Combination Vehicles: Coupling, Air Lines, and Handling
Colorado's Combination Vehicles test is 20 questions with 80% to pass. It covers coupling and uncoupling, air and electrical connections, and the handling differences that make a tractor-trailer less forgiving than a straight truck.
Coupling
- Back under the trailer squarely and slowly. Listen for the fifth-wheel jaws to close, then do a tug test: apply the trailer brakes, pull gently forward, and confirm the tractor is held.
- Get out and look: jaws closed around the kingpin, release handle in, safety latch engaged, no gap between the fifth wheel and trailer apron.
- Connect the air lines: red is the emergency line, blue is the service line. Connect the electrical cord, build pressure, and test the trailer brakes before raising the landing gear.
Uncoupling
- Chock the trailer wheels, lower the landing gear to firm ground, disconnect the lines, release the fifth-wheel handle, and pull out slowly. Never leave a trailer resting on landing gear alone on soft ground.
Driving
- Off-tracking: the rear wheels of a long trailer cut the corner. Make right turns wider and watch the trailer in the mirror.
- Rearward amplification: the last trailer in a set swings farther than the tractor; check mirrors before and during lane changes.
- A loaded combination needs more stopping distance than a car or straight truck. A bobtail (tractor without trailer) has poor braking because the rear axle is light; brake gently and expect wheel lockup.
- The trailer hand valve is for testing trailer brakes only - never for parking or normal stopping.
Worked example
You back under a trailer, hear the jaws click, and see the handle move. You skip the tug test because the sound was clear. At 40 mph the trailer separates on a bump. The click only proves the jaws contacted the kingpin; the tug test proves the lock. Correct sequence: back under, listen, tug with trailer brakes set, visually check jaws and latch, then connect lines and test.
Jackknife and brake failure
- If the tractor's rear wheels lock and the rig starts to jackknife, release the brakes and steer where you want to go. Hard braking makes it worse.
- If the trailer breaks away, the emergency line loses pressure and the trailer spring brakes apply.
Traps
- Reversing the air-line colors: red is emergency, blue is service.
- Using the trailer hand valve as a parking brake.
- Forgetting the tug test or the visual jaw check.
- Taking a right turn like a car and clipping the curb with the trailer.