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CDL Class A · 8 Essential Terms
CDL combination-vehicle themes: fifth wheel and kingpin, landing gear, trailer swing, jackknife versus trailer skid, rollover, offtracking, and the tractor protection valve.
License law is state-specific and manuals are revised. Confirm every item against the candidate handbook for the state where you will test.
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The coupling device on the tractor. Its locking jaws close around the trailer's kingpin. A proper couple has no daylight between the fifth wheel and the trailer apron, and it survives a tug test.
Flip cards, the timed match, and the quiz open in this panel. The full glossary below is ready either way.
Every term, definition, memory hook, and exam trap is written out here so you can review the whole set or print it.
The coupling device on the tractor. Its locking jaws close around the trailer's kingpin. A proper couple has no daylight between the fifth wheel and the trailer apron, and it survives a tug test.
Mnemonic. The fifth wheel is the tractor's lock. The kingpin is the trailer's pin.
Exam trap. The trap is a couple that looks closed from the cab. Get out, look at the jaws, look for daylight, and tug. A high fifth wheel can drop a trailer.
The hardened pin on the trailer's upper coupler that the fifth-wheel jaws lock around. A bent or poorly lubricated kingpin is a dropped-trailer risk.
Mnemonic. The kingpin is what the jaws bite. If the pin is damaged, the bite is a guess.
Exam trap. Grease on the apron is not a cosmetic detail. A dry or damaged pin is an exam cause for a coupling that will not hold. Do not tow it and hope.
The support legs under the front of a trailer. They hold the trailer when it is not coupled, and they must be fully raised, with the crank secured, before you move.
Mnemonic. Landing gear holds a parked trailer. It is not cargo equipment you leave down for the highway.
Exam trap. Moving with the legs partway down is the trap: they catch, they bend, and they can throw the combination. After coupling, raise them all the way and stow the handle.
Rearward amplification: a quick steering move in the tractor becomes a larger sideways move at the rear of the trailer, especially in a sudden lane change.
Mnemonic. The back of the trailer exaggerates the swerve you started in the cab.
Exam trap. The trap is a fast lane change at highway speed to avoid a small obstacle. The tractor clears it and the trailer arrives later, farther sideways, and possibly on its side.
A fold at the fifth wheel, often when the tractor brakes lock or lose grip and the trailer keeps pushing from behind. It is not the same as a trailer skid.
Mnemonic. A jackknife folds the hinge. A trailer skid lets the back of the trailer swing out.
Exam trap. Stabbing the brakes on a slippery surface is the usual setup. The exam wants you to recognize which unit lost traction. If the tractor is the unit that slid, think jackknife.
The tendency of a high, heavy combination to fall onto its side in a curve or a sudden swerve. Speed in the curve matters more than speed on the straight.
Mnemonic. Slow before the curve. A tall load does not forgive a late decision.
Exam trap. Braking hard in the middle of the curve is the trap. The manual wants the speed bled off before you turn the wheel.
In a turn, the rear wheels follow a path inside the path of the front wheels. The longer the combination, the more the rear cuts the corner.
Mnemonic. The back cuts the corner. Swing wide early so the trailer has a path.
Exam trap. The trap is turning as soon as the cab clears the corner. The trailer has not cleared, and it takes the sign, the curb, or the car on the inside.
A valve that protects the tractor's air supply if the trailer breaks away or the trailer lines lose air badly, so the tractor does not lose the air it needs to stop.
Mnemonic. The valve's job is to save the tractor's air when the trailer cannot be trusted to keep it.
Exam trap. The trap is a driver who keeps trying to recharge a catastrophically leaking trailer and drains the tractor tanks. The valve, and the decision to stop, exist so the tractor can still brake.