Illinois CDL Doubles and Triples key terms

A fixed-order check of the whole combination performed before driving: coupling, air lines, lights, tires, brakes, cargo securement and the dolly. Keeping the same sequence every time prevents a defect from being missed.

The measured groove depth in a major tread groove. Minimums apply to steer, drive and trailer tires, and each tire must also be free of cuts, bulges and exposed cord.

A flexible shield hung behind a tire to control spray and thrown debris. It must hang square, be mounted securely, and keep the ground clearance the rules require.

A valve under a trailer reservoir used to blow out condensed moisture at the start of the day. Opening it also shows whether the tank holds air and can be recharged.

The steel plate on a tractor or dolly whose jaws lock around the trailer kingpin. It carries the trailer's nose weight and forms the pivot point of the combination.

The hardened vertical pin under a trailer nose that the fifth wheel jaws grip. Inspect it for wear, looseness and cracks, because a worn pin lets the trailer shift or break loose.

A one- or two-axle frame with its own fifth wheel and a drawbar at the front. It supports the nose of a second or third trailer and turns it into a full trailer.

The heavy hook and latch on the rear of a trailer or dolly that catches the drawbar eye of the unit behind it. The latch must close fully and be secured with a pin.

The ring at the end of a dolly tow bar that fits over the pintle hook. Check it for cracks, a worn or elongated bore, and a bent shank before coupling.

Secondary retainers that keep a towed unit attached if the pintle or drawbar fails. They must be crossed, long enough to allow turning, and rated for the load.

A spring-loaded coupler at the end of an air line that twists onto its matching half. Blue marks the service line and red the emergency line so they are not easily confused.

The small rubber ring inside a coupler that stops air escaping at the joint. A missing, cracked or doubled seal leaks air and must be replaced before the lines are connected.

The blue air line that carries the driver's brake application to the trailer relay valve. A kink, leak or wrong connection makes trailer braking weak, delayed or uneven.

The red line that fills the trailer air tanks and carries the breakaway signal. If it loses pressure, the tractor protection valve closes and the trailer spring brakes apply.

A valve that shuts off the tractor's air supply when the emergency line is disconnected or badly leaking. It saves enough air to stop the tractor and sets the trailer brakes.

Brakes applied by a strong spring and released by air pressure. They set automatically when trailer air pressure falls too low or the lines separate, so the unit cannot roll away.

The arm that converts a brake chamber's pushrod stroke into rotation of the camshaft. Too much free stroke means the brake is out of adjustment and stopping distance grows.

With full pressure built and the engine off, hold the brakes released and watch the gauges for one minute. A drop past the allowed amount shows a leak that must be fixed.

Hold a full service brake application with the engine off and watch the gauge or listen for escaping air. It checks chambers, hoses and trailer lines while they are pressurized.

The light and buzzer that must come on before system pressure drops below about 60 psi. It warns that spring brakes are about to apply, so the vehicle must be stopped safely.

The coiled cord and plug that feed stop, turn, tail, marker and ABS power to the trailer. Check the pins, socket and cord for corrosion, bent pins and chafing.

A yellow trailer light that should come on at start-up and then go out. If it stays lit, the anti-lock system has a fault that must be reported and repaired.

Lights along the top and sides of a trailer or dolly that show the unit's width and height. All must burn, and the wiring must be supported rather than hanging loose.

Direction and hazard lights on the tractor and on each trailer, including the dolly. Verify every one from outside the cab, since a dead bulb can hide a lane change or breakdown.

The greatest load a single tie-down, chain, strap or fitting is rated to hold, as marked by its maker. It is used to work out how many securements a load needs.

The total of the working load limits of all tie-downs holding a load. It must be at least half the cargo weight, and more when the load is likely to shift.

A strap, chain, cable or rope plus its tensioning device used to hold cargo on the trailer. Each must be rated, pulled tight, and free of knots, cuts and wear.

A sturdy vertical wall or header board at the front of a trailer that stops cargo sliding forward into the tractor during hard braking. It must be anchored to the trailer structure.

The most weight one axle and its tires are designed to carry. Loading must keep every axle of the tractor, trailers and dolly within its own rating.

A federal limit on the weight carried by a group of axles that depends on how far apart they are spaced, so heavy loads are spread instead of concentrated on a short span.

Movement of cargo during braking, turning or rough road. It can break tie-downs, change axle loads and upset handling, so loads are blocked, braced and rechecked en route.

A coupling check: with the trailer brakes set, pull gently in low gear. If the trailer holds firm while the tractor moves, the jaws are locked around the kingpin.

A wedge set against a wheel so the trailer cannot roll while it is uncoupled. Chock on both sides before the air lines come off or the fifth wheel is released.

The crank-driven legs under a trailer nose. Lower them until they touch the ground and take the weight before pulling the release handle, or the trailer will drop.

The lever that opens the locking jaws. Pull it only after the trailer brakes are set, the wheels chocked, the landing gear landed and the air lines disconnected.

The amount the rear wheels of a long combination cut inside the path of the front wheels in a turn. More room is needed on the inside, especially at corners and right turns.

The tendency of the last trailer in a multi-trailer set to swing wider and faster than the tractor in a quick lane change, raising rollover risk at highway speed.

The outward movement of a trailer's rear end or front corner when the combination turns sharply, which can strike poles, signs or vehicles standing alongside the road.