After coupling the tractor to the trailer, a driver performs a 'tug test' or 'pull test'. During this test, the tractor's parking brake is set, the trailer's parking brake is released, and the driver gently tugs forward against the trailer. What is the primary purpose of releasing the trailer's parking brake for this specific test?
- ATo verify that the trailer's service brakes are functioning correctly.
- BTo ensure the fifth wheel jaws are securely locked around the kingpin.
- CTo allow the trailer to roll freely if the fifth wheel is not properly locked.
- DTo prevent damage to the trailer's braking system during the tug.
Show answer & explanationAnswer & explanation
Correct answer: B. To ensure the fifth wheel jaws are securely locked around the kingpin.
The 'tug test' is specifically designed to confirm the mechanical connection between the tractor and trailer. By releasing the trailer's parking brake, the driver is testing the fifth wheel's hold on the kingpin. If the connection isn't secure, the trailer will roll forward, indicating a failure. If the trailer brakes were set, the test would only confirm the trailer brakes work, not the fifth wheel connection.
Why the other options are wrong
- A. This test is not for service brakes; it's for the coupling connection.
- C. While true that it would roll, the purpose is to confirm the *secure lock* of the fifth wheel, not merely to allow rolling.
- D. Damage is unlikely if done gently, but the primary purpose is not damage prevention.
Pull Test (Trailer Brakes Released)
During a pull test, releasing the trailer's parking brake allows the driver to verify that the fifth wheel jaws are securely locked around the kingpin by attempting to gently pull the stationary trailer with the tractor.
- Tractor brakes set, trailer brakes released.
- Gently tug forward with tractor.
- If trailer moves, connection is NOT secure.
- Tests mechanical coupling, not brake function.
Memory trick: Release for Roll, Lock for Goal.