Kentucky CDL Combination key terms

A power unit (tractor) that pulls a semi-trailer, with the trailer's weight partly carried on the tractor's rear frame through a fifth wheel coupling rather than entirely on its own axles.

The horseshoe-shaped plate mounted on the tractor's rear frame that locks around the trailer kingpin and carries part of the trailer's weight while allowing the trailer to pivot.

The vertical steel pin mounted under the front of a semi-trailer that locks into the fifth wheel jaw, holding the trailer to the tractor and providing the pivot point for turns.

The procedure of backing the tractor under a trailer and locking the fifth wheel around the kingpin, then connecting air lines, electrical cord, and landing gear as required.

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

The retractable legs under a semi-trailer front that support the trailer when it is not attached to a tractor; they must be fully raised before moving and firmly set before uncoupling.

The flat metal plate under the front of a semi-trailer around the kingpin; the fifth wheel must contact it fully so the coupling is solid and the trailer rides level.

A mechanism that lets the tractor's rear axle group or the trailer's axles be moved forward or backward to redistribute weight among axles and stay within legal axle limits.

A heavy hook-and-latch coupling on the rear of a truck or tractor used to tow a full trailer or converter dolly by its lunette eye; it allows pivoting but must be latched securely.

A small auxiliary axle unit with its own fifth wheel and drawbar that couples a second semi-trailer to the rear of a first trailer, creating a double or triple combination.

The ring-shaped fitting on the end of a trailer drawbar that drops over a pintle hook to form the towing connection on full trailers and converter dollies.

Backup chains that connect the towing vehicle to a towed unit so the combination stays linked if the primary hitch fails; they must be crossed and have enough slack to allow turning.

Combinations of two or three trailers pulled by one tractor; they require extra care because of increased off tracking, longer stopping distance, and amplified trailer sway.

A braking system that uses compressed air to apply the service brakes and spring pressure to apply parking brakes, with pressure loss causing automatic emergency application on most combinations.

The foot-operated brake used during normal driving; on air-braked combinations it applies air pressure to chambers that push brake shoes or pads against the drums or discs.

A parking and emergency brake held off by air pressure and applied by a strong spring when air pressure drops or the parking valve is set; it is not for routine stopping.

The dash valve that releases air from the spring brake chambers to set the parking brake; pulling it applies the brakes, and pushing it in releases them once pressure is built.

A valve that closes the tractor's air supply to the trailer lines if trailer pressure drops sharply, keeping enough air in the tractor to stop it after a trailer breakaway.

The two flexible hoses (service and supply/emergency) that carry air between tractor and trailer; they must be connected color-to-color and supported so they do not drag or chafe.

The coupler fittings at the ends of the tractor and trailer air hoses; their rubber seals must be in place and the locking ears engaged to prevent air leaks.

A buzzer and light that must activate before pressure falls below a set threshold (commonly around 60 psi) to warn the driver that brakes may soon apply automatically.

The engine-driven pump that builds and maintains air pressure in the brake system; it must be able to raise pressure fast enough to keep up with normal brake use.

A storage reservoir that holds compressed air for the brakes; tanks must be drained of moisture and oil regularly, and a buildup of liquid reduces available braking capacity.

A test with the engine off and brakes applied to confirm pressure loss stays within limits; a steady drop indicates a leak that must be fixed before driving.

A system that automatically applies the trailer's spring brakes if the trailer separates from the tractor, bringing the trailer to a stop and limiting runaway risk.

The short delay between pressing the brake pedal and the brakes actually applying, caused by the time air takes to travel through the lines; it lengthens total stopping distance.

The total distance a combination travels from the moment a hazard is seen until it stops, including perception, reaction, brake lag, and actual braking distance.

The gap kept behind the vehicle ahead; for heavy combinations it should be at least one second per ten feet of vehicle length plus extra time for speed and conditions.

An electronic system that senses wheel lockup and rapidly modulates brake pressure to keep wheels rolling, helping the driver retain steering control during hard braking.

A component at each monitored wheel that reports rotation speed to the ABS controller; a damaged sensor can disable antilock function on that wheel or axle.

A yellow dash light that stays on when the antilock system detects a fault; normal brakes still work, but the wheels may lock and the fault should be repaired.

Loss of traction in which tires slide instead of rolling, often from braking too hard, accelerating too fast, or turning too sharply for the speed and surface.

A skid in which the tractor and trailer fold toward each other at the coupling, usually from locking the drive or trailer wheels while braking on a slippery surface.

Side-to-side fishtailing of a trailer, often triggered by speed, crosswinds, poor load distribution, or abrupt steering; easing off the accelerator helps regain control.

The tendency of the rear wheels of a long combination to track inside the path of the front wheels during a turn, so the trailer cuts the corner and can strike curbs or objects.

The total width of road a combination occupies as it turns, including the trailer's inside cut and the rear of the unit swinging outward; it is wider than the vehicle itself.

The size of the circle a vehicle needs to complete a turn; long combinations require a much larger radius, so turns must be started wide and taken slowly.

The way steering or sway movements at the tractor are magnified at the rear of a multi-trailer combination, making the last trailer swing more than the first.

A tip-over caused when centrifugal force from turning too fast exceeds the vehicle's stability, especially with a high center of gravity or a shifting load.

The point where the vehicle's weight is concentrated; raising it by loading heavy freight high makes the combination more likely to tip in turns or sudden maneuvers.

Keeping cargo secured, balanced, and low so it cannot shift during turns or braking; shifting loads change axle weights and can cause loss of control or rollover.

The tie-downs, straps, and blocking used to hold freight in place; required devices and working load limits must match the weight and type of load being hauled.

Spreading load weight properly among tractor and trailer axles so no axle is overloaded and steering and braking remain effective; sliding tandems and load placement adjust it.

The maximum weight allowed on a single axle or axle group by law and by tire rating; exceeding it reduces safety and can result in fines or out-of-service orders.

A systematic walk-around check of the combination before driving, covering coupling, air lines, brakes, tires, lights, and load securement to catch defects early.

Reversing a tractor-trailer, which requires steering opposite the desired trailer direction; pull-ups and GOAL (get out and look) reduce blind-side risk.

A person outside the vehicle who guides a backing maneuver with hand signals and stays visible in the mirrors, stopping the driver if the path is unsafe.