Coupling, handling, and safety concepts for the CDL doubles and triples endorsement.
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- What is a doubles combination?
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A tractor pulling two trailers, one behind the other, connected by a converter dolly.- What is a triples combination?
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A tractor pulling three trailers, connected by two converter dollies.- What is the fifth wheel?
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The coupling device on the tractor that connects to the kingpin on the first trailer's upper coupler.- What is the kingpin?
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The vertical pin on the upper coupler of a trailer that locks into the fifth wheel on the tractor.- What is a converter dolly?
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A small tractor unit with a fifth wheel used to connect a second or third trailer in a doubles or triples combination.- What does the tractor protection valve do?
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It prevents air from the tractor from being lost if the air lines between the tractor and trailer are severed.- When should you inspect the converter dolly air lines before coupling?
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During the pre-trip inspection, look for cracks, cuts, breaks, or improper connections before hooking up.- What air lines connect the tractor and first trailer?
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The service line and the emergency line, each with proper glad-hand connections.- What is the proper coupling procedure for a doubles combination?
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Lower the fifth wheel, back straight under the trailer kingpin, lock the coupling securely, and connect all air lines and lights.- What is the first step when uncoupling a doubles trailer?
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Apply the parking brake on the tractor and chock the trailer wheels.- Why should you always chock the wheels when uncoupling?
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To prevent the trailer from rolling backward or forward when the tractor is separated.- What is the main hazard when backing a doubles or triples combination?
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The rear trailers may not follow the same path as the tractor, causing jackknife or sideswipe.- What is off-tracking?
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The tendency of trailers behind a turning tractor to follow a tighter, shorter path than the tractor, potentially hitting curbs or objects.- What is jackknifing?
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When the tractor or trailer skids sideways and folds up at the coupling point, often caused by excessive braking or loss of traction.- How should you brake in an emergency with a doubles or triples combination?
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Apply the brakes in a smooth, steady manner; avoid hard braking which can cause jackknife, then downshift if safe.
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