Nebraska CDL Combination Vehicles key terms

A power unit towing one or more other units, such as a tractor pulling a semitrailer or a truck pulling a full trailer. The driver must manage the weight and braking of every unit together.

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

A trailer whose front rests on the towing vehicle's fifth wheel rather than on its own front axle. It has landing gear for when it is uncoupled.

A trailer that carries its load on its own axles at both front and rear, connected to the towing vehicle by a drawbar or hitch. It does not rest on the towing unit.

The horseshoe-shaped coupling plate mounted on the tractor's frame that locks onto the trailer's kingpin. It supports the trailer's front weight and allows pivoting in turns.

The vertical steel pin under the front of a semitrailer that locks into the fifth wheel's jaws. It is the main connection holding the trailer to the tractor.

The spring-loaded metal jaws inside the fifth wheel that close around the kingpin. A pull test confirms they are fully closed before the vehicle is driven.

A check made after coupling in which the driver pulls forward gently with the trailer brakes set. If the trailer holds the tractor back, the jaws are locked.

The retractable legs under the front of a semitrailer that hold it up when uncoupled. They must be fully raised before driving and fully lowered before dropping a trailer.

The process of connecting a tractor to a trailer, including aligning the fifth wheel, backing under, locking the jaws, and connecting air and electrical lines. It ends with a safety check.

The process of separating a tractor from a trailer: parking on level ground, lowering landing gear, disconnecting lines, pulling the release handle, and easing out from under.

The interlocking couplers that join the tractor's air supply and brake control hoses to the trailer's. They must be seated and sealed to prevent air loss.

The air hose, usually color-coded red, that supplies air to the trailer's tanks and applies the trailer spring brakes if pressure is lost. Also called the supply line.

The air hose, usually color-coded blue, that carries the driver's brake application signal to the trailer's relay valve. It does not supply the trailer's air tanks.

A trailer valve that uses the service line signal to send stored air to the trailer brake chambers. It shortens the time needed for brakes to apply and release.

Brake chambers that use a heavy spring to apply the brakes when air pressure drops. They set automatically on the trailer if the emergency line is disconnected or leaks.

A valve that keeps a tractor's air system from being drained by a trailer leak and seals off a broken line. It protects the tractor's ability to brake.

A lever on the steering column or dash, also called the trolley valve, that applies only the trailer brakes. It is used to test trailer brakes and to straighten a jackknife.

A check that the trailer brakes apply and hold when the hand valve or pedal is used and that air pressure does not drop too fast. It confirms the lines are connected and sealed.

A check of how fast air pressure falls with the brakes applied and the engine off. A rapid drop points to a leak in the tractor, trailer, or connecting hoses.

The rigid bar connecting a full trailer to the towing vehicle. It transmits pulling and stopping forces and must be strong enough for the loaded trailer.

A hook-and-latch coupling on the rear of a truck that grabs a trailer's lunette eye. The safety latch must be closed and a safety chain or cable attached.

The ring on the end of a full trailer's drawbar that fits over a pintle hook. It allows the trailer to pivot while staying attached.

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

A separation of trailer from tractor that triggers the trailer's emergency brakes through a lanyard and switch. The trailer brakes should set and stay set.

The tendency of a trailer's rear wheels to cut a shorter path than the tractor's front wheels in a turn. The longer the combination, the wider the sweep needed.

The total width of road a turning combination vehicle occupies, from the outside of the tractor's front to the inside of the trailer's rear wheels. It grows with vehicle length.

The radius of the circle a vehicle follows in a turn. A long combination needs a larger radius, so the driver must swing wide without crossing into oncoming lanes.

A skid in which the tractor and trailer fold toward each other at the coupling, often from locked wheels or hard braking on slick pavement. It can happen with or without a trailer.

Loss of traction by the trailer wheels, causing the trailer to swing sideways out of line with the tractor. Releasing the brakes and steering can help straighten it.

The way the last unit in a multi-trailer combination swings wider and faster than the tractor in a quick lane change. It makes sudden steering moves dangerous at speed.

The exaggerated sideways motion of the rear trailer in a double or triple combination when the driver steers sharply. It can tip the rear unit or push it out of its lane.

A combination of a tractor pulling two trailers, usually a semitrailer plus a full trailer on a converter dolly. It requires more following distance and smoother steering.

A combination of a tractor pulling three trailers. It is the least stable arrangement and needs the longest following distance and the gentlest steering inputs.

A small axle unit with a fifth wheel and drawbar that turns a semitrailer into a towable full trailer behind another trailer. It must be coupled and checked like any other hitch.

The short delay between pressing the pedal and the trailer brakes actually applying, because air must travel through the lines. It lengthens stopping distance, so following distance must increase.

The total distance traveled from seeing a hazard to coming to a stop, including perception, reaction, and braking. A loaded combination needs far more room than a car.

The gap kept behind the vehicle ahead, measured in seconds. A combination vehicle should keep at least one second per ten feet of length, and more in poor conditions.