FL CDL Doubles/Triples key terms

A separate axle and frame unit with a fifth wheel and a drawbar that couples a semitrailer to the rear of another trailer, turning a single semitrailer into the second unit of a double or triple combination.

The rigid arm projecting forward from a converter dolly that attaches to the towing trailer's rear coupling, transmitting pulling and braking forces between the two units.

A hook-and-latch coupling mounted on the rear of a trailer or truck that receives the dolly drawbar's lunette eye, allowing pivoting while holding the units together.

The ring welded to the end of a dolly drawbar that drops over and locks into the pintle hook on the trailer ahead, forming the articulated connection between units.

Grade-rated chains that cross under the drawbar from dolly to towing trailer, sized to hold the units together if the pintle hook or lunette eye fails.

The greased plate-and-jaw assembly on top of a converter dolly that the kingpin of the following semitrailer locks into, letting the trailer pivot as the combination turns.

The hardened vertical pin under a semitrailer's nose that locks into a fifth wheel jaw, securing the trailer to the tractor or dolly and carrying the trailer's vertical load.

The spring-loaded closing mechanism in a fifth wheel that wraps around the kingpin; a pull test with the trailer brakes released confirms it has fully latched.

A check performed after coupling in which the tractor is pulled gently forward against released trailer brakes to confirm the kingpin is locked and will not pull out.

The fixed order of steps for joining units: align and back under, check the jaws closed on the kingpin, connect air lines and electrical cord, secure the dolly, then test before driving.

The retractable legs under a semitrailer's nose that support it when uncoupled; they must be fully raised and the crank secured before the combination moves.

The ordered steps for separating units: park on level ground, set brakes and chock wheels, lower landing gear, disconnect lines, release the coupling, then pull away slowly.

Wedges placed against trailer tires to prevent rolling when a unit is uncoupled or parked on a slope; required before releasing any coupling on a grade.

The color-coded hoses (service, emergency, and supply) that carry compressed air between tractor, trailers, and dolly to operate brakes and charge reservoirs.

Interlocking coupler fittings on the ends of air lines; they must be seated, sealed, and locked so air cannot leak out of the brake system.

The air line that charges trailer reservoirs and signals a breakaway; if it separates or loses pressure, trailer spring brakes apply automatically.

Brake chambers with a heavy spring that applies the brakes when air pressure drops, so a trailer with lost air will stop rather than roll free.

Separation of a trailer from its towing unit; the emergency line pulls apart and the trailer's spring brakes set automatically to stop the loose unit.

A check with the engine off and brakes applied that watches air pressure gauges for excessive drop, revealing leaks in lines, glad hands, or chambers.

Any drop in system pressure from leaks or a separated line; with multiple trailers the volume is larger, so pressure builds and recovers more slowly.

The walk-around check of couplings, air lines, tires, lights, mud flaps, and dolly hardware required before driving a double or triple combination.

The flat frame area of a converter dolly that carries its fifth wheel and axle assembly; it must be free of cracks, loose bolts, and debris before use.

How closely the rear trailer follows the path of the tractor; in a multi-trailer combination each unit tracks inside the one ahead, so the last trailer cuts the corner most.

The sideways drift of a trailer's rear wheels toward the inside of a turn relative to the tractor's path, which increases with each added trailer and with sharper turns.

The total width of pavement a turning combination occupies, from the outside front corner of the tractor to the inside rear wheels of the last trailer.

The technique of swinging the tractor out and starting the turn later so the rear trailers clear curbs, poles, and the inside lane without off-tracking into them.

A visual reference the driver uses in the mirrors to judge where the last trailer's tires are running, helping keep them inside the lane and clear of obstacles.

The tendency of the last trailer's sideways swing to be larger than the tractor's in a quick lane change, growing with speed and with the number of trailers.

Loss of a unit onto its side when lateral force in a turn exceeds stability; doubles and triples roll over at lower speeds than a single trailer.

The point where a load's weight is concentrated; raising it or loading it unevenly makes a trailer more likely to sway or roll over in a turn.

The sideways push on a moving vehicle during a turn or lane change; it grows with speed and steering sharpness and is resisted by tire grip and low load height.

A speed chosen for conditions so that steering, braking, and curve entry stay well within the combination's stability limits, often below the posted limit.

A sudden rollover caused by a quick steering correction that sets the rear trailer swinging, common when a driver overcorrects after drifting off the pavement.

Steering too hard in the opposite direction to fix a drift or sway; with multiple trailers it can whip the rear units and cause loss of control.

Side-to-side weaving of the trailers, often triggered by speed, crosswinds, or poor load balance; easing off the accelerator is the first response, not braking hard.

Placing cargo so weight is balanced side to side and spread properly over the axles, keeping each unit stable and within axle and kingpin limits.

The maximum allowed load on axles, tandems, and the combination set by law; overloading any unit of a double or triple reduces braking and stability.

Setting slack adjusters so each brake chamber has correct pushrod travel; on a combination every unit's brakes must work or stopping distance grows sharply.

The total distance from perceiving a hazard to a full stop; a loaded multi-trailer combination needs far more room than a car or single trailer.

The gap kept behind the vehicle ahead; with doubles and triples it should be at least one second per ten feet of combination length, and more in bad weather.

A skid in which the tractor and trailer fold against each other at an angle, usually from locking drive wheels or braking hard on a slippery surface.

The actions to regain grip in a slide: stop braking, steer in the direction the vehicle needs to go, and avoid sudden throttle or steering inputs.

The frequent scan of both mirrors to watch the trailers' tires, lane position, and swing, especially before and during turns and lane changes.

The correct hookup order of service, emergency, and supply hoses between tractor, first trailer, dolly, and rear trailer so each brake system is charged and released properly.