Is there a skills test for the doubles and triples endorsement?

No. Federal rules list a knowledge test only for the double/triple trailers endorsement. Check with your state licensing office for how it issues the endorsement.

How many questions are on the doubles and triples test?

Your state sets the number of questions. Federal rules set 80 percent as the minimum passing score for each knowledge test. Check your state's CDL manual.

Which trailer goes first in a set of doubles?

The more heavily loaded semitrailer goes in first position behind the tractor. The lighter trailer goes in the rear.

CDL Doubles and Triples Test: What the T Endorsement Covers

The CDL doubles and triples test is a knowledge test for the T endorsement. Federal rules list a knowledge test only for this endorsement, with no skills test.

It covers coupling and uncoupling more than one trailer, inspecting the rig, checking air to the last trailer, and driving a rig that is less stable than a single trailer.

What federal rules say the test covers

49 CFR 383.115 says you need knowledge covering:

Procedures for assembly and hookup of the units
Proper placement of heaviest trailer
Handling and stability, including off-tracking, response to steering, braking, oscillatory sway and rollover in steady turns
Problems the rig creates for other drivers, such as slower speeds on steep grades and longer passing times

Section 7 of the manual says to also study Sections 2, 5 and 6. Section 6 is Combination Vehicles. See our combination vehicles test guide and our CDL air brakes test guide.

Your state sets the number of questions. The federal minimum passing score is 80 percent.

The points drivers miss

Topic	What the manual says
Heaviest trailer	The more heavily loaded semitrailer goes in first position behind the tractor. The lighter trailer goes in the rear.
Crack-the-whip	Doubles and triples are more likely to turn over than other combination vehicles. The last trailer is most likely to turn over. Steer gently and go slowly around corners, ramps and curves.
Rearward amplification	Triples have a rearward amplification of 3.5. You can roll the last trailer of triples 3.5 times as easily as a five-axle tractor.
Converter dolly	A coupling device of one or two axles and a fifth wheel that forms a double bottom rig.
Dolly ABS	Dollies built on or after March 1, 1998, must have antilock brakes. They have a yellow lamp on the left side of the dolly.
Valves	Shut-offs at the rear of front trailers: OPEN. Rear of last trailer: CLOSED. Dolly air tank drain: CLOSED.
Pintle hook	Never unlock the pintle hook with the dolly still under the rear trailer.

Coupling doubles, in order

Section 7.2.1 gives the steps.

Secure the rear trailer. If it has no spring brakes, drive the tractor close, connect the emergency line, charge the trailer air tank, then disconnect the emergency line. This sets the trailer emergency brakes, if the slack adjusters are correctly adjusted. Chock the wheels if you have any doubt about the brakes.
Position the dolly in front of the second trailer, in line with the kingpin. Release dolly brakes by opening the air tank petcock, or use the dolly parking brake control if it has spring brakes.
Connect the dolly to the front trailer. Back the first semitrailer in front of the dolly tongue. Hook the dolly, lock the pintle hook, and raise the support.
Connect the dolly to the rear trailer. Lock the trailer brakes or chock the wheels. Trailer height must be slightly lower than the center of the fifth wheel.
Back the dolly under the rear trailer. Raise the landing gear slightly. Test the coupling by pulling against the pin. Check that there is no space between the upper and lower fifth wheel.
Connect the lines. Safety chains, air hoses and light cords.
Set the valves. Close the dolly air tank petcock and the shut-offs at the rear of the second trailer. Open the shut-offs at the rear of the first trailer, and on the dolly if it has them.
Charge and check. Push the air supply knob in. Open the emergency line shut-off at the rear of the second trailer to check for air. No air means the brakes will not work.

For triples, couple the tractor to the first trailer. Then move the dolly into position and couple the first trailer to the second, using the method for doubles.

Uncoupling doubles

Park in a straight line on firm, level ground and set the parking brakes. Chock the second trailer's wheels if it has no spring brakes. Lower its landing gear enough to take some weight off the dolly. Close the air shut-offs at the rear of the first trailer. Disconnect and secure the dolly lines. Release the dolly brakes and the dolly fifth wheel latch. Slowly pull forward to take the dolly out from under the rear trailer. Only then unhitch the dolly from the trailer in front. For triples, uncouple the third trailer first.

Checking air to the last trailer

You must have air all the way to the back for all the brakes to work. Section 7.4.1 lists these checks:

Air flows to all trailers. Hold the rig with the tractor parking brake or chocks. At normal pressure, push in the red trailer air supply knob. Use the trailer handbrake to send air to the service line.
Emergency line. Open the emergency line shut-off at the rear of the last trailer. You should hear air escaping. Close it.
Service line. With the trailer handbrake or brake pedal on, open the service line valve, then close it. No air from both lines? Check that the shut-offs on the trailers and dollies are OPEN.
Tractor protection valve. Charge the system and shut the engine off. Step on and off the brake pedal to bring pressure down. The trailer air supply control should pop out at the pressure the manufacturer sets, usually 20 to 45 psi.
Trailer emergency brakes. Pull out the trailer air supply control and pull gently on the trailer with the tractor. The trailer brakes should hold.
Trailer service brakes. Move forward slowly and apply the trailer brakes with the hand control. You should feel them come on.

Try 10 exam-style questions built from the official CDL manual.

How to study it

Read Section 7 once, then say the coupling steps out loud in order. Write the open and closed valve list from memory. Then take practice tests and read why each wrong answer is wrong.