How to Prepare for the CDL Doubles/Triples Test

Before you Start

This study overview is quite short due to the fact that it build on info that is presented in 2 of our various other study guides: air Brakes test Study overview and combination Vehicles Test. Check out those 2 guides (in that sequence) prior to you check out this study guide—or girlfriend may end up being confused.

You are watching: Should the shut-off valves on the rear of the last trailer be open or closed

Hint: evaluation the info in the examine guides: Air Brakes Test and Combination Vehicles Test prior to taking your composed DMV check on Doubles/Triples. This shouldn’t be a problem—because typically all the created tests space taken at the same time.

General Information

Some chauffeurs will pull twin or triple trailers. This examine guide offers the simple information important to know how to pull doubles and also triples.

Doubles and triples are connected using converter gear (“converter dolly” or “dolly”). A dolly is like a really short trailer, through a ring hitch top top the front and a 5th wheel top top top. A dolly has lights, waiting tanks, waiting relays, waiting lines, waiting brakes, ABS, electrical connections, etc. Prefer a trailer, a dolly equipped through ABS will have actually a yellow indicator light on the left side.

Converter dollies have actually spring brakes. (Hint: This fact might be top top your created DMV test.) one extremely old converter dolly might not have actually spring brakes. If you room uncertain, use chocks.

Not every mix of trailers is legit in every state. Because that instance, triples are not legal in California—but right throughout the border in Nevada, castle are legal.

Helpful Illustrations


General plan of converter dolly in between lead trailer (left) and also following trailer (right).

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

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Ring Hitch (Military Version): Ring hitches are an installed on the front that converter dollies and also trailers.

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LCV stands for “Longer mix Vehicle*.

A pup trailer is a trailer the is between 26 ft and also 29 ft; typically used in doubles and also triples.

Double/Triple security Considerations

Doubles and also triples pose several special concerns for motorists due to their size and length. Here are few of them:


Double and also triple trailers are at a greater risk because that rollovers than solitary trailers, v the last trailer gift the most most likely to it is in affected. Sudden movements can make the trailer waver out and then back again, much like the pointer of a whip. This is why this swinging is called the crack-the-whip effect. Friend can assist prevent trailer rollovers by steering slowly around corners, controlling your speed, and braking early.

Managing Space

Double and also triple trailers are much longer than single trailers and require an ext space to it is in maneuvered. Since of this, more care need to be required to avoid possibly hitting the curb or an additional vehicle while turning or an altering lanes. Slowing down or transforming lanes need to be done slowly to stop jackknifing or shedding control. Watch far ahead. Keep aware of web traffic conditions, so braking have the right to be gentle. Lane alters can it is in challenging. Make particular that girlfriend have sufficient room prior to moving over.

With much more axles, twin or triple trailers are more likely come skid or jackknife. * constantly park in a point out you have the right to pull v to help prevent possible accidents. * Drive very closely in adverse conditions.

Pintle Hook and Ring Hitch

Never detach the pintle hook if 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 require to take into consideration a few extra things during the traditional seven-step investigate of double and triple trailers:

The shutoff valves in ~ the rear of the front trailer(s) and also dollies need to be open and also the shutoff valves at the behind of the ago trailer must be closed. (All the valves should be open, except the valves at the very rear the the truck.)

Confirm that the converter dolly waiting tank drain valve is closed.

Ensure that air lines space supported and also that the glad hands room connected.

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

Additionally, once loading a double/triple trailer, constantly ensure that the lightest trailer is in the rear and also the heaviest is in the front.

Converter dollies room (in plenty of ways) like an extremely short trailers. Numerous of the parts are similar.

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

Coupling and also Uncoupling

Double Trailers

The steps for coupling double trailers space as follows:1. Secure second trailer. 2. Position converter dolly in front of 2nd trailer. 3. Couple tractor and an initial semi-trailer (follow standard procedure). 4. Connect converter dolly come front trailer (pintle hook and also ring hitch). 5. Connect converter dolly to rear trailer (fifth wheel)

To uncouple a twin trailer, start by lowering the landing gear the the 2nd trailer, climate uncoupling the rear trailer (at the 5th wheel). Uncouple the converter dolly by lowering the landing gear of the dolly, disconnecting the safety chains, applying the feather brakes or wheel chocks, release the pintle hook, and also pulling far from the dolly.

Never detach the pintle hook while the dolly is under a trailer. The load of the trailer deserve to (almost certainly will) reason the ring hitch to fly up. This can be an extremely dangerous.

Make sure the trailer height is correct. The trailer need to be raised slightly when the dolly is donate under. (Hint: This information could be ~ above your composed DMV test.)

There need to be no gap in between the optimal of the fifth-wheel and the bottom the the trailer.

To secure a trailer prior to coupling: (Hint: The adhering to information could be ~ above your written DMV test.)

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

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

Use chocks.

Triple Trailers

The procedures for coupling triple trailers space the exact same as with twin trailers, through the complying with extra steps:

Couple second and third trailers.

Uncouple tractor and also pull far from trailers.

Couple tractor and an initial semi-trailer (following conventional procedure).

Couple tractor and an initial semi-trailer to second and third trailers.

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To uncouple triple trailers, very first uncouple the 3rd trailer by pulling the end the dolly and unhitching it, using the same technique you would for twin trailers.