Ohio CDL Doubles and Triples Endorsement Test key terms

A short frame with one or two axles and a fifth-wheel plate that attaches to the rear of a lead trailer and supports the front of the trailer behind it, letting one tractor pull multiple trailers.

A heavy curved hook on the rear of a trailer or dolly that catches the lunette eye of the trailer being towed behind it, forming the coupling for the rear trailer's drawbar.

The rigid ring on the end of a trailer drawbar that drops over and locks onto the pintle hook of the vehicle ahead, making the towing connection for the rear trailer.

The rigid arm extending from the front of a full trailer or dolly that carries the lunette eye and transmits pulling and braking forces to the towing vehicle's pintle hook.

The horseshoe-shaped coupling plate on a tractor or dolly that the trailer's kingpin locks into, allowing the trailer to pivot and transferring its weight to the towing unit.

The vertical steel pin under the front of a semitrailer that locks into the fifth-wheel jaws, securing the trailer to the tractor or dolly while allowing it to swivel.

The two curved metal pieces inside a fifth-wheel plate that close around the trailer kingpin; they must be fully closed with the release handle secured before the combination is moved.

A coupling check where the driver sets the parking brakes on the trailer, puts the tractor in a low gear, and gently pulls forward to confirm the locked jaws hold the kingpin.

The ordered steps for connecting a tractor to a trailer: inspect, back under squarely, raise and lock, connect air lines and electrical cord, secure the fifth-wheel release, test the connection, and check landing gear clearance.

The ordered steps for separating a tractor from a trailer: park on firm level ground, chock wheels, lower landing gear, disconnect lines and cords, release the fifth-wheel jaws, and pull straight out slowly.

The retractable support legs under a semitrailer's nose that carry its weight when it is not attached to a tractor; they must be fully raised and pinned before travel.

Wedges placed against trailer tires to stop rolling while a combination is being coupled, uncoupled, or parked, especially on any slope or soft surface.

Chains that cross under the drawbar and connect the towing vehicle to the towed trailer's frame, keeping the trailer attached if the pintle hook or lunette eye fails.

The line, usually red, that supplies air to a trailer's emergency relay valve; if it separates, the trailer's spring brakes apply automatically and stop the trailer.

The line, usually blue, that carries the driver's brake application signal to the trailer's relay valve so the trailer brakes apply in step with the tractor brakes.

A small valve on each dolly or trailer air line that can be closed to keep air from escaping while a trailer is disconnected, protecting the combination's air supply.

The self-sealing coupler at the end of an air line that joins tractor and trailer hoses; mismatched or dirty glad hands can leak air or cross the emergency and service lines.

A trailer-mounted valve that responds to the service line signal by releasing air from the trailer reservoir into the brake chambers, and applies the trailer brakes if supply air is lost.

Brake chambers with a powerful spring that applies the brakes whenever air pressure is released, so a trailer with a broken air line or low pressure stops on its own.

A test where the driver applies the brakes and watches the air gauge to confirm the combination, including all trailers and dollies, holds pressure without excessive leakage.

The systematic walk-around check of a multi-trailer combination before driving, covering coupling devices, air lines, tires, lights, brakes, and load security on every unit.

The tendency of the rear wheels of a long combination to cut inside the path of the front wheels during a turn, so the last trailer swings closer to the curb than the tractor.

The total width of road a combination covers as it moves through a turn, from the outside of the tractor's arc to the inside track of the last trailer's wheels.

An imaginary line extending straight ahead from the lane you are turning into; keeping the tractor behind it until the rear trailers clear prevents cutting the corner.

A turning technique for multi-trailer rigs where the driver swings out before the turn and keeps the tractor back from the corner so the trailing units stay out of the curb or opposing lane.

How closely the rear trailer's wheels follow the path of the tractor's wheels when traveling straight; poor tracking causes the last trailer to drift toward lane edges or shoulders.

The side-to-side wander of a trailing unit caused by road crown, wind, or poor alignment, which is amplified in doubles and triples and must be corrected with small smooth steering inputs.

Side-to-side rolling motion of the trailers that grows when speed is too high, the load is unbalanced, or the driver overcorrects; the cure is to slow down smoothly, not to jerk the wheel.

A sudden whipping motion of the rear trailer that can occur when a driver steers too sharply or brakes hard in a turn, potentially causing a jackknife or rollover.

A small, smooth, early steering input used to keep a multi-trailer combination centered; large or late corrections amplify trailer sway and can start a snap.

A loss of control where the tractor and trailer fold toward each other at an angle, often from hard braking on a slippery surface or from overcorrecting during a skid.

The response to a skid: stop braking, stop accelerating, and steer in the direction you want the front of the vehicle to go while keeping the wheels rolling.

The delay between the driver pressing the pedal and the trailer brakes applying, because air must travel through the lines; doubles and triples have longer lines and more lag.

The total distance a combination travels from the moment a hazard is seen until it stops, including perception, reaction, brake lag, and the braking distance itself.

Loss of braking power when brakes overheat from repeated or prolonged use, common on downgrades; drivers should slow down and use a lower gear instead of riding the brakes.

Using a lower gear so the engine's compression slows the vehicle, reducing reliance on the brakes and helping control speed on long downgrades with multiple trailers.

The gap kept behind the vehicle ahead, measured in seconds; doubles and triples need at least one second per ten feet of vehicle length plus reaction time.

Placing cargo so weight is spread correctly over each trailer's axles and the tractor's drive and steering axles, keeping every unit stable and within axle limits.

The maximum weight allowed on a single axle or axle group, such as 20,000 pounds on a tandem; overloading an axle on any trailer makes the combination unstable and illegal.

The total weight of the tractor, all trailers, cargo, fuel, and driver; it must stay within the registered and rated limits for the combination and its tires.

The tiedowns, straps, and blocking used to keep cargo from shifting on each trailer; a shifting load in a rear trailer can cause sway or a rollover.