Michigan CDL Doubles/Triples key terms

The horseshoe-shaped plate on the tractor that locks around the trailer kingpin to hold the trailer. Its jaws must be closed and secured by a release handle before you drive.

The vertical steel pin under the front of a trailer that the fifth wheel jaws grip. It is the single point that transfers trailer pull to the tractor.

The two curved metal fingers inside the fifth wheel that close around the kingpin. A pull test confirms they are fully closed, not resting on top of the pin.

The procedure of backing the tractor under a trailer so the fifth wheel captures the kingpin, then connecting air lines, electrical cord, and landing gear. Done in a set order and checked before moving.

The reverse of coupling: parking on firm level ground, chocking wheels, lowering landing gear, disconnecting lines and cords, releasing the fifth wheel, and pulling slowly forward clear of the trailer.

After coupling, you pull forward gently with the trailer brakes set. If the tractor stops and the trailer does not move, the jaws are locked; if the tractor creeps, the coupling failed.

The retractable legs under a trailer front that support it when uncoupled. They must be fully raised before towing and firmly on the ground before the tractor is pulled out.

The flat steel plate on the underside of a trailer nose around the kingpin. It must sit flat on the fifth wheel; a gap or tilt means the trailer is not properly seated.

A small axle unit with its own fifth wheel and drawbar that couples a second or third trailer to the one ahead. It carries the kingpin of the following trailer.

The rigid arm on a converter dolly that reaches forward to the pintle hook of the trailer ahead. It keeps spacing and transmits pulling force to the dolly.

A heavy hook-and-latch coupling on the rear of a trailer or tractor that accepts the dolly drawbar eye. Its latch must be closed and pinned before you move.

Chains that back up the pintle hook connection between trailers. They are crossed where possible and must be short enough to hold the drawbar if the hook opens.

The quick-coupling fittings on the ends of air lines, one for the service line and one for the emergency line. They seal with rubber gaskets and lock when turned.

The air line that carries brake application pressure from the tractor to the trailer brakes. On doubles and triples it must run to every trailer in the combination.

The air line that supplies air to the trailer tanks and triggers trailer brakes if the line separates or loses pressure. Also called the supply line.

The rubber rings inside glad hands that seal the air connection. Cracked, missing, or flattened gaskets cause leaks, so they are checked at every coupling and pretrip.

The coiled cable carrying lights and ABS power from tractor to trailers. It is plugged in with the alignment tab matched, and the cord must have slack for turns.

The socket on the tractor or trailer where the electrical cord plugs in. Its cover should be closed when unused to keep out water and dirt.

A check with the engine off and brakes released: watch the gauge for a steady drop, and listen at glad hands, hoses, and valves. Any continuous leak must be fixed before towing.

A valve that closes and seals the tractor air supply if a trailer line breaks away, so the tractor keeps enough air to stop. Its test button is part of the pretrip.

Moving the trailer axles or fifth wheel to shift weight between axles and adjust bridge formula compliance. Must be done with the trailer brakes set and the locking pins confirmed.

The federal rule limiting how much weight may be carried on a group of axles based on their spacing. Doubles and triples are especially sensitive because short spacings reduce allowed gross weight.

A pair of axles grouped close together on a tractor or trailer. Their spacing, and the spacing between trailers, determines legal axle weights in a multi-trailer combination.

How cargo and trailer position place load on steer, drive, and trailer axles. Poor distribution overloads axles, lightens the steer axle, and makes the combination unstable.

The sideways whipping of the rear trailer in a multi-trailer rig at higher speed, caused by steering input traveling back through the units. It worsens as speed and trailer count rise.

The degree to which a steering movement at the tractor is magnified at the last trailer. It is larger for triples than doubles and grows with speed, so steering inputs must be gentle.

The tendency of rear axles to follow a shorter path than the front wheels in a turn. Doubles and triples off-track more, so you swing wider and watch mirrors for the last trailer.

The total width of road a turning combination occupies, from the outside of the front to the inside of the rear trailer wheels. Multi-trailer rigs need more room than a single trailer.

Side-to-side oscillation of the trailers that can grow until control is lost. Easing off the accelerator and avoiding hard steering or braking is the usual recovery.

Making small, gradual steering corrections so movement does not amplify through the trailers. It is the main defense against crack-the-whip and snaking in doubles and triples.

Approaching a right turn wide enough to keep the rear trailer clear of the curb, then turning slowly and tracking the last trailer in the mirrors. Tight setups clip curbs and poles.

The delay between pressing the pedal and trailer brakes applying, because air must travel through the lines. In multi-trailer rigs the lag is longer, so following distance must increase.

The gap you keep behind the vehicle ahead. Towing doubles or triples calls for more than a single trailer, roughly one second per ten feet of vehicle length plus extra margin.

Total distance covered from perceiving a hazard to a full stop, including reaction, brake lag, and braking. It is longer for heavier combinations and on downgrades or slick pavement.

Choosing a speed that fits load, weather, curves, and grade. In doubles and triples, lower speeds reduce rearward amplification and shorten the distance needed to stop.

Brakes applied by heavy springs when air pressure drops, used as parking brakes and as the emergency stop if a line breaks. They release only when air pressure builds.

A dash control that applies only the trailer brakes, used during the coupling pull test and to check trailer brake operation. It is not a substitute for the foot brake in normal driving.

Wedges placed against trailer tires before uncoupling so the trailer cannot roll. They are required whenever a trailer is left standing, especially on any slope.