AK CDL Combination key terms

A power unit (truck tractor) towing one or more trailers, where the vehicles are connected so the tractor pulls and steers the load. The connection points and air lines make it behave differently from a single-unit truck.

The horseshoe-shaped coupling plate mounted on the tractor frame that grips the trailer's kingpin. It carries the trailer's weight and pivots so the combination can turn without binding.

The vertical steel pin under the front of a semitrailer that locks into the tractor's fifth wheel. It is the main load-bearing connection and must be fully seated and locked before moving.

The two curved metal pieces inside the fifth wheel that close around the kingpin to hold the trailer. A pull test and visual check confirm they are closed and the release handle is in.

After coupling, the driver pulls forward gently with the trailer brakes set to confirm the kingpin is locked and will not slip out. The tractor should not move; if it does, the coupling is wrong.

The full procedure of backing under a trailer, locking the kingpin, connecting air and electrical lines, raising the landing gear, and testing the connection. Each step must be completed in order.

The reverse of coupling: parking on firm level ground, setting brakes, lowering landing gear, disconnecting lines and the kingpin, then pulling clear. Chock wheels and confirm the trailer cannot roll.

The retractable legs under a trailer's front that support it when it is not attached to a tractor. They must be raised fully before driving and lowered to the ground before uncoupling.

The interlocking couplers that join the tractor's air lines to the trailer's. They are color-coded (service and emergency/supply) and must be seated with seals intact to prevent air leaks.

The air line (often blue) that carries the driver's brake application pressure to the trailer. It controls how hard the trailer brakes apply in normal braking.

The air line (often red) that supplies air to the trailer's reservoir and signals a breakaway. If it loses pressure, the trailer brakes apply automatically as a safety feature.

A valve on the trailer that sets the trailer brakes if the trailer separates from the tractor and the emergency line is pulled apart. It prevents a loose trailer from rolling away.

A trailer valve that speeds up brake application and release by sending reservoir air directly to the brake chambers. It helps the trailer brakes respond quickly and evenly.

The cylinder that converts air pressure into mechanical force to apply a wheel's brakes. Each wheel end has one, and a leak or damaged diaphragm weakens braking.

The linkage between the brake chamber pushrod and the brake shoe that takes up wear. Too much pushrod travel means the brakes are out of adjustment and stopping distance increases.

How far the brake chamber rod extends when the brakes are applied. Excessive travel signals worn or out-of-adjustment brakes and is checked during a pre-trip inspection.

A check with the engine off and brakes applied to see whether air pressure drops faster than allowed. A steady loss means a leak in lines, glad hands, or chambers that must be fixed.

With the system charged and the engine off, watch the gauge for a set time with brakes released and then applied. Pressure should hold within limits; a faster drop indicates a leak.

A buzzer and light that activate when air pressure falls below a safe level, warning the driver before brakes fail. It must be tested during the pre-trip inspection.

Emergency/parking brakes that use a strong spring to apply the brakes when air pressure is lost. They release only when enough air pressure is built up.

The trailer's own spring or parking brake system that holds it when parked or when air is lost. They must be set before uncoupling so the trailer stays put.

A systematic walk-around check of the tractor, trailer, coupling, air system, lights, tires, and load before driving. It catches defects that could cause a breakdown or crash.

The ties, straps, and blocking used to keep a load from shifting, falling, or spilling. A shifting load can upset a combination vehicle, especially in turns or hard braking.

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 path the rear of the trailer sweeps.

The total width of road a turning combination occupies, including the trailer's inside cut and any rear overhang swing outside. Drivers must allow room for both before starting a turn.

The size of the arc a vehicle needs to complete a turn. A combination vehicle needs a much wider radius than a single truck, so turns must be set up wide and slow.

The way a steering input at the tractor can grow into a larger sideways motion at the rear of the trailer, especially at higher speed. It makes quick lane changes and sudden corrections dangerous.

Side-to-side fishtailing of the trailer that can grow until control is lost. It is caused by speed, poor load balance, wind, or abrupt steering and is corrected by slowing gradually, not by hard braking.

A sudden rollover caused when a combination vehicle's trailer swings and the load shifts hard to one side. Sharp steering at speed or an off-balance load can trigger it.

Movement of cargo within or on the trailer during driving. It changes weight distribution and can make the trailer unstable or cause a rollover, so loads must be blocked and tied.

A lever (also called the trolley valve) that lets the driver apply only the trailer brakes. It is used to test trailer brakes and to help straighten a jackknife in some situations.

A loss of control where the tractor and trailer fold against each other at an angle. It often results from locking the tractor wheels on a slippery surface or braking hard in a turn.

Loss of traction where tires slide instead of roll. In a combination vehicle, a tractor skid can fold the rig, so drivers release the brakes and steer to regain grip.

A system that rapidly releases and reapplies brake pressure to prevent wheel lockup. On a combination vehicle, the tractor and trailer each have ABS, and the trailer light must be checked.

A heavy hook-and-latch coupling used to tow trailers with a lunette eye, common on straight trucks and some doubles. It allows pivoting but must be latched and safety-chained.

Backup chains that connect a towed unit to the towing vehicle in case the main hitch fails. They must be crossed and attached securely so the trailer stays connected.

The removable handle used to raise and lower landing gear. It must be stored securely after use so it does not fall into the road or strike the tractor.

A flexible guard behind the wheels that reduces spray and thrown debris. Missing or damaged flaps are an inspection item on both tractor and trailer.