Ohio CDL Combination key terms

A power unit towing one or more other units, such as a tractor pulling a semitrailer. The coupling between units changes how the rig brakes, turns, and responds to steering.

The powered front unit of a tractor-trailer combination. It carries the fifth wheel, the air supply lines, and the electrical cord that connect it to the trailer it pulls.

A trailer whose front end rests on the towing vehicle rather than on its own axle. It has no front wheels and depends on the fifth wheel for support and steering force.

A trailer that carries its load on its own axles at both front and rear, attached to the towing vehicle by a drawbar or hitch. It follows differently than a semitrailer.

The horseshoe-shaped coupling plate mounted on the tractor that locks onto the trailer kingpin. Its jaws must close fully around the pin before the vehicle is moved.

The vertical steel pin under the front of a semitrailer that drops into the fifth wheel jaws. Its apron and pin must be inspected for cracks, wear, and secure mounting.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A firm pull on the trailer with the tractor brakes released confirms they have locked.

The flat steel plate under the front of a semitrailer that surrounds the kingpin. It must sit flat and square on the fifth wheel, with no gap or wobble.

A coupling check in which the driver sets the trailer brakes, releases the tractor brakes, and pulls gently forward. If the rig does not move, the jaws are locked on the kingpin.

The process of connecting the tractor to the trailer, including aligning the fifth wheel, backing under, locking the jaws, and confirming the connection. Done incorrectly, the trailer can separate.

The process of separating the tractor from the trailer. The steps include parking on level ground, chocking wheels, lowering landing gear, disconnecting lines, and pulling clear of the kingpin.

The retractable legs under the front of a semitrailer that support it when unhitched. They must be lowered until they touch the ground and then cranked a few extra turns.

The couplers that join the tractor's air lines to the trailer's air lines. They must be locked together and sealed so the trailer brakes and air supply work properly.

The air line that carries the driver's brake application signal to the trailer. It is usually the line with the blue glad hand on a standard color-coded setup.

The air line that supplies air to the trailer tanks and applies the trailer brakes if the line breaks or separates. It is usually color-coded red at the glad hand.

A control in the cab that lets the driver apply the trailer brakes alone, independent of the tractor brakes. It is used to check trailer brake operation and to help control a skid.

Brakes held open by air pressure and applied by heavy springs when air pressure drops or is released. On a trailer they apply automatically if the emergency line separates.

The tendency of the rear wheels of a long combination to follow a path inside the front wheels on a turn. It can cause the trailer to strike curbs, signs, or other vehicles.

The total width of road the vehicle and its trailer occupy during a turn. In a combination it is wider than the tractor's path, so mirrors and space must be checked carefully.

The way the last trailer in a multi-trailer combination swings farther than the tractor in a quick lane change or steering maneuver. It makes the rear unit more likely to roll over.

The whipping motion that builds toward the rear of a long combination when the driver makes a quick steering correction. It can push the last trailer sideways out of control.

A loss of traction by the trailer wheels, often caused by overbraking or by a sudden steering correction. The correct response is to stop braking and steer in the direction the trailer should go.

A condition in which the tractor and trailer fold toward each other at an angle too sharp to recover. It usually happens when the drive wheels lock and the trailer keeps pushing forward.

The powered rear axles of the tractor that push the combination forward. Losing traction there while braking can let the trailer push the tractor sideways into a jackknife.

A heavy hook-and-latch coupling used to tow a full trailer or other unit by its lunette eye. The latch must be closed and secured before the vehicle is moved.

The ring on the end of a trailer drawbar that fits over a pintle hook. It must be inspected for cracks, wear, and a properly closed latch.

The rigid arm connecting a towed unit to the towing vehicle. Its length and condition affect how closely the trailer follows and how much it sways.

Chains that connect a towed unit to the towing vehicle in case the main hitch fails. They should be crossed, with enough slack to allow turns but not so loose they drag.

The flexible hoses that carry compressed air between tractor and trailer. They must be supported, not rubbing or kinked, and coupled with the correct glad hand connections.

The cable that carries power and signals between the tractor and trailer for lights and antilock brakes. It should be plugged in firmly and routed so it does not rub or drag.

An electronic system that senses wheel lockup and releases and reapplies brake pressure to prevent skids. On combinations it helps keep the units in line during hard braking.

The setting that keeps brake shoes close enough to the drums for full stopping power. Pushrod travel beyond the limit means the brakes are out of adjustment and must be corrected.

The distance a brake chamber pushrod moves when the brakes are applied. Too much travel means the brake is out of adjustment and the vehicle may not stop as expected.

The braking system on the towed unit, supplied by air from the tractor. They must be checked separately, since a trailer that does not brake can push the tractor into a skid.

A tip-over of the tractor or trailer, often caused by high speed in a curve or a quick steering move. Combinations have a higher center of gravity and are more prone to it.

The point where the vehicle's weight is balanced. A high or shifting load raises it, making the combination more likely to tip in turns or sudden maneuvers.

The tiedowns, straps, and blocking that keep a load from shifting. A shifting load changes the trailer's balance and can make the combination unstable or hard to steer.

The total distance traveled from the moment a hazard is seen until the vehicle stops. A loaded combination needs far more room than a passenger car at the same speed.

The gap kept behind the vehicle ahead, measured in seconds. A heavy combination should leave at least one second for each ten feet of vehicle length, plus more in bad weather.