Maryland CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor frame that locks around the trailer kingpin to connect tractor and trailer and allow pivoting in turns.

The vertical steel pin under the trailer nose that the fifth wheel jaws grip, forming the pivot point between tractor and trailer.

The procedure of backing the tractor under a trailer and locking the fifth wheel around the kingpin so the two units are joined securely.

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

The retractable legs under the trailer nose that support it when uncoupled; they must be fully raised before driving and firmly on the ground before uncoupling.

The procedure of parking a trailer securely, lowering landing gear, releasing the fifth wheel jaws, and pulling the tractor clear without damaging either unit.

A fifth wheel that can be moved forward or back on rails to shift weight between tractor axles and improve axle loading or turning clearance.

Trailer axles that can be repositioned along the trailer frame to balance weight across axles and adjust off-tracking behavior.

The two (or three with electrical) flexible hoses connecting tractor and trailer: service line for brakes and supply line for air, color-coded and sealed with couplers.

Interlocking coupler fittings that join the tractor and trailer air hoses; they must be locked together and checked for leaks after coupling.

The emergency air line that fills the trailer's air tanks and signals a breakaway; loss of pressure here triggers trailer emergency braking.

The control air line that carries the driver's brake application signal to the trailer's relay valve so trailer brakes apply with the tractor's.

A trailer valve that uses the small service-line signal to release stored air from the trailer tank into the brake chambers quickly.

The cylinder where air pressure pushes a piston or diaphragm to apply the brake shoes or pads at each wheel.

The linkage between brake chamber pushrod and brake shoe that converts pushrod stroke into shoe movement; too much free play means weak brakes.

How far the brake chamber pushrod extends when the brakes are applied; excessive stroke indicates misadjustment and must be corrected before driving.

Checking that air pressure holds steady with brakes applied and released; a steady drop signals a leak that must be fixed before the trip.

A buzzer and light that activate when air pressure drops to about 60 psi, warning that braking ability is being lost.

Emergency and parking brakes held off by air pressure; when air drops low, strong springs apply them automatically to stop the vehicle.

The dashboard instrument showing how much air pressure remains in the brake system; it must be monitored constantly while driving.

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

The total width of road a combination vehicle occupies through a turn, including the trailer's inward cut and the front end's outward swing.

The outward movement of the trailer's rear end when a combination vehicle turns sharply, which can strike objects beside the road.

The tendency of the last trailer in a multi-trailer combination to swing or sway more than the tractor during lane changes or steering corrections.

Side-to-side fishtailing of the trailer caused by speed, wind, steering input, or poor weight distribution; ease off the accelerator rather than braking hard.

A crash in which the vehicle tips onto its side, usually caused by taking a curve or evasive action too fast for the load's high center of gravity.

The point where the vehicle's weight is concentrated; a high or shifting center of gravity makes a combination vehicle much easier to roll over.

Movement of cargo during driving, which can change weight distribution, destabilize the trailer, and increase rollover or sway risk.

Tying down and arranging freight so it cannot slide, shift, or fall, keeping axle loads legal and the trailer stable in turns and braking.

The load carried by a single axle or axle group; it must stay within legal limits, and excess weight on one axle worsens handling and braking.

Loss of braking power when brakes overheat from repeated or prolonged use, common on long downgrades if speed is not controlled by gearing.

Using a lower gear so the engine's compression slows the vehicle, reducing reliance on wheel brakes and helping prevent fade on downgrades.

Choosing and holding a safe speed for conditions, curves, and downgrades so the combination vehicle never outruns its braking or steering ability.

Loss of tire grip so the wheels slide instead of roll, caused by braking too hard, accelerating too fast, or steering too sharply on a slippery surface.

A wheel stopping rotation while the vehicle still moves, which removes steering control and lengthens stopping distance on slick roads.

Antilock braking system that automatically releases and reapplies brake pressure to prevent wheel lockup, helping the driver keep steering control during hard stops.

A skid in which the tractor and trailer fold toward each other at the coupling, often after the drive or trailer wheels lock on a slippery road.

The driver's planned reaction to a sudden danger, such as an escape route, controlled braking, or steering choice made before the situation becomes an emergency.