LA CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor frame that locks onto the trailer's kingpin and carries the trailer's weight while allowing it to pivot.

The thick steel pin hanging beneath the trailer's front that locks into the fifth wheel jaw, holding the trailer to the tractor and letting it swivel in turns.

The process of backing the tractor under a trailer, locking the fifth wheel around the kingpin, connecting air lines and electrical cord, and raising the landing gear.

The sequence of parking on level ground, lowering the landing gear, disconnecting air and electrical lines, releasing the fifth wheel jaw, and pulling the tractor clear of the trailer.

After coupling, pulling forward gently with the trailer brakes set to confirm the fifth wheel jaw is locked and the trailer will not slide off the kingpin.

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

The interlocking couplers that join the tractor's air supply and service brake lines to the trailer's brake system; they must be sealed and locked.

The line that carries air pressure from the foot brake valve to apply the trailer brakes during normal stops; it is usually color-coded blue.

The supply line that charges the trailer's air tanks and signals a breakaway; if it loses pressure, the trailer brakes apply automatically. Usually color-coded red.

A system that automatically applies the trailer brakes if the trailer separates from the tractor and the emergency air line is pulled apart.

Brake chambers with strong springs that apply the brakes when air pressure drops below a safe level, providing parking and emergency braking on heavy vehicles.

A buzzer and light that activate when air pressure falls below about 60 psi, warning the driver to stop safely before spring brakes apply.

The engine-driven pump that builds and maintains air pressure in the brake system; it must build pressure fast enough to meet the leak-down test standard.

With the engine off and brakes released, watching the gauge for a minute to confirm air pressure does not drop more than the allowed amount.

Holding the brake pedal firmly with the engine off to check that pressure loss from the applied system stays within the limit for the vehicle.

The lever that links the brake chamber pushrod to the brake shoes; its angle and travel show whether the brakes are properly adjusted.

How far the brake chamber rod extends when the brakes are applied; too much travel means the brakes are out of adjustment and stopping power is reduced.

The canister that converts air pressure into mechanical force to apply the brakes; it is checked for cracks, dents, and secure mounting.

The tendency of a trailer's rear wheels to follow a shorter path than the tractor's front wheels, cutting inside the curve during a turn.

The outward movement of the rear of a long trailer during a tight turn, which can strike people, poles, or vehicles beside it.

A hazard where a combination vehicle swings left before turning right, inviting a car to pass on the right and be crushed against the curb.

A control that applies only the trailer brakes; used carefully to straighten a jackknife or test trailer brakes, never as a substitute for the foot brake.

A skid in which the tractor and trailer fold against each other, usually caused by locking the drive wheels or braking hard on a slippery surface.

The technique of releasing the brakes and steering in the direction the vehicle is sliding to regain traction, then reapplying brakes gently once rolling.

Loss of stopping power when brakes overheat from repeated or prolonged use, common on long downgrades if the service brake is used continuously.

An engine or driveline braking device that slows the vehicle on downgrades without using the service brakes, helping prevent fade and overheating.

A downgrade technique of applying the brakes firmly to reduce speed, then releasing them to cool, repeating as needed instead of riding the pedal.

The total distance traveled from perceiving a hazard to a full stop, including perception, reaction, and braking distance; it grows with speed and weight.

The gap kept behind the vehicle ahead, measured in seconds; combination vehicles need at least one second per ten feet of vehicle length plus extra margin.

The maximum total weight a vehicle or combination is designed to carry safely, including the vehicle, fuel, cargo, and passengers.

The maximum load a single axle and its tires are designed to support; exceeding it overloads tires and can cause blowouts or steering loss.

Placing cargo so that weight is spread properly across axles, keeping the steer axle loaded for control and staying within axle limits.

The point where the vehicle's weight is balanced; raising it by loading heavy cargo high makes a rollover much more likely in turns.

A crash in which the vehicle tips onto its side, most often caused by taking a curve or ramp too fast with a high center of gravity.

The safe rule of slowing before a curve and accelerating gently through it, because a combination vehicle's high center of gravity limits safe cornering speed.

Using tiedowns, straps, and blocking to keep cargo from shifting, falling, or spilling; required by federal rules and checked before and during the trip.

Movement of an unsecured load during driving that can change weight distribution, damage the trailer, or throw the vehicle out of control.

The systematic walk-around check of coupling, air lines, brakes, tires, lights, and cargo before driving, required for safety and by regulation.

After coupling, checking that glad hands are locked, lines are not crossed or chafed, and the trailer brakes release and apply with the tractor controls.