Braking and Skid Control with Trailer
Braking and Skid Control with Trailer
This domain tests stopping a combination: brake lag, total stopping distance, controlled braking, trailer brake use, and recovery from jackknife, trailer swing, and wheel skids. The exam wants the correct action under pressure, not a list of parts.
Core rules
- Total stopping distance includes perception, reaction, brake lag, and actual braking. Brake lag adds distance because air must travel to the trailer.
- Use controlled braking: apply firm, steady pressure without locking wheels. If you need more, release and reapply.
- Do not ride the brakes; heat fade reduces stopping power.
- Downshift and use engine braking on long grades, but do not coast.
- The trailer hand valve is for testing trailer brakes, not normal stopping.
- ABS may cycle; keep steady pressure and steer.
Skids
- Jackknife: the tractor's drive wheels lose traction and the tractor swings. Release the brakes to regain rolling traction and steer in the direction you want the tractor to go.
- Trailer swing: the trailer's wheels lose traction. Release the brakes and steer to counter the swing.
- Front-wheel skid: you lose steering. Release the brakes and let the wheels roll again.
Worked example
You are descending a grade and the trailer begins to swing left. You brake harder, and the swing worsens. The correct response is to release the brakes, let the trailer tires regain grip, steer gently to keep the tractor aligned, and slow with engine braking and light, controlled brake pressure. Hard braking on a slippery grade is what caused the swing.
Traps
- Using the trailer hand valve as a service brake.
- Braking hard enough to lock wheels.
- Steering away from the skid.
- Forgetting brake lag at highway speed.
- Overheating brakes on a downgrade.
Key points
- Air brake lag lengthens stopping.
- Controlled braking, not locked wheels.
- Release brakes to recover a skid.
- Trailer hand valve is a test device.
- Slow before grades; use engine braking.