CDL Doubles and Triples practice test

For the T endorsement, needed to pull two or three trailers. The real test has 20 questions in most states.

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Study all 18 questions with answers

Which trailer in a set of triples is most likely to turn over?
The last trailer
Rearward amplification, the crack-the-whip effect, is strongest at the back of the combination.
Where should the heaviest trailer go in a set of doubles?
Directly behind the tractor
Put the heavier trailer up front. A heavy rear trailer increases the risk of sway and rollover.
What is a converter dolly?
A coupling device with one or two axles and a fifth wheel used to connect a second trailer
The converter dolly turns a semi-trailer into a full trailer so it can be pulled behind another trailer.
Before coupling the second trailer, how do you keep it from moving?
Make sure its spring brakes are set or chock the wheels
If the second trailer doesn't have spring brakes, charge its air tanks and disconnect the emergency line to set its brakes, or chock it.
After coupling doubles, how should the shut-off valves be set?
Open on the first trailer, closed at the rear of the last trailer
Open valves let air flow through to every trailer. The valves at the very rear must be closed so air doesn't escape.
How can you check that air is flowing to all trailers?
Open the emergency line shut-off valve at the back of the last trailer and listen for air escaping
Air rushing out of the rear valve shows the whole system is charged. Close the valve again before driving.
Compared to a single trailer, doubles and triples need:
More following distance and more space to change lanes
Longer combinations take more distance to stop and are less stable. Leave more room in every direction.
What usually connects the converter dolly to the trailer in front of it?
A pintle hook
The dolly's eye attaches to a pintle hook at the back of the trailer ahead.
Why should steering be extra smooth when pulling doubles or triples?
Sudden movements are amplified at the rear trailer and can cause rollovers
A quick lane change that feels minor in the tractor can whip the last trailer hard enough to flip it.
When pulling doubles, you should check that the converter dolly air tank is:
Drained of moisture and its petcock closed before driving
Drain moisture from the dolly tank, then close the petcock so the dolly brakes can work.
Compared to tractor-trailers, doubles and triples are:
More likely to roll over
Longer combinations with more trailers are more likely to tip over than single trailers.
Why should you never unlock the pintle hook with the dolly still under the rear trailer?
The dolly tow bar may fly up and hurt you
Unhooking the pintle hook with the dolly under a trailer can make the tow bar spring up.
When coupling the dolly to the second trailer, the trailer should be:
Slightly lower than the center of the dolly's fifth wheel
Set the trailer so it lifts slightly when the dolly backs under, just like coupling to a tractor.
Does a tractor without a trailer (bobtail) stop faster than a loaded combination?
No, a bobtail tractor can take longer to stop
With little weight on the drive axle, a bobtail tractor has less traction and can take longer to stop.
Why should you look far ahead when pulling doubles or triples?
So you can make gradual moves instead of sudden ones
Spotting hazards early lets you slow and steer smoothly, which keeps the rear trailers stable.
Before driving doubles, the converter dolly's air tank petcock should be:
Closed
Drain the dolly tank, then close the petcock so the dolly brakes can hold air.
In bad weather, doubles and triples are:
More affected by wind and slippery roads
More trailers mean more sway and more chance of skidding. Use extra caution in wind, rain and snow.
When uncoupling the rear trailer, before disconnecting it you should:
Park in a straight line and make sure the trailer is secured
Park straight on firm, level ground and secure the trailer with brakes or chocks before uncoupling.

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