FL CDL Combination key terms

A power unit (tractor or truck) coupled to one or more trailing units, such as a semi-trailer or a trailer plus a converter dolly. It bends at the coupling points, so it handles and brakes differently from a single-unit truck.

A short-wheelbase power unit with a fifth wheel mounted over the rear axles, built to pull a semi-trailer. It has no cargo body of its own; the trailer carries the load and part of its weight rests on the tractor.

A trailer whose front end rests on the towing vehicle through a kingpin and fifth wheel rather than on its own front axle. Landing gear supports it only when it is uncoupled.

A trailer that carries its own weight at both front and rear on its own axles and attaches to the towing vehicle with a drawbar and hitch. It does not impose vertical load on the towing unit's fifth wheel.

A small axle assembly with a fifth wheel and a drawbar that lets a tractor pull two semi-trailers. It turns the second trailer into a full trailer, and it must be coupled and checked as carefully as any other connection.

A combination of a tractor pulling two semi-trailers, usually with a converter dolly between them. Doubles have two articulation points, so they track poorly and are prone to rearward amplification and off-tracking.

A combination of a tractor and three trailers, joined by two converter dollies. Triples sway the most and are restricted or prohibited in many jurisdictions; the rear trailer swings far outside the path of the tractor.

The horseshoe-shaped coupling plate on the tractor that grips the trailer kingpin. It pivots to allow turning and must be locked, greased, and checked for slack before every trip.

The vertical steel pin under the front of a semi-trailer that locks into the tractor's fifth wheel. It transmits pulling force and holds the trailer's front end in place.

The two curved metal jaws inside the fifth wheel that close around the kingpin. The release handle and safety latch must be in the locked position, and a pull test confirms the jaws have actually engaged.

After coupling, the driver engages a low gear and gently pulls against the trailer with the trailer brakes set. If the trailer does not move and the tractor does not jump, the kingpin is locked in the fifth wheel.

The full sequence of backing under a trailer, locking the kingpin, connecting air lines and electrical cord, raising landing gear, and testing the connection. Skipping a step is a leading cause of dropped trailers.

The reverse of coupling: park on firm level ground, set the trailer brakes, lower the landing gear, disconnect air and electrical lines, and pull out slowly. The trailer must be chocked or braked before the kingpin is released.

The retractable legs under the front of a semi-trailer that support it when uncoupled. Crank them down until they touch the ground and the tractor is free of the weight before pulling out.

The quick-coupling fittings that join the tractor's air lines to the trailer's, usually color-coded for service and emergency lines. They must seat fully with no leaks and the seals must be intact.

The air line that carries the driver's brake application pressure to the trailer brakes. It is typically the blue line on a standard color-coded coupling.

The air line that supplies air to the trailer's reservoirs and signals a breakaway. It is typically the red line; if it loses pressure, the trailer brakes apply automatically.

A valve that closes the tractor's air supply to the trailer if a line breaks or the trailer separates. It keeps enough air in the tractor to stop it and causes the trailer brakes to set.

The system that applies the trailer brakes automatically if the trailer separates from the tractor and the emergency line is pulled apart. It is why a trailer left uncoupled must have its spring brakes set.

After coupling, the driver charges the trailer system, shuts off the engine or releases the parking brake, and listens for leaks. Watching the trailer brake chambers or gauges confirms both lines are connected and holding.

The coiled cable that carries lighting and brake signals from tractor to trailer. It must be plugged in, routed so it will not rub or catch, and checked for function at both trailers on a double or triple.

Brake chambers that use a heavy spring to apply the brakes when air pressure drops or is released. They are the reason an uncoupled trailer stays braked and why releasing them requires full air pressure.

The valve on the trailer that sets its spring brakes independently of the tractor. It must be set before the air lines are disconnected and before the kingpin is released.

Chains that connect a towed unit to the towing vehicle as a backup if the primary hitch fails. They must be crossed, tight enough not to drag, and rated for the load.

The rigid bar that connects a full trailer or converter dolly to the towing vehicle. Its length and play affect how the towed unit tracks and how much it sways.

A hook-and-latch coupling used with a lunette eye on a drawbar. The latch must be closed and secured, and the hook must be rated for the load it pulls.

The tendency of the rear wheels of a long combination to cut inside the path of the front wheels in a turn. The longer the combination, the wider the swept path, so turns must be set up wide.

The tendency of the last trailer in a multi-trailer combination to swing wider and faster than the tractor during a quick lane change or evasive move. It makes sudden steering inputs dangerous in doubles and triples.

The total width of road a combination covers as it turns, including the overhang of the tractor's front corner and the rear trailer's tail swing. It is wider than the vehicle's static width and must be kept clear.

The outward movement of the rear of a trailer as the combination turns. It can strike pedestrians, signs, or parked vehicles on the inside of the turn, so the driver must watch the mirrors through the whole turn.

Any point where the combination articulates, such as the fifth wheel or a dolly hitch. Each pivot adds a place where the units can fold, so the driver must keep the combination stretched out when backing.

A loss of control in which the tractor and trailer fold against each other at the fifth wheel. It is commonly caused by hard braking on a slippery surface or by oversteering during a skid.

Loss of traction at the trailer wheels, often triggered by an empty trailer and aggressive braking. The trailer swings out and pushes the tractor, so the driver should release the brakes to let the wheels roll and regain grip.

An electronic system that rapidly releases and reapplies brake pressure to prevent wheel lockup. On a combination, the tractor and trailer each have their own ABS, and a trailer ABS warning lamp must be checked.

A trailer valve that speeds up brake application and release by sending reservoir air directly to the brake chambers. A faulty relay valve can leave trailer brakes slow or uneven.

Driving a tractor without a trailer. The tractor stops shorter and handles differently when bobtailing, and some jurisdictions require the tractor's own brakes to be adequate without trailer assistance.

Using both flat and convex mirrors to watch the trailer, the blind spots, and the tail swing during turns, lane changes, and backing. Mirrors are the driver's only view of what the trailing units are doing.

The systematic walk-around of tractor, trailer, and all couplings before driving. On a combination it covers fifth wheel, kingpin, air lines, electrical cord, landing gear, tires, brakes, and lights on every unit.

The total distance needed to bring a loaded combination to a stop, including perception, reaction, and braking distance. A loaded combination needs far more room than a car, and weight, speed, and road surface all increase it.