CDL General Knowledge TestCombination Vehicles (if applicable)Hard
A driver is inspecting the kingpin and fifth wheel assembly. What is a critical sign of wear or damage that would render the coupling unsafe?
- AA kingpin that is slightly discolored from grease.
- BA fifth wheel release handle that is stiff but functional.
- CMinor scratches on the fifth wheel plate.
- DExcessive wear on the kingpin, reducing its diameter.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive wear on the kingpin, reducing its diameter.
Excessive wear on the kingpin, which reduces its diameter, can lead to a loose connection within the fifth wheel locking jaws. This increases the risk of the trailer separating from the tractor during operation, a catastrophic failure.
Why the other options are wrong
- A. Discoloration from grease is normal and expected, not a sign of damage.
- B. A stiff but functional release handle might indicate a need for lubrication or adjustment, but not immediate unsafe operation unless it fails to lock or unlock.
- C. Minor scratches are cosmetic and do not compromise the structural integrity of the fifth wheel.
Kingpin Wear
Deterioration of the kingpin's material, specifically a reduction in its diameter, due to friction and stress during coupling and driving.
- Compromises coupling security.
- Increases risk of trailer separation.
- Must be within specified tolerances.
Memory trick: King's pin must be strong, not thin.