Coupling, handling, and safety concepts for the CDL doubles and triples endorsement.
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- What is a doubles combination?
- A tractor pulling two trailers, one behind the other, connected by a converter dolly.
- What is a triples combination?
- A tractor pulling three trailers, connected by two converter dollies.
- What is the fifth wheel?
- The coupling device on the tractor that connects to the kingpin on the first trailer's upper coupler.
- What is the kingpin?
- The vertical pin on the upper coupler of a trailer that locks into the fifth wheel on the tractor.
- What is a converter dolly?
- 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?
- 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?
- 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?
- The service line and the emergency line, each with proper glad-hand connections.
- What is the proper coupling procedure for a doubles combination?
- 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?
- Apply the parking brake on the tractor and chock the trailer wheels.
- Why should you always chock the wheels when uncoupling?
- 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?
- The rear trailers may not follow the same path as the tractor, causing jackknife or sideswipe.
- What is off-tracking?
- 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?
- 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?
- Apply the brakes in a smooth, steady manner; avoid hard braking which can cause jackknife, then downshift if safe.