NC CDL Combination Vehicles key terms

The sideways force, usually expressed as a fraction of gravity, at which a vehicle's center of gravity shifts far enough outward to lift the inside tires off the road. In a loaded tractor-trailer it is often only about 0.35 g.

The single point where the vehicle's total weight is considered to act. The higher it sits above the road, the easier the vehicle tips over in a turn, on a ramp, or during sudden steering.

The rate at which centrifugal force grows with the square of velocity, so doubling speed in a curve roughly quadruples the tipping force. Drivers must slow before entering a curve, not while inside it.

The outward push a vehicle feels when it turns. It grows with speed squared and with how sharply the path curves, and it is what tries to roll the combination over or slide it sideways.

How high the cargo sits above the road surface. Tall, heavy loads raise the center of gravity and sharply reduce the speed at which the combination will tip in a curve or on a ramp.

Loss of stopping power caused by overheated brake drums or shoes, common after long downgrades where brakes are used continuously. Gears and engine braking should carry most of the load instead.

Using the engine's compression and exhaust restriction to slow the vehicle while in a lower gear. It saves service brakes on long grades and helps keep brake temperatures in a safe range.

A downhill technique where the driver applies the brakes firmly to drop speed several mph, then releases them fully to let drums cool. It is repeated as needed rather than riding the brakes.

Choosing a gear low enough to hold a safe speed before starting down a hill, and keeping that speed constant. The rule of thumb is to descend in the same gear that would climb the grade.

The total distance traveled from the moment a hazard is seen until the vehicle stops, including perception, reaction, and braking. A loaded combination needs far more room than a passenger car, especially on wet pavement.

The gap kept behind the vehicle ahead. Combination vehicles need at least one second per ten feet of vehicle length at speeds under 40 mph, plus an extra second above that.

Side-to-side fishtailing of the trailer, often triggered by speed too high for conditions, improper loading, or abrupt steering. The safe response is to ease off the accelerator and slow gradually, not to brake hard.

The tendency of each trailing unit in a multi-trailer combination to swing wider than the unit ahead of it during a quick lane change or steering input. It makes doubles and triples far more prone to the last trailer rolling over.

A sudden rollover of the rear trailer caused by a fast steering correction, where the amplified motion of the last unit flips it before the driver can react. It is the classic hazard of quick lane changes in doubles.

The difference between the path of the front wheels and the path of the rear wheels in a turn. The rear of a long combination cuts the corner, so the driver must swing wide and watch the trailer's inside wheels.

The total width of pavement a turning combination occupies, from the outside of the front bumper to the inside of the rear trailer wheels. It is wider than the vehicle itself, so pedestrians and signs in the gap are at risk.

The point about which a trailer rotates relative to the towing unit, usually the kingpin or pintle hook. Because the pivot sits ahead of the trailer axles, the trailer's rear wheels track inside the tractor's path.

A heavy-duty towing hook with a spring-loaded latch used to pull trailers with a lunette eye rather than a kingpin. It allows more articulation than a fifth wheel and must be checked for a closed, locked latch before moving.

The ring on the front of a towed trailer's drawbar that drops over a pintle hook. It must be fully seated and the hook latch closed and secured, or the trailer can separate in motion.

Backup chains that cross under the drawbar and connect trailer to towing vehicle in case the primary hitch fails. They must be crossed to cradle the tongue and be short enough not to drag.

Combinations pulling two or three trailers, joined by converter dollies. They require special skill because of rearward amplification, longer stopping distances, and much greater off-tracking in turns.

A small axle-and-frame unit with its own fifth wheel and drawbar that couples a second or third trailer to the one ahead. Its air lines and electrical cord must be connected and its locking jaws checked.

The vertical steel pin under a trailer's front that locks into the tractor's fifth wheel. The fifth wheel jaws must close fully around it, and a visible gap or high hook means it is not seated.

The horseshoe-shaped plate on the tractor that carries the trailer's weight and locks onto the kingpin. Its jaws, mounting bolts, and pivot pins must be inspected for wear and secure attachment.

After coupling, the driver applies gentle forward pull with the trailer brakes set to confirm the kingpin is locked and will not slide out. The tractor should not move if the coupling is secure.

The walk-around check after hooking up: fifth wheel jaws closed, no gap, air lines and electrical cord connected, landing gear raised, mud flaps clear, and trailer brakes tested before driving away.

The retractable legs supporting an uncoupled trailer's front. They must be fully raised after coupling, with the crank handle secured, or they strike the road and can cause a serious crash.

The ordered steps for dropping a trailer: park on firm level ground, set trailer parking brakes, lower landing gear, disconnect air lines and cord, pull the fifth wheel release, then pull slowly out.

A spring-applied brake that locks the trailer wheels when air pressure is released, either by the dash valve or by disconnecting the supply line. It must be set before the trailer is uncoupled.

The two flexible hoses, service and emergency, that carry air between tractor and trailer. They must be connected color-to-color, free of cracks or chafing, and supported so they do not drag.

The interlocking couplers at the ends of the air lines. Their rubber seals must be intact and the couplers locked together so the lines cannot separate and trigger a trailer brake lockup.

The air line that supplies the trailer's reservoir and signals a breakaway. If it separates or loses pressure, the trailer brakes apply automatically, which is why a leak here can lock the trailer wheels.

The slight delay before trailer brakes apply after the tractor's, caused by air traveling through the lines. It lengthens the combination's stopping distance and can push the trailer forward in a hard stop.

A system that senses wheel lockup and rapidly releases and reapplies brake pressure to keep wheels rolling and steering. In a combination, the tractor and trailer each have their own ABS, and a trailer ABS lamp must be working.

When a braked wheel stops rotating and slides, losing steering control and increasing stopping distance. ABS prevents it, but drivers must still brake firmly and steer rather than pump the pedal.

The tiedowns, straps, chains, and blocking used to keep freight from shifting, sliding, or falling. Shifting cargo changes axle loads and can tip a combination over or tear open the trailer.

Placing cargo so the load is balanced across tractor and trailer axles and stays within axle and gross weight limits. Too much weight on the rear axle lightens the steering axle and makes the rig hard to control.

Spreading weight evenly side to side and front to back, keeping heavy items low and centered. Uneven loading causes poor handling, tire overload, and a greater chance of rollover or trailer sway.

The maximum load allowed on any single axle or axle group, set by law and by tire ratings. Exceeding it damages roads, invites fines, and makes braking and steering unpredictable.