Alabama CDL Combination Vehicles key terms

A truck or tractor that pulls one or more trailers, so the whole unit is steered by the front axle and articulated at the coupling point. Handling and braking differ sharply from a single-unit vehicle.

The pivot where a trailer attaches to the towing vehicle, such as a fifth wheel or pintle hook. The vehicle bends here, so the trailer tracks inside the path of the tractor during turns.

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 swing and the more room needed.

The total width of pavement a turning combination vehicle occupies, including the tractor, the trailer's inside cut, and any rear overhang swing. Drivers must keep the whole path clear of curbs and traffic.

The outward arc traced by the back corner of a long trailer or rear overhang as the vehicle turns. It can strike signs, poles, or pedestrians standing outside the turn.

The way sideways motion at the tractor's rear is magnified at the trailer's rear, especially in quick lane changes. Sudden steering can whip the trailer and start a rollover.

The sideways force, usually expressed in g's, at which a vehicle tips over. Loaded tankers and high-center-of-gravity trailers tip at lower forces, so they need slower speeds in curves.

The point where a vehicle's weight is balanced. A high or off-center load raises this point and makes the combination easier to tip in a curve or sudden maneuver.

How cargo weight is spread and secured on a trailer. Heavy loads should sit low and centered over the axles; shifting loads can change steering, braking, and rollover behavior.

The tiedowns, straps, and blocking used to keep freight from moving. Shifting cargo can push a trailer sideways, overload an axle, or cause a jackknife during braking.

The horseshoe-shaped coupling plate on the tractor that locks onto the trailer's kingpin. It carries the trailer's weight and lets the combination pivot during turns.

The heavy pin mounted under the trailer's nose that locks into the tractor's fifth wheel. A properly seated kingpin is confirmed by a firm tug test before driving.

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

The process of separating a tractor from a trailer: parking on level ground, chocking wheels, lowering landing gear, disconnecting lines, and releasing the fifth wheel.

A check after coupling where the driver pulls gently forward with the trailer brakes set. If the tractor stops firmly, the fifth wheel is locked; if it moves, the coupling is not secure.

The retractable legs under a trailer's nose that support it when uncoupled. They must be fully raised before driving and firmly on the ground when the trailer is parked.

A heavy hook-and-latch coupling used to tow trailers with a lunette eye instead of a kingpin. The latch must be closed and secured before the vehicle moves.

Backup chains that connect a towed trailer to the towing vehicle in case the main hitch fails. They should be crossed and long enough to allow full turning without binding.

The couplers that join the tractor's air hoses to the trailer's brake system. They must be locked, sealed, and free of leaks, or trailer brakes may not release or apply correctly.

The air line, usually color-coded, that carries air from the tractor to charge the trailer's reservoir and release its spring brakes. A loss of supply pressure applies the trailer brakes.

The air line that carries the driver's brake application signal to the trailer's relay valve. It tells the trailer when and how hard to brake.

A trailer valve that uses a small service-line signal to send reservoir air to the trailer brake chambers. It speeds up brake response on long trailers.

A parking and emergency brake held applied by a strong spring and released by air pressure. If air pressure drops too low, the spring brakes apply automatically.

A drop in system air pressure from a leak or failure. Warning buzzers sound and spring brakes apply at low pressure, so the driver must stop safely before losing control.

A check for air leaks made by charging the system, shutting off the engine, and watching the gauge. A steady pressure drop beyond the allowed rate means a leak must be fixed.

A leak test done with the brakes released and again with the brakes applied, comparing gauge readings. Each phase has its own acceptable pressure-loss limit.

The engine-driven pump that builds air pressure for the brake system. The gauge should recover to normal operating pressure within a reasonable time after the compressor cuts in.

A buzzer and light that activate when pressure falls below a safe level. The driver must stop and find the problem before continuing.

The canister where air pressure pushes a diaphragm and pushrod to apply the wheel brakes. On combination vehicles, trailer chambers must be checked during a walk-around inspection.

A valve that closes off the tractor's air supply if a trailer breaks away or a line fails. It keeps enough air in the tractor to stop it.

A pre-trip walk-around that checks coupling, air lines, landing gear, lights, tires, and brakes on both units. Problems found here prevent most breakdowns and coupling failures.

The required safety check before driving, covering the engine compartment, walk-around items, and in-cab controls. For combination vehicles it must include the coupling and trailer brake checks.

A skid where the tractor and trailer fold toward each other at the coupling, often caused by locked wheels or braking hard on slippery roads. It can be avoided by smooth braking and steering.

Loss of traction at the trailer wheels, letting the trailer swing sideways. The driver should release the brakes to let the wheels roll and regain grip rather than steer harder.

Changing lanes in a long vehicle requires checking mirrors on both sides, signaling early, and steering gradually. Quick steering whips the trailer and can cause a rollover.

Areas around a combination vehicle that the driver cannot see in the mirrors, including alongside the trailer and directly behind. Mirrors must be adjusted and checked often.

Moving a tractor-trailer in reverse using small steering corrections to push the trailer where it should go. Back slowly, use a spotter, and get out to check if unsure.

A person outside the vehicle who guides the driver during backing. Agree on hand signals and stop immediately if you lose sight of the spotter.

Choosing a speed that lets the driver stop within the visible clear distance and stay below rollover limits in curves. Downshift before downgrades and brake smoothly.