Braking and Stopping with Multiple Trailers
Braking and Stopping with Multiple Trailers
This domain tests stopping distance, weight transfer under braking, and how the air brake system of a combination behaves. Expect questions on brake testing after coupling and on the trailer hand valve.
Stopping rules
- Total stopping distance is perception distance plus reaction distance plus brake lag plus braking distance. Weight and speed increase every one of these.
- Brake early, smoothly, and progressively. Hard braking lets the rear trailer push the tractor and first trailer, which can cause a jackknife or a swing.
- The combination has a dual air system. Each trailer has spring brakes, so a parted emergency line locks that trailer's brakes automatically.
- After coupling, test the brakes: apply the trailer brake valve, pull gently forward, and confirm the combination holds.
- On a downgrade, select a low gear before the grade and use a steady, light brake application with engine braking. Do not ride the brakes.
- The trailer hand valve is a test device. Do not use it for normal braking or for parking.
Worked example
A combination has weak brakes on the front trailer only. Under hard braking, the rear trailer's mass keeps moving forward with little retardation, so it pushes the tractor's drive axles and the combination folds into a jackknife. Reasoning: braking force must be balanced across all units; one weak trailer changes the geometry of the stop. The correct action is smooth, early braking and a repair, never harder pedal pressure.
Traps
- Riding or fanning the brakes down a long grade instead of gearing down.
- Using the trailer hand valve to stop or hold the vehicle.
- Skipping the brake test after coupling.
- Forgetting brake lag and the extra stopping distance of multiple trailers.