OR CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor frame that locks around the trailer kingpin to hold the trailer attached and let it pivot during turns.

The sturdy vertical steel pin under the front of the trailer that slides into the fifth wheel jaws and is the main connection holding tractor and trailer together.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A firm pull on the trailer with the tractor braked confirms they are fully closed.

A coupling check where you set the trailer brakes, release the tractor brakes, and gently pull forward. If the trailer holds, the jaws are locked; if the tractor rolls free, recouple.

The full procedure of backing the tractor under a trailer, locking the fifth wheel, connecting air lines and electrical cord, raising landing gear, and verifying all connections before driving.

The reverse of coupling: parking on firm level ground, chocking wheels, lowering landing gear, disconnecting lines, and releasing the fifth wheel so the tractor can pull away safely.

The retractable legs under the front of a trailer that support it when it is not attached to a tractor. They must be cranked down before uncoupling and fully raised after coupling.

The quick-coupling fittings that join the tractor and trailer air lines. They must be sealed and locked so supply and service lines do not leak or separate in motion.

The tractor-to-trailer air line (often color-coded) that charges the trailer air tanks and signals the trailer emergency brake system. Loss of pressure here triggers trailer brakes.

The air line that carries the driver's brake application signal from the tractor to the trailer relay valve, causing the trailer brakes to apply along with the tractor.

Spring-applied brakes that set automatically when trailer air pressure drops or the supply line separates, stopping the trailer if it breaks away from the tractor.

A valve that closes off the tractor's air supply if the trailer line fails or separates, keeping enough air in the tractor tanks to stop the power unit.

A trailer valve that receives the service line signal and quickly sends stored tank air to the trailer brake chambers, speeding up brake response on a long combination.

A pre-trip sequence testing air pressure build-up, static leak rate, warning buzzer and light, tractor protection valve pop-out, and trailer brake response before the vehicle is moved.

With the engine off and brakes released, you watch the gauge for a minute to confirm pressure loss stays within limits; then you hold the brake pedal and check loss again.

A buzzer and light that activate before air pressure falls too far, warning the driver that brakes may soon apply automatically. You must know the pressure at which it triggers.

Brake chambers with a powerful spring that applies the brakes whenever air pressure is released, so a combination cannot be driven with insufficient air pressure.

The tendency of the rear trailer wheels to cut inside the path of the front wheels during a turn, so the trailer sweeps a wider arc and can strike curbs or nearby objects.

The outward sweep of the rear of a long trailer during a turn, which can strike signs, poles, or vehicles beside the trailer if the driver does not allow room.

Approaching a right turn by moving into the left portion of your lane or an adjacent lane so the trailer's off-tracking path clears the curb and the corner.

Loss of stopping power when brakes overheat from repeated or prolonged use, such as on a long downgrade, causing the lining and drum to lose friction.

A downhill technique of applying the brakes firmly to reduce speed well below the limit, then releasing them so they cool, rather than riding them continuously.

A device such as an exhaust or compression brake that slows the vehicle using engine resistance, sparing the service brakes on long descents and helping control speed.

The total distance a combination travels from the moment the driver perceives a hazard until it stops, including perception, reaction, and braking distance.

The gap kept behind the vehicle ahead, measured in seconds. A loaded combination needs at least one second per ten feet of vehicle length plus reaction time.

A loss of control where the tractor and trailer fold against each other at an angle, often caused by locking the drive or trailer brakes on a slippery surface.

Sliding of tires that have lost traction, so steering or braking no longer responds normally. Recovery requires releasing the brake or easing the accelerator and steering where you want to go.

A wheel that stops rotating and slides because the brake force exceeds available traction, which removes steering control and can start a jackknife or flat-spot a tire.

When trailer brakes grab harder than tractor brakes on slick pavement, the trailer swings around the tractor. Properly balanced brake adjustment helps prevent it.

The sideways force at which a combination tips over. Loaded trailers tip at lower forces than cars, so speed in curves and ramp turns must be reduced well below posted limits.

The point where the vehicle's weight is concentrated. Raising it by loading heavy cargo high makes a rollover much more likely in turns or sudden maneuvers.

Positioning cargo so weight sits low and centered over the axles, keeping the center of gravity down and preserving steering and braking balance on the combination.

The maximum legal weight allowed on each axle or axle group. Overloading one axle reduces steering or braking control and can put the vehicle out of legal compliance.

Spreading cargo weight across tractor and trailer axles so no axle is overloaded and the fifth wheel carries its proper share, keeping handling predictable.

Using tiedowns, straps, chains, and blocking to keep cargo from shifting or falling. Cargo must be secured to withstand expected forces during braking and turning.

A strap, chain, or web assembly anchored to the vehicle and attached to the cargo, with a working load limit that must meet the required aggregate strength for the load.

Setting slack adjusters so each brake chamber pushrod stroke stays within its limit, ensuring all brakes on the combination apply evenly and resist lockup.

The speed chosen for a curve based on its sharpness, superelevation, and the vehicle's rollover threshold, not simply the posted advisory sign meant for cars.