Basic Control of Combination Vehicles
Basic control - what is tested
The exam checks whether you understand how a combination vehicle behaves differently from a car: slower acceleration, longer stopping distance, and heavier steering inputs. Questions mix procedures with numbers.
Getting moving
- Complete the pre-trip, build air to governor cut-out (about 125 psi), and release the spring brakes.
- Start in the lowest gear that will move the load; a loaded combination accelerates slowly, so plan a large gap before pulling into traffic.
- Shift progressively. With a non-synchronized transmission, double-clutch: clutch to neutral, clutch again into the next gear.
- Do not ride the clutch or skip gears climbing a grade with a heavy load.
Steering and speed
- The wheelbase is long and the center of gravity is high: slow down before a curve, not in it.
- The trailer amplifies every steering input, so make smooth corrections rather than sharp ones.
Stopping distance
Total stopping distance = perception distance + reaction distance + brake lag + braking distance.
- At 55 mph, perception and reaction each take about 60 feet.
- Air-brake lag adds roughly 32 feet at 55 mph before the brakes begin to work.
- Braking distance grows with the square of speed: 40 mph needs about four times the distance of 20 mph.
- Use controlled braking: firm, steady pressure that does not lock the wheels.
Worked example
A 65-foot combination at 55 mph: reaction plus brake lag alone eats about 150 feet before the brakes bite. If total stopping distance is roughly 420 feet and the hazard appears 350 feet ahead, stopping is impossible. The correct answer is to reduce speed until the required distance fits inside the space you can see.
Traps
- Assuming a light trailer always shortens stopping distance; out-of-adjustment trailer brakes lengthen it.
- Coasting downhill in neutral or with the clutch depressed.
- Expecting the trailer to follow a sharp low-speed turn immediately.
- Using the trailer hand valve as a service brake.