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Page 1 Combination Vehicles Test Study Guide for the CDL

General Information

Most people taking the CDL exam will drive a combination vehicle regularly. This study guide gives a brief overview of the most important information needed to drive common combination vehicles.

Rollover Risk

The risk of rollover greatly increases when driving a combination vehicle. To help decrease the risk, always keep cargo as close as possible to the ground. Gentle steering is also essential because of the “crack-the-whip” effect that happens with vehicles pulling trailers. Quick lane changes cause rearward amplification, which can overturn the trailer even without overturning the truck.


Always brake early when driving a combination vehicle. Empty trucks take much longer to stop than those that are fully loaded and are at a greater risk for jackknifing. To avoid jackknifing, try to be cognizant of what you see in your mirrors. Seeing your trailer in your mirrors is a good sign it is skidding, so if this occurs, release the brake immediately and try to regain control of your vehicle.

Backing Up

Backing a combination vehicle can be especially tricky. To back up safely, always look at your path, back slowly using both mirrors and turn toward the driver’s side. If possible, get a helper to guide you as you drive. If you see the trailer veering off path, correct it immediately by turning the wheel or pulling forward.

Driving Around Corners

When driving a combination vehicle, you should always turn wide when going around a corner. Combination vehicles are prone to “off tracking,” which means the rear wheels do not follow the same track as the front. Longer vehicles off track more than shorter vehicles, so it is important to steer wide enough that you will not hit anything in your path, such as a curb or another vehicle.

Combination Vehicle Care

Always follow the standard seven-step inspection procedure when inspecting a combination vehicle. During the walk-around inspection, you should also look at the coupling system areas (the lower and upper fifth wheel, air and electric lines to trailer, and the sliding fifth wheel), as well as the landing gear.

During the air brake systems inspection, the trailer air brakes must also be tested. This involves checking air flow to all trailers, testing the tractor protection valve, testing the trailer emergency brakes, and testing the trailer service brakes. Combination vehicles have additional parts to control the trailer brakes. These parts include the trailer air lines, trailer air supply control, hose couplers (glad hands), tractor protection valve, trailer air tanks, the trailer hand valve, shut-off valves, and the trailer service, parking, and emergency brakes. It is necessary to familiarize oneself with all of these parts before operating a combination vehicle.

Coupling and Uncoupling

Learning how to couple and uncouple a combination vehicle is essential for safe driving. Procedures vary from rig to rig, but there are common steps to be followed.

The steps for coupling are as follows:

1) Inspect fifth wheel
2) Inspect area and chock wheels
3) Position tractor
4) Back slowly
5) Secure tractor
6) Check trailer height;
7) Connect air lines to trailer
8) Supply air to the trailer
9) Lock trailer brakes
10) Back under trailer
11) Check connection for security
12) Secure truck
13) Inspect coupling
14) Connect the electrical cord and check air lines
15) Raise front trailer supports (landing gear)
16) Remove and store trailer wheel chocks.

The steps for uncoupling are:

1) Position rig
2) Ease pressure on locking jaws
3) Chock trailer wheels
4) Lower the landing gear
5) Disconnect air lines and electrical cable
6) Unlock the fifth wheel
7) Pull tractor somewhat clear of the trailer
8) Secure tractor
9) Inspect trailer supports
10) Pull tractor clear of the trailer.