CDL Combination Vehicles key terms

The tendency of a trailer's rear wheels to follow a path closer to the inside of a turn than the tractor's front wheels, so the vehicle sweeps a wider arc than the driver may expect.

The total width of road surface the entire combination occupies as it turns, bounded by the outside of the tractor's front corner and the inside of the trailer's rear tires.

An imaginary line drawn on the pavement from the tractor's drive tires to the trailer's rear tires; watching it helps a driver judge how far the trailer cuts inside a turn.

A hazard in which a vehicle or object beside the trailer is caught between the trailer's rear wheels and the curb because the trailer swings left before turning right.

Outward movement of the rear of a trailer in the direction opposite the turn, caused by the pivot point at the fifth wheel or kingpin.

The coupling location where the tractor and trailer articulate, either the fifth wheel on a semi-trailer or the drawbar and hitch on a full trailer.

The horseshoe-shaped plate on the tractor's frame that locks around the trailer kingpin and carries the front weight of a semi-trailer.

The hardened vertical pin under the front of a semi-trailer that the fifth wheel jaws grip to connect the trailer to the tractor.

The procedure of backing the tractor under a trailer, engaging the fifth wheel or hitch, and confirming a secure connection before moving.

A deliberate low-speed pull forward with the trailer brakes set to confirm the kingpin is locked and the trailer cannot separate from the tractor.

The retractable legs under the front of a semi-trailer that support it when uncoupled; they must be fully raised before the vehicle moves.

The sequence of parking on firm level ground, setting brakes, lowering landing gear, disconnecting air lines and electrical cord, then releasing the fifth wheel.

The interlocking couplers that join the tractor's air supply and service lines to the trailer's brake system.

The air line that carries the driver's brake application pressure to the trailer, usually coded blue on a combination vehicle.

The air line that charges the trailer's reservoirs and signals a trailer breakaway, usually coded red on a combination vehicle.

A trailer valve that releases stored air to apply the trailer brakes when supply-line pressure drops sharply, as in a breakaway or line separation.

The system that applies the trailer brakes automatically if the trailer separates from the tractor, powered by air stored in the trailer's own reservoir.

Brakes held applied by heavy mechanical springs and released by air pressure, so they set automatically when air pressure is lost.

A light and buzzer that activate before air pressure falls too far to stop the vehicle safely, alerting the driver to a developing brake system failure.

The pre-trip checks of air pressure buildup, static leak rate, applied leak rate, and warning and spring-brake activation points on a combination vehicle.

A valve that closes off the tractor's air supply when trailer lines separate, keeping enough pressure for the tractor to stop on its own.

An intersection signal that has been green since it first came into view, meaning it may change before the vehicle clears the crossing.

The margin of clear road kept around a combination vehicle, larger than for a car because of greater stopping distance and blind spots.

The gap behind the vehicle ahead, measured in seconds; a loaded combination needs at least one second for each ten feet of vehicle length.

The total distance traveled from perception of a hazard to a full stop, combining reaction distance and braking distance for the loaded weight.

The tendency of the rear trailer in a multi-trailer combination to swing sideways farther than the tractor during a quick lane change or steering input.

A small axle-and-fifth-wheel unit that couples a second trailer to the rear of the first in doubles or triples configurations.

Combinations pulling two or three trailers, which require special care because they amplify sway, require more stopping distance, and demand gentle steering.

Sideways sliding of trailer tires caused by excessive speed, abrupt steering, or poor traction, often the first stage before a jackknife.

A loss of control in which the tractor and trailer fold against each other at an angle too sharp to steer out of, usually from locked drive wheels or trailer brakes.

The technique of releasing hard braking, steering toward the intended path, and reapplying brakes gently once the tires regain rolling traction.

A skid in which only the front wheels of the tractor lose traction, so steering control is lost while the vehicle continues straight.

A rollover caused by a rapid steering maneuver at speed, where rearward amplification tips the rearmost trailer onto its side.

The lateral acceleration at which a vehicle tips instead of sliding; it drops as the center of gravity rises and as speed in a turn increases.

The point where the vehicle's weight is concentrated; raising it with high or off-center cargo makes a rollover far more likely.

The use of tiedowns, blocking, and bracing to keep freight from shifting, which protects axle loading, steering, and rollover stability.

Placing cargo so that weight is shared properly among tractor and trailer axles and stays within axle and gross weight limits.

A trailer axle assembly that can be moved forward or rearward to shift weight between the tractor and trailer axles.

A pre-trip check that confirms the trailer brakes apply and hold when the trailer hand valve or supply-line test is used.

A control that applies only the trailer brakes, useful for testing trailer brake operation and for steering a skidding trailer in some situations.