I85PREP
⌘ K
Ohio CDL Combination study materials
Ohio CDL Combination study resources

Ohio CDL Combination: Combination Vehicle Knowledge - Ohio Class A

Ohio CDL Combination · Work through your exam, one topic at a time.

All guide sections

Combination Vehicle Knowledge - Ohio Class A

Combination Vehicle Knowledge

Ohio's Combination Vehicle Knowledge domain tests the Class A driver's ability to inspect, couple, uncouple, and control a tractor-trailer or other combination safely. The questions emphasize the fifth wheel, kingpin, air lines, trailer brakes, off-tracking, and weight distribution, not general car driving. Ohio's test follows the federal CDL standards in 49 CFR Part 383; a Class A combination has a GCWR of 26,001 lb or more and a towed unit over 10,000 lb.

Core rules

  • The fifth wheel must be greased and its jaws fully closed around the kingpin. After coupling, do a tug test with the trailer parking brake set and a visual check of the jaws.
  • Air lines are color-coded: service (blue) and emergency (red). Do not cross them or use the trailer hand valve for parking.
  • The tractor protection valve seals the tractor air system if trailer pressure drops; know how to test it.
  • Check landing gear is fully raised before moving; secure the trailer with chocks during uncoupling.
  • Combination vehicles off-track: the rear wheels of the trailer cut inside the tractor's path. Swing wide and watch mirrors.
  • Total stopping distance includes perception, reaction, and brake lag; a loaded combination needs more space than a bobtail.
  • With ABS, keep firm, steady pressure; do not pump the brake.

Worked example

A driver backs under a trailer, hears the fifth-wheel jaws click, and connects the red and blue lines. He then releases the tractor parking brake and pulls forward. The trailer stays put. Reasoning: the tug test was invalid because the trailer parking brake was not set; the tractor can move while the trailer is held by its own brakes, so the test proves nothing. Correct sequence: set trailer brakes, tug gently, then visually confirm the jaws are locked around the kingpin.

Traps

  • Trusting the click instead of the tug test and visual check.
  • Leaving the trailer hand valve applied or using it as a parking brake.
  • Forgetting to chock wheels or lower landing gear before uncoupling.
  • Cutting turns too tight and clipping curbs with trailer tires.
  • Assuming a bobtail stops like a loaded combination.