Question 1

What is the correct order for connecting air lines when coupling a second trailer to a converter dolly in a doubles combination?

Accepted Answer

Connect the service (blue) and emergency (red) air lines to the trailer's glad hands before moving the dolly under the trailer. This ensures the trailer brakes are controlled before the coupling pin is engaged, preventing the trailer from rolling free.

Question 2

What is rearward amplification and how does it affect doubles and triples combinations?

Accepted Answer

Rearward amplification is the tendency for the rear trailer in a combination to sway or whip more violently than the front trailer when the driver steers or swerves. Each trailer in the combination amplifies the movement of the unit ahead of it, making the rearmost trailer the most dangerous in evasive maneuvers.

Question 3

What is off-tracking and why is it more severe in triple combinations than in doubles?

Accepted Answer

Off-tracking is when the rear wheels of a trailer follow a shorter, inner path through a turn than the front wheels, meaning the rear of the vehicle cuts the corner. Triples experience more severe off-tracking because the additional trailer length means the rearmost axles cut even further inside the intended path, increasing the risk of striking curbs, medians, or pedestrians.

Question 4

What specific inspection items must be checked at each trailer connection point in a doubles or triples combination?

Accepted Answer

At each connection point, the driver must inspect the fifth wheel coupling for proper locking and no gap between plates, the safety chain or cable attachment, the air line connections at the glad hands, the electrical cord connections, and the security of the converter dolly pintle hook to the lead trailer's drawbar eye.

Question 5

Why do doubles and triples have longer stopping distances than single trailer combinations of similar total weight?

Accepted Answer

Doubles and triples have longer stopping distances because air brake application pressure must travel through more brake lines and glad hand connections to reach the rear trailer brakes, creating greater brake lag. Additionally, any imbalance in brake adjustment across multiple trailers can reduce total braking efficiency.

CDL Commercial Drivers License General Knowledge — Doubles & Triples Endorsement

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