Braking and Skid Control with Combinations
What this domain tests
This domain tests stopping distance, brake lag, controlled and stab braking, ABS use, skid recovery, and downgrade control in a tractor-trailer.
Core rules
- Total stopping distance includes perception, reaction, brake lag, and actual braking. Brake lag is longer in a combination because air must reach the trailer.
- Controlled braking means pressing the pedal firmly without locking wheels; stab braking means applying fully, releasing when wheels lock, then reapplying. With ABS, brake normally and steer.
- On downgrades, select a low gear before the descent and use the brakes progressively; do not coast.
- If wheels lock, release the brake to regain rolling traction and steer; do not hold a locked wheel.
Worked example
You are descending a long grade with a loaded trailer and smell hot brakes. You have been riding the foot valve instead of using a lower gear. The correct response is to slow, select a lower gear, and use progressive braking rather than continuous light pressure. The reasoning: continuous braking builds heat and fade, while engine braking and controlled applications help control speed without overheating the drums or discs.
Traps that cost marks
- Pumping the brake during ABS activation.
- Holding a locked wheel instead of releasing to steer.
- Coasting downhill in neutral or a high gear.
- Ignoring trailer brake condition before a downgrade.