Pennsylvania CDL Combination key terms

A power unit towing one or more other units, such as a tractor pulling a semitrailer. It bends at the coupling points, so it turns and stops differently from a single truck.

The powered front unit of a tractor-trailer combination. It carries the engine, steering axle, and drive axles and supports the front of the semitrailer on its fifth wheel.

A trailer whose front rests on the towing vehicle instead of its own axle. It has no front wheels and attaches through a kingpin and fifth wheel.

A trailer that carries its load on its own axles at both front and rear and connects to the towing vehicle through a drawbar or hitch rather than a fifth wheel.

The horseshoe-shaped coupling plate mounted on the tractor that locks around the trailer kingpin and carries the trailer's front weight. It must be greased and latched securely.

The vertical steel pin under the front of a semitrailer that locks into the fifth wheel jaws. Its condition and the mounting bolts should be checked before coupling.

The two curved metal pieces inside the fifth wheel that close around the kingpin. A pull test confirms they are locked before the tractor moves.

The flat steel plate around the kingpin on the underside of a trailer. It rides on the fifth wheel, so a bent or cracked apron means the trailer should not be used.

Another name for the fifth wheel latches that grip the kingpin. They must close fully around the pin, and the safety latch must be engaged.

A small lever or pin on the fifth wheel that keeps the locking jaws from opening. It must be closed and secured after coupling.

After coupling, the driver applies low power in a low gear with the trailer brakes set and feels whether the tractor tugs against a locked kingpin. It confirms the coupling is secure.

The retractable legs under the front of a semitrailer that support it when uncoupled. They must be fully raised before driving and checked for cracks and secure mounting.

The removable handle used to raise and lower landing gear. It should be stored securely so it cannot strike the tractor or be lost.

A fifth wheel mounted on rails so its position can be moved forward or back. Moving it shifts weight between the steering and drive axles and changes turning clearance.

A trailer axle assembly that can be moved along the trailer frame to adjust how much weight rests on the tractor's drive axles and on the trailer axles.

How a combination's total weight is shared among steering, drive, and trailer axles. It affects steering control, braking, and whether axle limits are exceeded.

A crosswise assembly that carries wheels and supports part of the vehicle's weight. Each axle has a maximum load rating that must not be exceeded.

The front axle that turns the vehicle. It needs enough weight on it for control, but not more than its rated limit.

A powered rear axle that pushes the tractor forward. Too little weight on the drive axles reduces traction, especially on wet or slippery pavement.

Two axles mounted close together as a unit. They are common on tractors and trailers and can often be slid as a group to adjust weight.

The maximum total weight a single vehicle is designed to carry, including its own weight. It is set by the manufacturer and shown on the certification label.

The maximum total weight allowed for the tractor and all towed units together, including cargo. Exceeding it overloads the combination even if each unit is under its own rating.

A heavy hook-and-latch coupling on the rear of a truck or trailer that holds a drawbar eye. It needs a safety pin or latch to prevent the eye from jumping out.

The rigid bar linking a towed unit to the towing vehicle. It can be a straight bar or a pivoting A-frame, and its eye fits over the pintle hook.

Chains that connect the towed unit to the towing vehicle as a backup if the main hitch fails. They should be crossed under the tongue and have enough slack to turn.

A small axle unit with a fifth wheel that lets a tractor pull two semitrailers. The first trailer couples to the dolly, and the dolly couples to the second trailer.

A combination of a tractor and two trailers, usually semitrailers joined by a converter dolly. It requires more care in braking and turning than a single trailer.

A combination of a tractor and three trailers. It is allowed only on certain roads and needs extra following distance and gentle steering.

The tendency of the rear wheels of a long combination to follow a shorter path than the front wheels when turning. It can cause the trailer to strike curbs, signs, or other vehicles.

The total width of pavement a combination covers while turning, from the outside of the front to the inside of the rear wheels. Wide vehicles need extra room.

The way the last trailer in a multi-trailer combination swings farther sideways than the tractor during a quick lane change or turn. It makes rollover more likely.

A turn that requires swinging wide to the left before starting so the trailer's rear wheels clear the corner. Check mirrors throughout and watch for vehicles on the right.

Using the side mirrors to watch the trailer and the space beside it, especially before turns, lane changes, and backing. Mirrors should be adjusted before driving.

An area beside or behind the combination that the mirrors do not show. Large vehicles have big blind spots, so the driver must check carefully before changing lanes.

Moving from one lane to another. In a combination vehicle, signal early, check mirrors and blind spots, and change lanes smoothly to avoid trailer sway.

Driving in reverse. A combination vehicle must be backed slowly with the trailer watched in the mirrors, and a helper should guide from a visible position.

A person outside the vehicle who guides the driver while backing. The driver should stop if the spotter is not visible in the mirror.

A dangerous condition in which the tractor and trailer fold against each other at an angle. It can happen from hard braking on slippery pavement or from backing too sharply.

When the trailer's wheels lose traction and the trailer swings sideways out of line. Ease off the accelerator and steer gently to correct it rather than braking hard.

Loss of tire grip on the road. In a combination vehicle, release the brakes to regain rolling traction, then steer in the direction you want to go.

The braking system on the towed unit, usually air-operated. It should be checked during the walk-around and used with the tractor brakes, not instead of them.

A braking system that uses compressed air to apply the brakes. In a combination, air lines and couplings must be connected and free of leaks before driving.

The coupler that joins the tractor's air hoses to the trailer's. The service line is usually blue and the emergency line red, and both must seat firmly.

A flexible hose carrying compressed air between the tractor and trailer. Lines should be supported so they do not drag, rub, or kink.

The cord that carries power for trailer lights and signals from the tractor. It must be plugged in, supported, and checked for damaged pins.

A lever on the steering column or dash that applies only the trailer brakes. It is used to test trailer brakes and to help control trailer swing.

A parking and emergency brake that uses a strong spring to apply the brakes when air pressure drops. It releases only when the system is charged with air.

A brake that holds a vehicle when parked. On air-braked combinations, the spring brakes set automatically when air pressure is lost.

The check of the combination before driving, covering coupling, air lines, tires, lights, brakes, and load security. It must be done every day the vehicle is used.

A slow circle around the combination looking for damage, leaks, loose parts, and burned-out lights. It is part of the pre-trip inspection.

Separating the tractor from the trailer. The procedure includes setting the trailer brakes, lowering the landing gear, and pulling straight forward slowly.

Connecting the tractor to the trailer. The driver backs under the trailer, locks the kingpin, connects air lines and lights, raises the landing gear, and does a pull test.

The bolts that fasten the fifth wheel to the tractor frame. Loose or missing bolts mean the fifth wheel can shift or break loose, so they must be checked.

Tying down cargo so it cannot shift, slide, or fall. Shifting cargo changes weight distribution and can make a combination unstable.