Kansas CDL Combination key terms

A power unit (truck or tractor) coupled to one or more towed units, such as a semi-trailer or a truck and full trailer. The coupling hardware and the added weight change how the rig brakes, turns, and stops.

The front power unit of a tractor-trailer combination. It carries the engine, steering axle, drive axles, and the fifth wheel that supports and locks the front of the semi-trailer.

A towed cargo unit whose front rests on the towing vehicle's fifth wheel rather than on its own axle. Its rear axles carry the back of the load, and it has a kingpin at the nose.

The horseshoe-shaped coupling plate mounted on the tractor frame. It supports the trailer's front weight and locks around the kingpin so the trailer pivots and pulls with the tractor.

The vertical steel pin under the trailer's nose that drops into the fifth wheel. The coupling jaws grip it to hold the trailer to the tractor, and it is the pivot point for turns.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A locking mechanism keeps them shut; a pull handle releases them when uncoupling.

The lever on the side of the fifth wheel that opens the coupling jaws. It is pulled to release the kingpin during uncoupling and must be in the locked position before towing.

The flat steel plate under the trailer's nose around the kingpin. It slides across the fifth wheel during coupling and helps the trailer ride level on the plate.

The retractable legs under the front of a semi-trailer that support it when it is not attached to a tractor. They must be fully raised before towing and lowered before uncoupling.

The quick-connect couplers that join the tractor's air lines to the trailer's brake lines. One line is the service (control) line and the other is the emergency (supply) line.

The air line that carries the driver's brake application pressure from the tractor to the trailer brakes. It is usually the line with the smaller fitting, often colored blue.

The air line that supplies air to the trailer's reservoirs and signals a breakaway. If it loses pressure, the trailer brakes apply automatically; it is usually red with a larger fitting.

The dash-mounted control, also called the trolley valve, that applies only the trailer brakes. It is used to test trailer brakes and to check for a firm hold during inspection.

The system that applies the trailer brakes automatically if the trailer separates from the tractor. Air escapes from the emergency line, and the trailer's spring brakes set without driver action.

Chains that connect a towed unit to the towing vehicle as a backup if the primary hitch fails. They must be crossed under the tongue and long enough to allow turning.

A heavy hook-and-ring coupling used to tow full trailers and heavy equipment. The ring on the towed unit drops over the pintle hook and is secured with a latch or pin.

A towed unit that carries its own weight on its own axles at both front and rear. It attaches to the towing vehicle through a drawbar or hitch rather than resting on a fifth wheel.

The rigid bar that connects a full trailer's front axle assembly to the towing vehicle's hitch. Its length affects how closely the trailer follows the path of the truck.

The procedure of connecting the tractor to the trailer: backing under the trailer, locking the fifth wheel, connecting air lines and electrical cord, raising landing gear, and testing the connection.

The procedure of separating the tractor from the trailer: parking on firm level ground, lowering landing gear, disconnecting lines, releasing the fifth wheel, and pulling away slowly.

A check after coupling in which the driver pulls gently against the locked trailer with the trailer brakes set. If the tractor moves and the trailer does not, the coupling is secure.

The tendency of a towed unit's rear wheels to cut a shorter path than the front wheels in a turn. Longer combinations and sharper turns increase how far the rear wheels track inside.

The total width of road a combination occupies as it turns, including the front unit's path and the trailer's inside cut. Drivers must allow room for both to avoid curbs and adjacent lanes.

The way a steering movement at the tractor is magnified at the rear of the combination, so the last trailer swings wider than the first. It grows with speed and with more towed units.

Side-to-side fishtailing of a towed unit, often triggered by sudden steering, strong crosswinds, or improper load distribution. Easing off the accelerator and steering smoothly helps regain control.

A loss-of-control condition in which the tractor and trailer fold against each other at a sharp angle. It often follows locked wheels or hard braking on a slippery surface.

Brakes held off by air pressure and applied by heavy springs when pressure drops. They set automatically on air loss and serve as the parking brake on many heavy vehicles.

Antilock braking system, which senses wheel lockup and rapidly adjusts brake pressure to keep wheels rolling. On a combination, the tractor and trailer may have separate ABS systems and warning lamps.

A check that the trailer brakes apply and hold when the driver uses the trailer hand valve or the brake pedal. It confirms the service line is connected and the brakes are working.

Placing cargo so weight is balanced over the axles and the trailer stays within axle and gross weight limits. Poor distribution can cause sway, poor steering, or overloaded tires.

Using tiedowns, straps, and blocking to keep cargo from shifting or falling. Shifting loads change axle weight and can make a combination unstable in turns or braking.

The systematic check of a combination before driving, covering coupling, air lines, brakes, tires, lights, and cargo. It is required by federal rules and is scored in the skills test.

The portion of the vehicle inspection test in which the candidate moves around the combination and identifies parts, defects, and safety items out loud to the examiner.

The skills-test step in which the candidate checks items inside the cab, such as seat belt, gauges, horn, wipers, and brake controls, while explaining what is being checked.

The hands-on portion of the CDL exam that includes vehicle inspection, basic control, and on-road driving. For combination vehicles it also covers coupling and uncoupling.

The skills-test segment that checks low-speed control of the combination, including straight-line backing, offset backing, and sometimes alley docking within marked boundaries.

A skills-test maneuver in which the driver backs the combination into a simulated dock area from an alley. The driver must use pull-ups and GOAL as needed without crossing boundary lines.

Get Out And Look: stopping to walk around and inspect the area before continuing a difficult backing maneuver. It is allowed on the skills test and prevents costly backing collisions.