PA CDL Doubles/Triples key terms

A short frame with one or two axles, a fifth wheel on top, and a drawbar, used to connect a second or third trailer to the rear of the trailer ahead of it.

The rigid arm extending forward from a converter dolly or trailer that ends in an eye or ring, through which it attaches to the towing vehicle's pintle hook.

A heavy hook-and-latch coupling mounted on the rear of a trailer or truck that captures the drawbar eye of the vehicle behind it and holds it during pulling and braking.

Chains that connect the towing unit to the towed unit as a backup, so the combination stays attached if the primary coupling fails; they must be crossed and have enough slack to allow turns.

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

The vertical steel pin under the front of a trailer that drops into the fifth wheel and is gripped by its locking jaws, forming the main towing connection.

The two curved metal pieces inside a fifth wheel that close around the kingpin; a pull test confirms they are fully closed before the combination is moved.

After coupling, the driver tugs forward gently with the trailer brakes set to confirm the kingpin is locked and will not slip out of the fifth wheel.

The flat plate under the front of a trailer around the kingpin; it must rest flat on the fifth wheel, not on top of the tractor frame or hoses.

The retractable legs under a trailer front that support it when uncoupled; they must be fully raised before moving and firmly on the ground when parked.

The sequence of steps that joins a tractor to a trailer or dolly to a trailer: alignment, backing under, locking, connecting air lines and lines, raising gear, and testing.

The reverse of coupling: parking on firm level ground, setting brakes, lowering landing gear, disconnecting lines and electrical cord, releasing the fifth wheel, and pulling out slowly.

The lever on the fifth wheel that, when pulled, opens the locking jaws so the kingpin can be released; it should be secured after coupling.

The interlocking couplers on the ends of trailer air lines that join the service and emergency lines between units; their seals must be clean and in good condition.

The flexible hoses carrying compressed air between towing and towed units; they must be supported, not dragging, and routed so they do not catch on the dolly or trailer parts.

The air line that carries the driver's brake application pressure to the trailer or dolly; a break or leak in it causes a gradual loss of braking on the towed unit.

The air line that supplies air to charge the trailer reservoirs and signals a breakaway; if it loses pressure, the trailer brakes apply automatically.

The tendency of the rear wheels of a long combination to follow a path inside the path of the front wheels, so the vehicle sweeps a wider area than the driver expects.

How well the wheels of each trailer follow the path of the unit ahead; poor tracking means the rear trailer drifts sideways, especially when backing or at speed.

The total width of road a turning combination covers, including the outside front corner and the inside rear wheels; it grows with vehicle length and sharpness of turn.

Side-to-side fishtailing of a trailer or the rear trailer, often caused by speed, crosswinds, poor loading, or a sudden steering correction; ease off the accelerator rather than braking hard.

A sudden, violent rollover that can occur when a combination with a high center of gravity is steered sharply while traveling too fast for conditions.

The way motion at the front of a long combination is amplified toward the rear trailers, so a small steering input can swing the last trailer far sideways.

The ratio of the sideways motion of the last trailer to that of the tractor in a sudden maneuver; it increases with speed and with the number of trailers.

The delay between the driver pressing the pedal and the brakes at the rear of a long combination taking hold, because air must travel farther through the lines.

The total distance traveled from the moment a hazard is seen until the vehicle stops, made up of perception, reaction, and braking distance; it grows with weight and speed.

The gap a driver keeps behind the vehicle ahead; for doubles and triples it should be at least one second for every ten feet of vehicle length at speeds under 40 mph.

The safety margin kept around the vehicle on all sides, enlarged for doubles and triples because they need more room to stop, turn, and change lanes.

Air-operated parking and emergency brakes held off by air pressure; they apply automatically when air pressure drops or the emergency line separates.

The system that applies the trailer's spring brakes if it separates from the towing unit, working through the emergency line and the trailer's air reservoir.

A valve that closes the tractor's air supply to the trailer lines when pressure drops or a line breaks, so the tractor keeps enough air to stop itself.

The systematic walk-around check of coupling, air lines, lights, tires, brakes, and load securement before driving; for doubles and triples it covers every unit and connection.

The ties, straps, and blocking that keep cargo from shifting; on doubles and triples a shifting load in any trailer can upset the whole combination.

Movement of freight within a trailer during driving, which can change axle loading, cause trailer sway, or make the combination unstable, especially in the rear units.

Spreading load among the axles of the tractor and each trailer so no axle is overloaded and each axle carries its share, keeping steering and braking effective.

The point where the vehicle's weight is concentrated; a high or off-center center of gravity makes a combination more likely to roll over in turns.

The maximum load allowed on a single axle, a tandem axle group, or the gross vehicle weight, set by federal and Pennsylvania rules and checked at weigh stations.

The federal formula that limits the weight carried on any group of axles based on the number of axles and the distance between them, to protect bridges.

The maximum total weight a vehicle or combination is designed to carry, including the tractor, trailers, fuel, and cargo, as set by the manufacturer.