Arkansas CDL Combination Vehicles Test key terms

A power unit towing one or more trailers or semi-trailers, such as a tractor pulling a trailer. It bends at the coupling point, so it handles and brakes differently from a single-unit truck.

Any place where a combination vehicle pivots, such as the fifth wheel or a dolly connection. The more articulation points, the more the units can fold or swing out of line during turns or braking.

The tendency of a trailer's rear wheels to follow a path inside the path of the tractor's front wheels. It is worst in tight turns, so you must swing wide and check mirrors before completing a turn.

The total width of pavement a turning combination vehicle occupies, including the tractor's front swing and the trailer's rear cut-in. Knowing it helps you avoid curbs, poles, and adjacent lanes.

A hazard created when a driver swings left before a right turn and a car moves into the open space on the right. The trailer can then crush the car as the turn tightens.

The increase in sideways movement of each successive trailer unit during a maneuver, so the last trailer swings the widest. It makes lane changes and curves riskier for multi-trailer rigs.

The horseshoe-shaped coupling plate mounted on the tractor that locks onto the trailer's kingpin. It carries the trailer's weight and lets the units pivot as one while turning.

The stout vertical pin under the trailer's front that the fifth wheel jaws grip. It must be fully seated in the jaws or the trailer can separate from the tractor.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A release handle and safety latch keep them shut until the driver deliberately pulls the handle.

After coupling, you pull forward gently with the trailer brakes set to confirm the kingpin is locked and will not slip out. If the tractor moves freely, the trailer is not properly hitched.

The retractable legs under a trailer's front that support it when uncoupled. They must be fully raised before driving and firmly on the ground before the tractor pulls out.

The sequence of lining up the tractor with the trailer, backing under it, locking the fifth wheel, connecting air and electrical lines, raising the landing gear, and testing the connection.

The reverse of coupling: park on level ground, set the trailer brakes, lower the landing gear, disconnect air and electrical lines, pull the release handle, and pull the tractor clear.

The interlocking couplers that join the tractor's air lines to the trailer's. They are color-coded so the emergency and service lines are not crossed, and their seals must be in good shape.

The air line that carries the driver's brake application signal to the trailer, usually the blue-coded line. It releases air to let the trailer brakes apply in normal driving.

The air line, usually red-coded, that supplies air to the trailer's reservoir and signals a breakaway. If it loses pressure, the trailer brakes apply automatically.

A system that applies the trailer brakes automatically if the trailer separates from the tractor and the air lines part. A charged reservoir and check valve make this work.

A valve near the trailer's wheels that speeds up brake application and release by sending reservoir air directly to the chambers. It is why trailer brakes can respond quickly to a small signal.

The cylinder where air pressure pushes a piston and pushrod to apply a wheel's brakes. On combination vehicles, each wheel end has its own chamber fed from the trailer reservoir.

The linkage that connects a brake chamber pushrod to the brake shoe mechanism. Too much free play means delayed, weak braking, so it must be checked during inspection.

The amount of pushrod travel allowed before the brakes engage. Excess travel on any chamber means that brake is out of adjustment and may not contribute full stopping power.

A parking and emergency brake that uses a strong spring to apply the brakes when air pressure is lost. It releases only when enough air pressure builds to compress the spring.

The control, often a red push-pull knob, that applies the trailer's spring brakes for parking or emergencies. It must be set before uncoupling and released before driving.

An electronic system that senses wheel lockup and rapidly adjusts brake pressure to keep wheels rolling. It helps you steer and stay upright during hard braking on slippery surfaces.

A device at each wheel that reports how fast that wheel is turning to the ABS controller. A failed sensor can disable ABS on that wheel or the whole unit.

The yellow dashboard light that shows the antilock system's status. It should come on at startup and go out; if it stays on, ABS is not working and needs service.

A valve that closes the tractor's air supply to the trailer if a line breaks or separates, preserving tractor air for its own brakes. It also helps set the trailer brakes on separation.

A loss of control where the tractor and trailer fold toward each other at a sharp angle. It often happens when drive wheels lock or the trailer pushes the tractor during hard braking.

A skid in which the trailer's rear wheels lose traction and the trailer swings sideways out of line. It can lead to a jackknife if not corrected with smooth steering and throttle control.

Loss of tire grip so the vehicle slides instead of rolling in the intended direction. On combination vehicles, a skid can involve the tractor, the trailer, or both.

Turning the steering wheel opposite the direction the vehicle is sliding to bring the front wheels back in line with travel. It must be smooth and matched to the slide to avoid overcorrecting.

The total distance traveled from perceiving a hazard to a full stop, including perception, reaction, and braking distance. A loaded combination vehicle needs far more room than a car.

The gap you keep behind the vehicle ahead, measured in seconds. Combination vehicles need at least one second per ten feet of vehicle length plus extra time in bad weather or heavy loads.

Loss of braking power when brakes overheat from repeated or prolonged use, such as long downgrades. Downshifting and controlled braking help prevent it.

A retarding device that uses engine compression to slow the vehicle without using the wheel brakes. It helps control speed on downgrades and reduces the risk of brake fade.

The system of tiedowns, straps, and blocking used to keep cargo from shifting or falling. On combination vehicles, secure cargo must also be checked and retightened as needed during the trip.

How a load's weight is spread across axles and between tractor and trailer. Poor distribution can overload an axle, lighten the steer axle, or make the rig unstable.

Movement of cargo during driving, which can change axle loads, damage the trailer, or affect steering and braking. Proper securement and blocking prevent it.

A strap, chain, or other device used to anchor cargo to the vehicle. Tiedowns must be rated for the load and arranged so cargo cannot move forward, back, or sideways.

The maximum weight the law allows on a single axle or axle group. Overloading an axle hurts handling and braking and can result in fines or being placed out of service.