CDL Doubles/Triples (T) Practice Test
Coupling and inspecting double and triple trailers, and managing their unique handling and rollover risks.
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About the CDL Doubles/Triples (T) test
The CDL Doubles/Triples (T) endorsement lets you pull more than one trailer using converter dollies. The test focuses on what makes these combinations harder to handle: they roll over more easily, they are subject to the crack-the-whip effect, and they have extra coupling points and air-brake connections to get right. The content comes from Section 7 of the federal Commercial Driver License Manual, so it applies in every state.
What the Doubles/Triples (T) test covers
- Pulling doubles and triples: why they need more following distance, smoother steering, and lower speeds than a single trailer.
- Rollover and the crack-the-whip effect: how a quick steering move is amplified down the rig so the rear trailer can swing wide and tip.
- Coupling and uncoupling in order, including the converter dolly, pintle hook, drawbar, and safety chains.
- Air brakes across multiple trailers: brake lag, the tractor protection valve, not crossing the glad-hand air lines, and confirming air reaches the last trailer.
- Inspecting the dolly and coupling points, plus antilock brakes and recognizing and recovering from a trailer skid.
Common mistakes to avoid
The most-missed questions are about the crack-the-whip effect and the air check on the rear trailer. Remember the last trailer is the most likely to roll over, so steer gently and slow down well before curves. After coupling, confirm air reaches the rearmost trailer by opening the emergency line shut-off (petcock) at the back of the last trailer until air escapes, then closing it. People also forget the coupling order (front trailer first, then the dolly, then the rear trailer) and that you must never cross the service and emergency air lines.
How to study for it
Drill the coupling and uncoupling order and the rear-trailer air check until they are automatic, since the test leans on them. Run the full practice test, then the exam simulator. This endorsement builds on the combination-vehicle and air-brake material, so review those first if they are not yet solid.
Other CDL tests
General Knowledge
Required for every CDL applicant (Class A, B, and C).
Air Brakes
Needed by nearly all Class A and most Class B drivers.
Combination Vehicles
Required for every Class A (tractor-trailer) applicant.
HazMat (H)
Drivers hauling placarded hazardous materials.
Passenger (P)
Bus, transit, and shuttle drivers.
School Bus (S)
School bus drivers (requires the Passenger endorsement first).
CDL Doubles/Triples (T) test FAQ
How many questions are on the CDL Doubles/Triples test?
Most states use about a 20-question Doubles/Triples endorsement test with an 80% pass mark. Our practice and exam-simulator modes use the same length so you rehearse the real format.
What is the crack-the-whip effect?
It is how a sudden steering movement is amplified as it travels back through the combination, so the rear trailer swings much wider and can tip over. It is why doubles and triples require gentle steering and lower speeds in curves.
How do you check that air reaches the rear trailer of a doubles unit?
After coupling and charging the system, open the emergency line shut-off (petcock) at the rear of the last trailer. If air escapes, the system is charged all the way through, then close it. If no air comes out, the rear brakes will not work and you must fix it before driving.
In what order do you couple a set of doubles?
Couple the tractor to the front trailer first and secure it, then connect the converter dolly, and finally couple the rear trailer. You uncouple in the reverse order, from the rear forward.