California CDL Combination Vehicles Test key terms

The horseshoe-shaped coupling plate mounted on the tractor frame. It locks around the trailer kingpin to hold the trailer securely and pivot as the combination turns.

The vertical steel pin under the trailer's front apron that drops into the fifth wheel. The locking jaws close around it to connect tractor and trailer.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A pull test and visual check confirm they have gripped it.

The flat steel plate under the front of the trailer that carries the kingpin. It must rest flat and firm on the fifth wheel with no gap.

The procedure of connecting a tractor to a trailer: inspecting, aligning, backing under, locking the fifth wheel, connecting lines, and testing the connection.

The procedure of separating tractor from trailer: parking on firm level ground, chocking wheels, lowering landing gear, disconnecting lines, releasing the fifth wheel, and pulling away slowly.

After coupling, pulling gently forward with the trailer brakes set to confirm the locking jaws hold the kingpin. The tractor should not move if the connection is secure.

The retractable support legs under the front of the trailer. They hold the trailer level when uncoupled and must be fully raised before driving away.

Space between the trailer apron and the fifth wheel after coupling. Any visible gap means the trailer is not seated and the connection must be corrected before driving.

A wedge placed against a wheel to prevent rolling. Chocks are required when uncoupling so the trailer cannot creep while the tractor is released.

The coupling fittings on the ends of the air and electrical lines between tractor and trailer. They must be sealed, locked, and free of leaks.

The air line that carries brake application pressure from the tractor to the trailer. Also called the control line; it is usually the line with the coupler that closes when disconnected.

The air line that supplies air to the trailer tanks and signals a breakaway. If it loses pressure, trailer brakes apply automatically as a fail-safe.

A system that applies the trailer brakes automatically if the trailer separates from the tractor. It works through the emergency line and trailer spring brakes.

Reservoirs on the trailer that store compressed air for its brakes. They must be drained of moisture and kept charged for brakes to release and apply properly.

A valve on the trailer that speeds up brake application and release by sending air directly from the trailer tanks to the brake chambers.

Brake chambers that use a heavy spring to apply the brakes when air pressure drops. They set automatically on low air and release only with sufficient pressure.

The cylinder where air pressure pushes a pushrod to apply the brakes. Combination vehicles have chambers on both tractor and trailer axles.

The lever that connects the brake chamber pushrod to the brake shoes. It should move no more than about one inch when the brakes are released.

The rod moved by air pressure in the brake chamber. Its travel is checked during inspection; too much travel means brakes are out of adjustment.

The setting that keeps brake shoes close to the drums. Poorly adjusted brakes on a combination vehicle cause long stopping distances and can fail an inspection.

A buzzer and light that activate when air pressure falls below about 60 psi. It warns the driver that spring brakes will soon apply.

A check that air pressure holds steady with the engine off and brakes released, then applied. A steady drop signals a leak needing repair.

The dashboard instrument showing air pressure in the brake system. Drivers must watch it and stop when pressure drops below a safe level.

A control that applies only the trailer brakes, used for testing the trailer brake system and for the pull test after coupling.

A systematic check of the combination vehicle before driving, covering coupling, air lines, brakes, tires, lights, and the fifth wheel connection.

The tendency of the rear wheels of a long combination to follow a shorter path than the front wheels. It makes turns wider and can strike curbs or pedestrians.

The total width and area the vehicle covers as it turns. Long combination vehicles need extra room because the trailer cuts inside the tractor's path.

The outward movement of the trailer's rear end during a turn. It can hit signs, poles, or other vehicles if the driver turns too sharply or too soon.

The most dangerous turn for a combination vehicle. Drivers should swing wide, check mirrors, and watch the trailer's rear wheels to avoid off-tracking over the curb.

Turning from a position farther from the corner than a car would use, so the trailer stays clear of the curb and oncoming traffic.

The tendency of the last trailer in a multi-trailer combination to swing wider than the tractor during a quick lane change. It increases rollover risk.

The total distance needed to stop, including perception, reaction, and braking distance. A loaded combination needs far more space than a passenger car.

The space kept behind the vehicle ahead. Combination vehicles need at least one second per ten feet of vehicle length plus extra time in poor conditions.

The safety margin kept around the vehicle on all sides. It gives room to react and is especially important for heavy, slow-stopping combination vehicles.

The rate of travel that must be reduced for curves, downgrades, rain, and heavy loads. Combination vehicles must slow before curves, not during them.

A valve on air tanks used to release moisture and oil. Tanks on both tractor and trailer should be drained daily to protect the brake system.

The handle used to raise and lower the landing gear. It must be stored securely after use so it does not strike the road or the tractor.

The brake that holds a parked vehicle. On combination vehicles it must be set, with wheels chocked if needed, before coupling or uncoupling.