CDL Doubles/Triples key terms

A small frame with one or two axles and a fifth wheel that attaches to the rear of one trailer and supports the front of the next trailer in a double or triple combination.

A heavy hook-and-latch coupling mounted on the rear of a vehicle or dolly that grabs a drawbar eye to connect a towed unit. It must be latched and secured before driving.

Chains that connect the towing unit to the towed unit as a backup in case the primary hitch fails. They should be crossed and long enough to allow turns but short enough to keep the drawbar off the ground.

A rigid bar that connects a towed trailer to a pintle hook or dolly. It pivots at the coupling so the combination can turn, and it must be free of cracks or bends.

A tractor pulling three trailers, usually linked by converter dollies. It needs more room to turn and stop and is more affected by wind and quick steering than a single trailer.

Couplers that join the tractor's air hoses to the trailer's air hoses. They have rubber seals and must be locked together so the service and emergency air lines do not leak or separate.

The horseshoe-shaped plate on the tractor that locks around the trailer kingpin to support and pivot the trailer. It must be greased, closed, and latched securely.

The vertical steel pin under the front of a trailer that locks into the tractor's fifth wheel. It holds the trailer to the tractor and allows the unit to pivot when turning.

A check after coupling where you pull forward gently with the trailer brakes set to confirm the fifth wheel jaws are locked on the kingpin and will not slip.

The retractable legs under the front of a trailer that support it when it is not attached to a tractor. They must be fully raised and pinned before driving.

The two curved metal pieces inside the fifth wheel that close around the kingpin and hold the trailer. A pull test confirms they are fully closed.

Blocks placed against trailer tires to keep the trailer from rolling during coupling, uncoupling, or inspection. They are required whenever the trailer is left standing alone.

The dash valve, usually a red knob, that controls air to the trailer. Pulling it out applies the trailer spring brakes; pushing it in charges the trailer air system.

The coiled cord that carries lights and turn signals from the tractor to the trailer. It must be plugged in fully and not sag or drag on the ground.

A systematic check of the tractor, trailers, coupling, air lines, brakes, tires, and lights before driving. It is required by law and helps catch defects before they cause a crash.

Checking the fifth wheel, kingpin, locking jaws, mounting bolts, and release handle for wear, cracks, or looseness. Any damaged part must be repaired before the vehicle is driven.

Standard colors that identify tractor-trailer air hoses: blue for the service line and red for the emergency line. Correct hookup prevents brake failure and loss of trailer air.

Checking each tire for correct pressure, tread depth, cuts, bulges, and matching size and type across an axle. Bad tires can fail under the heavy loads that doubles and triples carry.

Testing that the brake lights, turn signals, and clearance lights on every trailer work before driving. A helper or a wall reflection can confirm the lights are operating.

A valve that closes automatically if a trailer separates from the tractor, trapping air in the trailer tanks so the trailer spring brakes apply and stop the unit.

A valve on the trailer air line, usually near the glad hand, that lets you close off the trailer's air supply for testing or repairs. It must be open for the brakes to work normally.

A pump driven by the engine that builds air pressure for the braking system. It must be able to build pressure fast enough to keep the tanks full during normal driving.

A device that controls the compressor, turning it on when tank pressure drops to the cut-in level and off when it reaches the cut-out level, usually around 100 to 125 psi.

A pressure relief valve on the air tank that opens automatically if pressure gets too high, protecting the system from damage. It is usually set near 150 psi.

A buzzer and light that come on when air pressure drops below about 60 psi, warning the driver to stop before the spring brakes apply automatically.

Brakes held applied by strong springs and released by air pressure. If air pressure is lost, the springs apply the brakes and stop the vehicle.

The lever that connects the brake chamber pushrod to the brake camshaft. It adjusts to keep the brakes tight; too much free play means weak or delayed braking.

A valve that seals the tractor's air system if a trailer breaks away or an air line ruptures, so the tractor keeps enough air to stop safely.

A check with the engine off and brakes applied to see how fast air pressure drops. Leaks beyond the allowed rate mean the vehicle must be repaired before driving.

A test that measures how quickly the compressor raises tank pressure from a low reading to the cut-out level. Slow buildup means the compressor or governor needs service.

A cylinder that turns air pressure into mechanical force to apply the brakes. A ruptured chamber or leaking diaphragm causes weak braking and must be fixed.

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 turn, increasing the risk of a rollover.

The tendency of the rear wheels of a long combination to follow a tighter path than the front wheels during a turn, so the trailer cuts the corner and can strike curbs or objects.

A loss of control where the tractor and trailer fold against each other at a sharp angle. It often happens from braking too hard on a slippery surface.

Side-to-side swaying of trailers that grows worse if the driver steers sharply in response. Easing off the accelerator and holding the wheel steady usually helps.

The amount of space a vehicle needs to complete a turn. Doubles and triples need much wider turns than a single trailer because the rear units cut the corner.

The gap between your vehicle and the one ahead. With multiple trailers you need more room to stop, so add at least one second for each trailer behind the tractor.

A loss of braking power caused by overheated brakes, common on long downgrades. Using a lower gear and avoiding constant hard braking helps prevent it.

Choosing a lower gear before a steep downgrade so engine braking helps control speed. Shift before the descent, not during it, and stay off the brakes too much.