Doubles/Triples Test Study Guide for the CDL

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How to Prepare for the CDL Doubles/Triples Test

Before You Start

This study guide is quite short because it builds on information that is presented in two of our other study guides: Air Brakes Test Study Guide and Combination Vehicles Test. Read those two guides (in that sequence) before you read this study guide—or you may become confused.

General Information

Some drivers will pull double or triple trailers. This study guide gives the basic information necessary to know how to pull doubles and triples.

Doubles and triples are connected using converter gear (“converter dolly” or “dolly”). A dolly is like a very short trailer, with a ring hitch on the front and a fifth wheel on top. A dolly has lights, air tanks, air relays, air lines, air brakes, ABS, electrical connections, etc. Like a trailer, a dolly equipped with ABS will have a yellow indicator light on the left side.

Converter dollies have spring brakes. (Hint: This fact might be on your written DMV test.) An extremely old converter dolly might not have spring brakes. If you are uncertain, use chocks.

Not every combination of trailers is legal in every state. For instance, triples are not legal in California—but right across the border in Nevada, they are legal.

Helpful Illustrations


General arrangement of converter dolly between lead trailer (left) and following trailer (right).

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Pintle Hook (Military Version) in closed position. Pintle hooks are mounted on the rear of tractors and trailers.

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Ring Hitch (Military Version): Ring hitches are mounted on the front of converter dollies and trailers.

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LCV stands for Longer Combination Vehicle.

A pup trailer is a trailer that is between 26 ft and 29 ft; commonly used in doubles and triples.

Double/Triple Safety Considerations

Doubles and triples pose several special issues for drivers due to their size and length. Here are some of them:


Double and triple trailers are at a greater risk for rollovers than single trailers, with the last trailer being the most likely to be affected. Sudden movements can make the trailer swing out and then back again, much like the tip of a whip. This is why this swinging is called the crack-the-whip effect. You can help prevent trailer rollovers by driving slowly around corners, controlling your speed, and braking early.

Managing Space

Double and triple trailers are longer than single trailers and require more space to be maneuvered. Because of this, more care must be taken to avoid potentially hitting the curb or another vehicle while turning or changing lanes. Slowing down or changing lanes must be done gradually to avoid jackknifing or losing control. Watch far ahead. Keep aware of traffic conditions, so braking can be gentle. Lane changes can be challenging. Make certain that you have enough room before moving over.

With more axles, double or triple trailers are more likely to skid or jackknife.

  • Always park in a spot you can pull through to help prevent possible accidents.

  • Drive carefully in adverse conditions.

Pintle Hook and Ring Hitch

Never detach the pintle hook while the dolly is under a trailer. The weight of the trailer can (and almost certainly will) cause the ring hitch to fly up. This can be very dangerous.

Inspection Considerations

You will need to consider a few extra things during the standard seven-step inspection of double and triple trailers:

  • The shutoff valves at the rear of the front trailer(s) and dollies must be open and the shutoff valves at the rear of the back trailer must be closed. (All the valves must be open, except the valves at the very rear of the truck.)

  • Confirm that the converter dolly air tank drain valve is closed.

  • Ensure that air lines are supported and that the glad hands are connected.

  • Check that the spare tire is secured, the ring hitch of the dolly is in place, the pintle hook is latched, the safety chains are secured, and the electrical cords are firmly in their sockets.

  • Additionally, when loading a double/triple trailer, always ensure that the lightest trailer is in the rear and the heaviest is in the front.

  • Converter dollies are (in many ways) like very short trailers. Many of the parts are similar.

  • Coupling second and third trailers is the same as coupling doubles.

  • Double and triple brakes are the same as (or, very similar to) the brakes of a single combination vehicle—there are simply more brake parts. Instead of one trailer, there are two or three trailers, plus one or two dollies. The inspection process for inspecting doubles and triples is very similar to inspecting a normal combination vehicle. The steps are the same. It merely takes longer.

Note: The process for testing the shutoff valves of a combination vehicle (including doubles and triples) is in the Combination Vehicles study guide. This procedure can be found in the Test the Shutoff Valves section.

Coupling and Uncoupling

Double Trailers

The steps for coupling double trailers are as follows:

  1. Secure second trailer.

  2. Position converter dolly in front of second trailer.

  3. Couple tractor and first semi-trailer (follow standard procedure).

  4. Connect converter dolly to front trailer (pintle hook and ring hitch).

  5. Connect converter dolly to rear trailer (fifth wheel)

To uncouple a double trailer, begin by lowering the landing gear of the second trailer, then uncoupling the rear trailer (at the fifth wheel). Uncouple the converter dolly by lowering the landing gear of the dolly, disconnecting the safety chains, applying the spring brakes or wheel chocks, releasing the pintle hook, and pulling away from the dolly.

Never detach the pintle hook while the dolly is under a trailer. The weight of the trailer can (almost certainly will) cause the ring hitch to fly up. This can be very dangerous.

Make sure the trailer height is correct. The trailer should be raised slightly when the dolly is backed under. (Hint: This information might be on your written DMV test.)

There should be no gap between the top of the fifth-wheel and the bottom of the trailer.

To secure a trailer before coupling: (Hint: The following information might be on your written DMV test.)

  • Make certain the trailer has spring brakes and that there is no air in the emergency air line (red air line), or

  • Set the emergency brakes—by charging the emergency air line (red air line), filling the trailer’s air tank, then dumping the air out of the emergency air line, or

  • Use chocks.

Triple Trailers

The steps for coupling triple trailers are the same as with double trailers, with the following extra steps:

  1. Couple second and third trailers.

  2. Uncouple tractor and pull away from trailers.

  3. Couple tractor and first semi-trailer (following standard procedure).

  4. Couple tractor and first semi-trailer to second and third trailers.

To uncouple triple trailers, first uncouple the third trailer by pulling out the dolly and unhitching it, using the same method you would for double trailers.

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