Commercial Driver’s License (CDL) · CDL Manual Chapters
Combination Vehicles
On this page 6 sections
In 30 seconds
A combination vehicle is a tractor and trailer joined by a fifth wheel and two air lines, longer and less forgiving than a straight truck. Coupling is a fixed sequence, and uncoupling reverses it. Because the trailer's rear wheels follow a shorter path on every turn, called off-tracking, you swing wide going forward and back slowly using both mirrors.
Why this matters
Every Class A applicant must pass the 20-question Combination Vehicles test with 16 correct, and the same coupling, inspection, and backing skills are what the examiner grades in the pre-trip and the alley dock, the most failed part of the skills test.
The college version
In normal terms
- Couple in order: line up straight, chock, check height, connect air lines, back under until the jaws lock, tug test, inspect, connect the cord, raise the landing gear.
- Blue glad hand is service air, red is emergency air, and the red octagon knob is the trailer air supply valve.
- The trailer cuts inside the tractor's path on turns, so swing wide, and in reverse turn the wheel toward the mirror that shows the trailer drifting.
Concepts in this outline
- Coupling/uncoupling procedures — Line up straight, chock, check height, connect glad hands, back under, tug test, inspect, connect cord, raise landing gear; reverse to uncouple.
- Inspection specifics — Fifth wheel, kingpin, locking jaws, landing gear, glad hands, air lines, tractor protection valve, and separate trailer brake checks.
- Backing techniques and off-tracking — Rear wheels follow a shorter path on turns, so swing wide going forward and steer toward the mirror the trailer drifts into when backing.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of a bike pulling a wagon. The wagon never follows the bike's exact path, and neither does a trailer. The hitch is the fifth wheel, a greased steel plate on the tractor with a slot. It catches the kingpin, a steel post under the trailer's nose. The locking jaws snap shut around it. Two air hoses join with glad hands, snap-together couplers. Blue is service air, which works the trailer brakes with the pedal. Red is emergency air, which keeps the trailer's spring brakes released. A cord powers the lights.
Coupling is hooking up, and the test wants the order. Check the fifth wheel for grease, cracks, and open jaws. Line up straight in front of the trailer and chock its wheels. Back slowly until the plate just touches the trailer, which should sit slightly lower so it lifts. Connect the glad hands. Push in the red trailer air supply valve and watch the pressure hold. Pull it out to lock the trailer brakes. Back under until the jaws lock. Then pull forward in low gear against the locked trailer brakes, the tug test. Set the brakes and look under with a flashlight for a gap and closed jaws. Then connect the cord, raise the landing gear, pull the chocks, and check the lights.
Uncoupling runs backward. Park on firm, level ground and chock the trailer. Ease back to unload the jaws. Lower the landing gear until it takes weight. Stow the lines and pull the release handle. Pull forward only until the fifth wheel clears the kingpin, then recheck the landing gear.
Inspection adds parts a plain truck lacks. Check the fifth wheel mount, slider pins, kingpin, apron, landing gear, glad hand seals, and air lines. The tractor protection valve closes by itself to save the tractor's air if the trailer line breaks. Engine off, pump the pedal until the red knob pops out between 20 and 45 psi. Psi measures how hard air pushes. Pull the red knob and tug gently to confirm the trailer brakes hold. Roll slowly, apply the trailer hand valve, and feel the brakes grab. Then set each parking brake and pull against it.
Off-tracking is the wagon problem. The rear wheels take a shorter path than the front on every turn. On a right turn, swing wide enough to keep the trailer tires off the curb. But not so wide a car slips inside. In reverse, turn the wheel toward the mirror the trailer drifts into. Trailer in the right mirror means turn right. A jackknife is the trailer folding out of line like a pocketknife. Hard braking on slick pavement with a light trailer causes it, so brake smoothly.
Worked example
Rosa walks up to a dropped, chocked 53-foot trailer, sees the jaws open and the plate greased, and lines the tractor up dead straight. She backs until the plate touches the nose, which sits just below the fifth wheel. She hooks blue to service and red to emergency, pushes the red octagon knob, watches the gauge hold, and pulls it out again. She backs under, hears the jaws snap, and pulls forward in first gear against the locked trailer brakes until the rig stops dead: a good tug. Flashlight in hand she checks for a gap and locked jaws, then plugs in the cord, cranks up the landing gear, pulls the chocks, and checks the lights. Leaving the yard she swings wide on a right turn so the trailer tires clear the curb. At the alley dock the trailer drifts into her left mirror, so she turns left, then straightens, backing at idle and stopping once to get out and look.
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