Combination Vehicles: Coupling and Handling
Maryland's Combination Vehicles domain tests coupling, uncoupling, inspection, and the handling differences of tractor-trailers. A combination vehicle has more pivot points, more air lines, and more weight-transfer effects than a single unit.
Coupling and uncoupling
Inspect the fifth wheel, kingpin, locking jaws, and safety latch. Back under the trailer squarely, listen for the lock, and perform a tug test. Connect the red emergency line and blue service line; the red line charges the trailer and applies the trailer spring brakes when disconnected. Raise the landing gear fully, connect the electrical cord, and check lights. After coupling, check the trailer brakes by pulling against them gently. Uncoupling reverses the order: chock wheels, lower landing gear, disconnect lines and cord, release the fifth wheel, and pull out slowly.
Weight and handling
Know gross combination weight, axle weights, and the kingpin-to-rear-axle distance. Off-tracking means the trailer's rear wheels follow a shorter path than the tractor's front wheels. Rearward amplification means the rear of the combination swings wider during a quick lane change. Slow down before curves, and do not brake hard in a turn.
Worked example
A 53-foot trailer turns left from a two-lane road. The tractor's rear wheels stay 3 feet from the curb, but the trailer's rear wheels track several feet inside that path. The driver must swing wide enough to keep the trailer off the curb while checking the left mirror; waiting until the trailer is alongside a parked car is too late.
Traps
Forgetting the tug test; leaving landing gear partly down; crossing the red and blue lines; using the trailer hand valve as a parking brake; not checking trailer lights; and assuming a combination stops as quickly as a bobtail tractor.