Skid Control, Jackknifing, and Emergency Handling
Skid Control, Jackknifing, and Emergency Handling
This domain tests what causes a combination to skid or jackknife and what to do in the first two seconds. The most common cause is driving too fast for conditions combined with hard braking.
Skid types and responses
- Drive-wheel skid. The drive tires lock and the tractor's rear slides out - the start of a tractor jackknife. Release the brake pedal so the wheels roll again, and steer to keep the tractor aligned with the trailer.
- Trailer jackknife. The trailer swings out of line, usually because the trailer brakes are stronger than the tractor's or the drive wheels locked. Release the brakes, steer to pull the combination straight, and do not accelerate hard.
- Front-wheel skid. The steering tires lose grip and the vehicle will not turn. Release the brakes and let the wheels roll so steering returns. Do not steer harder.
- All-wheel skid. Release the brakes and steer; the vehicle straightens only when the wheels roll.
Prevention and ABS
Slow down before curves and slippery patches; brake gently and early; keep trailer brakes adjusted and balanced with the tractor's. With ABS, apply firm steady pressure and steer - do not pump the pedal. The trailer hand valve is not a service brake, and using it in a stop can cause a jackknife.
Worked example. Empty trailer, wet road, a light turns red. You brake hard, the drive wheels lock, and the tractor's rear swings left. Correct response: release the brake pedal to restore rolling, steer right to bring the tractor back in line with the trailer, then reapply pressure more gently. Reasoning: a locked wheel provides no directional control, and continued braking deepens the jackknife angle.
Traps that cost marks. Pumping ABS; braking harder when the trailer swings; using the trailer hand valve to stop; assuming an empty trailer stops shorter - its brakes are often over-adjusted for its weight.