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CDL · Inspecting Combination Vehicles

What should a driver check regarding the gap between the upper and lower fifth wheel during inspection?

Correct answer

There should be NO gap between the upper fifth wheel (trailer apron) and the lower fifth wheel (tractor plate) — a gap indicates the trailer is not properly seated and may be a high hitch

  1. A There should be a large visible gap
  2. B There should be NO gap between the upper fifth wheel (trailer apron) and the lower fifth wheel (tractor plate) — a gap indicates the trailer is not properly seated and may be a high hitch
  3. C A gap of 6 inches is normal
  4. D The gap should be filled with grease only

Why this is the answer

FIFTH WHEEL GAP INSPECTION: After coupling, there should be NO GAP between the upper fifth wheel (the trailer's apron plate) and the lower fifth wheel (the plate on the tractor). A GAP indicates: the trailer is not fully and properly seated; possibly a HIGH HITCH (where the kingpin rests on top of the locking mechanism rather than locked in the jaws) — extremely dangerous, can release at speed; the coupling is incomplete; PROPER COUPLING: The two plates should be flush against each other with the kingpin locked in the jaws; INSPECT from the side with a flashlight; LUBRICATION: The fifth wheel plate should be properly greased (reduces friction and wear, allows the plates to pivot) — but grease is not a substitute for proper seating; OTHER FIFTH WHEEL CHECKS: secure mounting to the frame (no missing/loose bolts); no cracks or damage; locking jaws closed around the kingpin shank; release handle in locked position; sliding fifth wheel locked in place; the no-gap check is a key indicator of proper coupling and is essential to verify before every trip.
Source: FMCSA CDL Manual — Fifth Wheel Gap Inspection

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