Why does a rough or broken road surface matter more to a set of triples than to a rig with a single trailer?
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Answer D: The set is upset more easily, and the rear trailer feels the movement most
A set of triples is less stable to begin with, so bouncing and jolting reach the least steady unit in the rig and make an already amplified swing worse.
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What can you learn by watching the rear trailer of your set in the mirrors as you go through a long curve?
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Answer B: Whether it is following properly and staying upright
The cab tells you almost nothing about the unit that leans and swings the most, so the mirrors are the only warning you get while the curve is being taken.
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During a hard stop without antilock brakes you see the rear trailer of your set beginning to swing out. What should you do?
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Answer C: Ease off the brakes so the wheels can roll again, and steer to keep the tractor straight
The swing began when a wheel locked and lost its sideways grip, so releasing enough brake to let it turn again is what lets the unit fall back into line.
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In a coupled set of doubles, what carries the front end of the rear trailer?
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Answer A: The dolly axle, through the dolly fifth wheel
The dolly fifth wheel takes the nose of the trailer and passes that weight straight down onto the dolly wheels, while the tow bar only pulls the dolly along.
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Why do the side marker lamps along a set of triples matter so much to other drivers at night?
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Answer C: They show how long the rig is, so a driver can judge when it has been passed
A passing driver has no other way at night to tell where a rig this long ends, and cutting back in too early is what a dark gap in the markers causes.
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At what point during the coupling are the safety chains, air hoses and light cord connected to the dolly and the rear trailer?
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Answer D: After the coupling has been tested with a pull and checked by eye
The mechanical connection is proved first, because a failed pull test would part the units and tear away anything already connected between them.
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You are uncoupling a set of doubles and the tractor, first trailer and dolly are about to be pulled forward. What must be released first?
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Answer A: The dolly brakes and the dolly fifth wheel latch
The dolly cannot roll with its brakes set and cannot slide out from under the trailer with its fifth wheel still locked on the kingpin.
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You are dropping the rear trailer of a set and that trailer has no spring brakes. What must be done before the dolly is pulled out?
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Answer D: Chock its wheels so that it cannot roll once the dolly has gone
Without spring brakes the trailer has nothing to hold it once the air lines are pulled off, so chocks are the only thing keeping it in place.
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You have used the tractor and first trailer to carry a heavy converter dolly to the nose of the rear trailer. What comes next?
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Answer B: Lower the dolly support, unhook it from the first trailer, and wheel it into position
The dolly is set down close to the trailer and then lined up with the kingpin by hand, after which it is coupled to the first trailer again for the final push.
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When uncoupling, why is the rear trailer landing gear lowered only far enough to take some weight off the dolly?
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Answer C: So weight comes off the dolly without the trailer being jacked up off the fifth wheel
The dolly has to be able to slide out cleanly, and lifting the trailer clear of the fifth wheel would bind the plates and put the trailer high on its legs.
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Another vehicle settles in alongside the middle of your set of triples on a four lane highway. What should you do?
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Answer A: Move ahead or drop back, because the set cannot be moved sideways quickly
A car sitting beside the trailers is in a blind area many car lengths long and takes away the only space a rig this long could use if it had to move over.
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A driver on a two lane road pulls out to pass your set of triples. How can you help?
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Answer D: Hold a steady speed and stay right, since the pass takes a long stretch of road
Passing a rig this long already uses a great deal of road, and any change in your speed or position leaves the other driver stranded in the oncoming lane.
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A driver couples a set of doubles and drives away with the converter dolly air tank petcock still open. What is the result?
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Answer C: The dolly tank cannot hold air, so the dolly brakes will not work properly
The petcock was opened to release the dolly brakes for positioning, and leaving it open means the reservoir that feeds those brakes drains as fast as it fills.
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The last trailer of your set is receiving no air, so it has no working brakes, while the units ahead of it brake normally. What happens on a hard stop?
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Answer B: That trailer pushes the units ahead of it and can shove the set out of line
An unbraked trailer keeps its momentum and drives into the coupling ahead of it, which lengthens the stop and can fold the set at a joint.
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You are building a set of doubles and one trailer is loaded low and heavy while the other is loaded high with light bulky freight. What is the concern about putting the tall load at the rear?
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Answer A: The rear position swings the most, so a top heavy trailer there is the likeliest to roll
A high center of gravity and the amplified swing at the back of the set are the two things that tip a trailer, and the rear position combines both of them.
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You switch on the ignition and the yellow antilock lamp for the trailers of your set comes on and then goes out. What does that mean?
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Answer D: That the system checked itself at start up and is working
The lamp is meant to light briefly during the self test, and a fault is shown by a lamp that stays on or comes on again while you are driving.
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What does a commercial driver need before pulling a set of doubles or triples?
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Answer C: The doubles and triples endorsement, obtained by passing a knowledge test
Pulling more than one trailer requires the doubles and triples endorsement, which is added to the license after the driver passes the written test for it.
Source: fmcsa.dot.gov