Iowa CDL Combination Vehicles Test key terms

Two metal couplers at the ends of the tractor and trailer air hoses that lock together and seal with a rubber gasket, joining the supply and service air lines between the units.

The heavy horseshoe-shaped plate mounted on the tractor's rear frame that receives the trailer kingpin, locks it in place, and lets the trailer pivot while being towed.

The thick vertical steel pin under the front of the trailer that drops into the fifth wheel and is gripped by the locking jaws, forming the pivot point of the combination.

Spring-loaded metal jaws inside the fifth wheel that close around the trailer kingpin and hold it until the release handle is pulled to uncouple.

A small spring latch on the fifth wheel release handle that must be closed so the handle cannot vibrate open and let the kingpin escape while driving.

The flat plate or collar around the trailer kingpin that rests on the fifth wheel and carries the trailer's weight on the coupling.

The crank-operated legs under the front of a trailer that support it when it is uncoupled; lower them before uncoupling and raise them fully before towing.

After coupling, with the trailer brakes applied, pull slowly forward; if the tractor moves without the trailer separating, the kingpin is locked and the coupling is secure.

Colored hoses that carry compressed air between tractor and trailer: the red supply (emergency) line and the blue service line, connected with glad hands.

The electrical plug and socket that join tractor and trailer wiring, carrying clearance lights, turn signals, brake lights, and ABS power between the units.

A small frame with one or two axles, a fifth wheel, and a drawbar that attaches behind a trailer so a second trailer can be towed in a double or triple combination.

A heavy hook with a spring latch mounted on the rear of a truck or dolly that grabs a drawbar ring to tow trailers and heavy equipment.

A valve that closes the tractor's air lines and applies the trailer spring brakes when trailer air pressure drops or the lines separate, protecting the tractor's air supply.

A lever on the dashboard or steering column that applies only the trailer brakes; it is used to test trailer brakes, not as a parking brake.

A valve mounted near the trailer axles that receives the control signal and quickly delivers or releases large amounts of air to the brake chambers.

Brake chambers containing a powerful spring that applies the brakes when air pressure is lost, so the vehicle stops by itself and stays parked.

Levers connecting the brake chamber pushrod to the brake camshaft; when they are out of adjustment the brakes apply weakly and stopping distance increases.

A device that switches the air compressor on at a set low pressure and off at a set high pressure, keeping the system within its safe operating range.

Quarter-turn valves at the tractor's air line connections that shut off air to the trailer lines so the tractor can be driven without a trailer.

A light and buzzer that come on before air pressure drops below the safe level, warning the driver to stop and find the leak.

An electronic system that senses wheel lockup and rapidly adjusts brake pressure so the wheels keep turning, helping the driver steer while braking hard.

A unit on the compressor discharge line that removes water and oil vapor from the air before it reaches the tanks, reducing corrosion and winter freeze-ups.

The dashboard push-pull valve that controls air to the trailer supply line; when it pops out or is pulled, the trailer spring brakes apply.

An emergency method of pressing the brake pedal fully, holding until wheels lock, then releasing and repeating; used on vehicles without ABS when stopping distance matters more than control.

Pressing the brakes as hard as possible without locking the wheels, holding steady pressure, and easing off only enough to steer if control is lost.

Loss of braking power when overheated drums and linings cannot grip, common after repeated or long hard braking on downgrades; gearing down helps prevent it.

Driving a tractor without a trailer; the combination is shorter, lighter, and brakes differently, and ABS may not cover the rear tractor axle.

Before crossing tracks, stop, look, and listen, and make sure the entire combination, including the last trailer, can clear the tracks without stopping on them.

The tendency of a combination vehicle's rear wheels, especially the trailer's, to follow a path inside the front wheels' path, so the trailer cuts corners.

Loss of control in which the trailer swings sideways out of line with the tractor, or the tractor's drive wheels skid and the tractor swings around, folding the combination.

The coupling location where the trailer turns relative to the tractor; the longer the distance from it to the trailer axles, the more the trailer cuts corners.

The tendency of the last trailer in a multi-trailer combination to swing wider and faster than the tractor during abrupt steering, raising its rollover risk.

The side-to-side whipping motion that grows toward the rear of a multi-trailer combination when the driver steers sharply, which can tip over the last trailer.

The sideways acceleration, usually given in g's, at which a vehicle will tip over; a high center of gravity lowers this value and makes rollover more likely.

The point where the vehicle's weight is concentrated; the higher it is, the more the vehicle leans in curves and the easier it rolls over.

A pre-trip check in which the driver circles the vehicle, checking lights, tires, coupling devices, air lines, and brake adjustment before driving.

A check made with the engine off and brakes released, measuring how much air pressure the system loses per minute to locate leaks.