NY CDL Doubles and Triples key terms

The horseshoe-shaped coupling plate mounted on the tractor that locks around the trailer's kingpin to hold the trailer and carry its weight while towing.

The vertical steel pin under the front of a semitrailer that the fifth wheel jaws grip; the pivot point between tractor and trailer.

The full procedure of backing a tractor under a trailer and locking the fifth wheel to the kingpin so the two units become one safe vehicle.

The reverse sequence of coupling: parking on firm level ground, chocking wheels, releasing lines and fifth wheel jaws, then pulling the tractor clear of the trailer.

After coupling, pulling gently forward with the trailer brakes set to confirm the fifth wheel jaws are locked and will not let the trailer separate.

The retractable support legs under a trailer's front that hold it upright when it is not attached to a tractor; must be raised before towing.

The spring-loaded lever on the fifth wheel that keeps the locking jaws closed; it must be secured after coupling to prevent accidental release.

A fifth wheel that can be moved forward or back on the tractor frame to shift weight between axles and improve handling with heavy or long combinations.

A small axle unit with its own fifth wheel and drawbar that couples a second or third trailer to the rear of the first, converting single trailers into doubles or triples.

The heavy hook-and-latch coupling on the rear of a truck or trailer that grabs the eye of a dolly drawbar to connect the next unit.

The rigid bar extending from a converter dolly or full trailer that ends in an eye and attaches to the pintle hook of the unit ahead.

Backup chains that connect the dolly or trailer to the towing unit so the drawbar stays attached if the pintle hook fails.

A combination of a tractor pulling two trailers, usually a semitrailer followed by a converter dolly and a second semitrailer.

A combination of a tractor pulling three trailers, which demands more planning for turns, braking distance, and lane changes than doubles.

Interlocking couplers that join the tractor's air supply and service lines to the trailer's brake lines; they must be sealed and locked to avoid leaks.

The air line that carries the driver's brake application signal to the trailer brakes; usually the line with the blue glad hand.

The tractor air line that charges the trailer's reservoirs with compressed air; usually the line with the red glad hand.

On trailers with a third line, the line that automatically applies the trailer brakes if the combination separates or loses air pressure.

The coiled cable carrying power and signals for trailer lights and antilock brakes; it must be plugged in, seated fully, and supported so it does not drag.

Charging the system, shutting off the engine, and watching the gauges to confirm pressure loss stays within limits before driving the combination.

A valve that closes the tractor's air supply if a trailer line breaks or separates, keeping enough pressure to stop the tractor and apply the trailer brakes.

Spring-applied, air-released brakes on the trailer that set automatically when air pressure drops, acting as a parking brake and emergency brake.

The short delay between pressing the pedal and the brakes taking hold; with multiple trailers the signal travels farther, so stopping distance grows.

The total distance traveled from seeing a hazard to coming to a full stop, including perception, reaction, brake lag, and braking with a heavier combination.

The gap kept behind the vehicle ahead; with doubles or triples it should be at least one second per ten feet of total vehicle length.

The tendency of rear trailer wheels to cut a shorter path than the tractor's front wheels, so the rear units swing inside the curve or over the curb.

The visual path the trailer wheels follow through a turn; watching it in the mirrors helps the driver keep rear units off curbs and out of adjacent lanes.

The total width of roadway a turning combination needs, from the outside of the front tractor to the inside of the last trailer's wheels.

The outward arc of the tractor's rear and the front of the first trailer during a turn, which can strike nearby vehicles, poles, or pedestrians.

Side-to-side fishtailing of the rear trailer units, often started by sudden steering, high speed, or poor weight distribution, and hard to correct once it grows.

The way a steering movement at the tractor grows larger at each following trailer, so the last trailer in triples swings the most.

A rollover caused when a rapid steering correction whips the rear trailer sideways hard enough to tip it over, common in lightly loaded doubles and triples.

The maximum sideways force a loaded combination can take before tipping; it drops when the center of gravity is high or the load is light.

The point where the vehicle's weight is concentrated; raising it by loading high or unevenly makes sway and rollover more likely.

How cargo weight is spread across axles and trailers; keeping each unit properly loaded prevents light trailers from swaying and overloading axles.

The removable handle used to raise and lower landing gear; it should be stored securely so it cannot strike the tractor or trailer during coupling.

The flexible hoses, color-coded for service and supply, that carry brake air between tractor and trailers and must be supported so they do not chafe or drag.

A hand valve on the dolly or trailer that lets the driver close off air to a unit so it can be tested or isolated when troubleshooting the combination.

A full circuit of the combination checking coupling, lines, lights, tires, and brakes before every trip; with multiple units it takes longer and must not be skipped.