Missouri CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor that locks around the trailer's kingpin to connect the two vehicles and allow them to pivot as one unit.

The sturdy vertical pin under the front of a semitrailer that slides into the fifth wheel and is gripped by the locking jaws to hold the trailer to the tractor.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A pull test confirms they are fully closed before you drive away.

The flat plate on the underside of a trailer's front that rests on the fifth wheel. It must sit flush so the coupling is solid and the trailer rides level.

An adjustable fifth-wheel mounting that lets you move the coupling point forward or back on the tractor frame to shift weight between the steering and drive axles.

A valve that closes automatically when trailer air pressure drops or the trailer separates, sealing the tractor's air supply so the tractor can still brake.

The interlocking couplers that join the tractor's air lines to the trailer's. They are color-coded: blue for service brakes and red for emergency supply.

The blue-coded line that carries air from the tractor's brake pedal to apply the trailer brakes during normal stopping. It is also called the control line.

The red-coded line that supplies air pressure to the trailer tanks. If it loses pressure or separates, the trailer brakes apply automatically.

Brakes held applied by heavy springs and released by air pressure. When air pressure falls too low, they set automatically, which is why you must build pressure before moving.

A buzzer and light that activate when air pressure drops below about 60 psi, warning you to stop and find the leak before the spring brakes lock up.

The engine-driven pump that builds and maintains air pressure in the brake system. It must be able to raise pressure faster than the system loses it.

The device that cuts the compressor in and out to keep system pressure within a set range, usually between about 100 and 125 psi.

A check where you charge the system, shut the engine off, and watch the gauge. Pressure should not drop more than a few psi per minute with brakes released or applied.

A lever on the steering column or dash that lets you apply the trailer brakes alone, often used to test trailer braking or to correct a jackknife in an emergency.

Driving a tractor without a trailer. The tractor brakes must still stop the unit, and bobtail stopping distance is shorter but the ride is rougher and less stable.

The common combination in which a powered tractor carries the front of a trailer on its fifth wheel while the trailer's rear axles carry the rest of the load.

A small frame with one or two axles and a fifth wheel used to attach a second trailer to a combination, forming doubles or triples.

A combination of a tractor pulling two trailers, joined by a converter dolly. They require extra care in turns and when backing, which is generally avoided.

A combination of a tractor pulling three trailers. These rigs need the most room to turn, stop, and change lanes and are limited to certain routes.

The tendency of a trailer's rear wheels to follow a path inside the tractor's front wheels in a turn. The longer the combination, the wider the swept path.

The total width of road a combination occupies during a turn, including the trailer's rear wheels. Mirrors and signs are common hazards because of it.

A loss of control where the tractor and trailer fold sharply against each other, often from hard braking on slick pavement or from oversteering.

Loss of stopping power when brakes overheat from repeated or prolonged use, common on long downgrades. Downshifting and using engine braking help prevent it.

A retarding device that uses engine compression to slow the vehicle without using the service brakes, helping control speed on downgrades and reduce brake fade.

A downgrade technique of applying the brakes firmly to reduce speed by about 5 mph, then releasing them, rather than riding them continuously.

A misspelling of off-tracking sometimes seen in study notes. The correct term describes trailer wheels cutting inside the tractor's path in a turn.

Two axles grouped together on a tractor or trailer. Sliding them changes how weight is shared among axles and affects steering and traction.

How a load's weight is spread across steering, drive, and trailer axles. Poor distribution can overload an axle or lighten the steering axle dangerously.

The maximum weight allowed on a single axle or axle group. Exceeding it is a violation and can make the vehicle unstable or damage the road.

A systematic walk-around check of the tractor, trailer, coupling, brakes, lights, and tires before driving, required by federal and state rules.

The inspection step that confirms the fifth wheel is locked, the apron is flush, and the pull test holds before you drive the combination.

After coupling, you gently pull forward with the trailer brakes set to confirm the kingpin is locked and will not slip out of the fifth wheel.

The retractable legs under a trailer's front that support it when uncoupled. They must be fully raised and the crank secured before driving.

The procedure for separating tractor and trailer: park on level ground, set brakes, lower landing gear, disconnect lines, then release the fifth wheel.

The measured groove depth on a tire. Steer tires need at least 4/32 inch and other tires at least 2/32 inch to pass inspection.

Checking tires for proper inflation, tread depth, cuts, bulges, and matching sizes, since a single bad tire can cause a blowout or loss of control.

A sequence of checks for leaks, warning devices, and spring brake operation that confirms the air system will stop the vehicle safely.

The skills of starting, stopping, steering, and positioning a combination smoothly in a controlled area, usually tested before on-road driving.

Backing a combination in a straight line using small steering corrections and both mirrors, keeping the trailer centered between the lane lines.

Backing into a lane that is offset left or right from the starting position, requiring you to judge trailer angle and correct with small movements.

A backing maneuver into a simulated dock from an alley, testing control of trailer angle and the ability to stop close to the barrier without hitting it.

A person outside the vehicle who guides a backing maneuver with hand signals. If you lose sight of the spotter, stop immediately.

The live traffic portion of the test where you demonstrate lane control, turns, lane changes, speed management, and safe following in a combination vehicle.

The gap you keep behind the vehicle ahead. For a loaded combination, allow at least one second per ten feet of vehicle length plus a second.

Moving to another lane after checking mirrors, signaling, checking blind spots, and confirming the trailer has cleared the lane before completing the move.

A turn that requires swinging wide enough for the trailer's rear wheels to clear the curb while staying in the proper lane and watching for cyclists.

An area beside or behind the combination that mirrors do not show. You must check it before lane changes and backing.