IN CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor frame that locks around the trailer's kingpin, allowing the trailer to pivot while pulling it.

The vertical steel pin under the front of the trailer that drops into the fifth wheel jaws; the jaws must fully close and lock around it before towing.

The two curved metal pieces inside the fifth wheel that close around the kingpin. They must be fully closed with no gap and the release handle must be secured.

After coupling, pull forward gently with the trailer brakes set to confirm the kingpin is locked and will not pull out of the fifth wheel.

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

The sequence of parking on firm level ground, setting the trailer brakes, lowering the landing gear, disconnecting lines, releasing the fifth wheel, and pulling the tractor clear.

The retractable legs under the front of the trailer that support it when it is not attached to a tractor; they must be fully raised before driving.

The coupling fittings on the ends of the tractor and trailer air hoses that lock together to supply the trailer's brake system. They must be sealed and free of leaks.

The hose, usually color-coded green, that carries compressed air from the tractor's compressor and tanks to charge the trailer's air tanks.

The hose, usually red, that signals the trailer to apply its brakes if it loses air pressure or separates from the tractor; it also supplies air for normal trailer braking.

The cable that carries power and signals to the trailer lights and ABS. It must be plugged in, secured, and checked for damaged pins or insulation.

The trailer's spring-applied brakes, set by the red trailer air supply knob or by the trailer's own control, that hold the trailer when parked or during coupling checks.

Brakes held applied by heavy springs and released by air pressure. If air pressure is lost, the springs apply the brakes automatically, so the trailer fails safe.

The anti-lock braking system on the trailer that prevents wheel lockup during hard braking. Its dash lamp should come on at start-up and go out once the vehicle moves.

The systematic walk-around check of the tractor, trailer, coupling, air lines, lights, tires, and brakes required before driving a combination vehicle.

The tendency of the rear wheels of a combination vehicle to follow a shorter path than the front wheels, cutting the corner on turns and sweeping wide on lane changes.

The total width and area a combination vehicle covers as it turns, including the trailer cutting inside and the rear of the trailer swinging outside its lane.

The hazard when a truck turning right swings left first, letting a car move into the space on the right; the trailer can then crush the car against the curb.

The way side-to-side movement of the tractor is magnified at the rear of the trailer, so a small steering input can swing the trailer's tail much farther.

Side-to-side oscillation of the trailer caused by sudden steering or speed changes; it worsens if the driver overcorrects and can lead to loss of control.

The lever that lets the driver apply the trailer brakes separately from the tractor brakes; used carefully to straighten a trailer that starts to swing.

The total distance a combination vehicle travels from the moment a hazard is seen until it stops, including perception, reaction, and braking distance; it grows with weight and speed.

The gap a driver keeps behind the vehicle ahead. For combination vehicles it should be at least one second per ten feet of vehicle length plus one second at speeds over 40 mph.

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

Perception distance plus reaction distance plus brake lag plus actual braking distance; the full measure of how far a loaded combination needs to stop.

A tip-over caused when centrifugal force in a turn exceeds the vehicle's stability, especially with a high center of gravity; it happens at lower speeds than many drivers expect.

The safe speed for a curve, which for a combination vehicle is often well below the posted advisory speed; the posted limit is for cars, not loaded trucks.

The compressed-air system that operates the service brakes, parking brakes, and trailer brakes, using reservoirs, a compressor, valves, and lines to apply and release pressure.

With the engine off and tanks charged, watching the air gauge for a set time to confirm pressure loss stays within limits; excessive loss means a leak that must be fixed.

A buzzer and light that activate when air pressure drops below a set level, warning that brakes may apply automatically and the vehicle must be stopped safely.

A check with the brakes released and then applied, watching the gauge to confirm the system holds pressure; leaks mean the vehicle should not be driven.

Loss of traction when tires slide instead of rolling, caused by braking too hard, accelerating too fast, or turning too sharply on a slippery or loose surface.

A wheel that stops rotating and slides during hard braking; it removes steering control and can start a skid or a jackknife.

A loss of control in which the tractor and trailer fold toward each other at an angle, often after hard braking or a drive-wheel skid on a slippery surface.

Steering too far in the opposite direction to correct a skid or swing, which starts a new skid the other way and can turn a small problem into a crash.

Applying the brakes firmly but smoothly enough to slow down without locking wheels, keeping the vehicle straight and steering available.

The point where the weight of the cargo is concentrated; the higher it sits, the more easily the combination can roll over in a turn or sudden maneuver.

A gravel or sand ramp built beside a steep downgrade for drivers who lose braking ability; it is meant to stop a runaway vehicle.