MN CDL Combination key terms

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

The vertical steel pin under the trailer's front that the fifth wheel jaws grip to hold the trailer attached to the tractor.

The retractable legs under the front of a trailer that support it when it is not attached to a tractor.

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

A pull check performed after coupling at low speed with the trailer brakes set, confirming the fifth wheel jaws have locked on the kingpin.

A self-sealing coupler on the end of a trailer air line that connects to the tractor's matching fitting to supply air for brakes and accessories.

The air line, usually color-coded, that carries air from the tractor to charge the trailer's reservoirs and release its spring brakes.

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

A valve that closes automatically when trailer air pressure is lost or the trailer breaks away, keeping enough tractor air pressure to stop the power unit.

A safety system that applies the trailer's spring brakes automatically if the trailer separates from the tractor and the air lines part.

An adjustable fifth wheel mounting on the tractor frame that can be moved forward or rearward to shift weight between the steering and drive axles.

The systematic walk-around check of a combination vehicle before driving, covering coupling security, air systems, lights, tires, and load.

A check that the compressor raises reservoir pressure to a normal operating range within a reasonable time, showing the air system is charging properly.

A check with the engine off and brakes released and applied that reservoir pressure does not drop faster than the allowed rate, revealing air leaks.

The engine-driven pump that supplies compressed air to the reservoirs used for braking and other air-operated systems on the combination vehicle.

A device that removes moisture and contaminants from compressed air before it enters the reservoirs, protecting valves and lines from corrosion and freezing.

A storage tank for compressed air that gives the brake system enough volume to make several applications even if the compressor stops.

A parking and emergency brake that uses a heavy spring to apply the brakes when air pressure is released, as in a breakaway or air loss.

A buzzer and light that activate before air pressure falls too low to stop safely, warning the driver to stop and find the problem.

A trailer valve that uses a small control signal from the service line to send reservoir air to the trailer brake chambers quickly.

The air cylinder at each wheel that converts air pressure into the mechanical force that applies the foundation brakes.

The linkage between the brake chamber pushrod and the brake shoe that converts and adjusts the applied force; excessive pushrod travel means brakes need adjustment.

Loss of braking power caused by overheating of drums and shoes from repeated or prolonged use, common on long downgrades if brakes are not used correctly.

A downhill technique of applying the brakes firmly to reduce speed about 5 mph, then releasing them to cool, repeated as needed.

A retarding device that uses engine compression to slow the vehicle on downgrades, reducing reliance on the service brakes and helping prevent fade.

The tendency of a combination vehicle's rear wheels to follow a path inside the front wheels' path, greater on longer wheelbases and in tight turns.

The total width of road a combination vehicle occupies while turning, including the trailer's inside cut and any rear overhang swing.

The hazard of a right turn where the trailer cuts the corner and can strike or trap a vehicle or pedestrian on the inside of the turn.

The distance between the steering axle and the drive axles of the tractor; a longer wheelbase makes the vehicle track straighter and turn wider.

The size of the circle a combination vehicle needs to complete a turn, which grows with wheelbase and affects where the driver must position the tractor.

The fifth wheel location about which the trailer rotates; the driver must swing the tractor wide so the trailer clears curbs and obstacles.

A loading dock reached by backing from a narrow street or alley, requiring a sight-side or blind-side approach with careful setup and GOAL checks.

Backing a combination vehicle toward the driver's right, where the trailer blocks the view and the driver must rely on mirrors and a spotter.

Get Out And Look: the practice of stopping and walking around the vehicle when visibility or clearance during backing is uncertain.

A person outside the vehicle who guides the driver during backing, using agreed hand signals and staying visible in the mirrors.

Side-to-side movement of the trailer, often caused by improper load distribution, speed, or steering corrections, that can lead to a rollover.

A loss of control in which the trailer swings sharply out of line with the tractor, usually from braking on a slippery surface or oversteering.

The corrective actions after a skid: stop braking, steer in the direction the vehicle needs to go, and avoid sudden inputs until traction returns.

The grip between tires and road surface, reduced by rain, ice, snow, oil, or worn tread, which lengthens stopping distance and raises skid risk.

The space kept behind the vehicle ahead, measured in seconds at highway speed and increased for heavy loads, poor weather, or low visibility.

The distance a combination vehicle travels between the moment a hazard appears and the moment the driver applies the brakes.

The sum of perception distance, reaction distance, and braking distance; it grows dramatically with speed and weight for a combination vehicle.

Placing cargo so weight is balanced and within axle limits, keeping the center of gravity low and avoiding excessive weight on the rear of the trailer.

The share of total weight carried by each axle and tire, adjusted by cargo placement and slider position to stay within legal limits.