Off-tracking and Jackknifing
What this domain covers
Off-tracking and jackknifing are related to articulation but have different causes. Off-tracking is normal geometry: the rear wheels track inside the front wheels. Jackknifing is a loss of control in which the tractor and trailer fold toward each other. The exam tests both because they affect turns, braking, and skid recovery.
Off-tracking rules
- The longer the wheelbase and trailer, the more the rear wheels cut inside the turn.
- Use the mirrors and watch the trailer’s rear wheels through the turn.
- Start wide enough to clear the corner, but do not cross lane lines or swing into another lane.
- Do not turn too soon; the trailer can hit a curb or a vehicle beside you.
Jackknifing rules
- A tractor jackknife happens when the drive wheels lock and the trailer pushes the tractor sideways. It is common on slippery surfaces under hard braking.
- A trailer jackknife happens when the trailer swings out to the side.
- Hard braking on ice, snow, or wet pavement is a major cause. Engine braking on a slippery surface can also contribute.
- If a jackknife starts, release the brakes to let the wheels roll and steer in the direction you want the front of the vehicle to go. Do not keep holding a locked brake.
- Prevent it by braking early and gently, reducing speed before curves, and avoiding sudden steering.
Worked example
You are crossing an icy bridge and brake hard. The tractor’s drive wheels lock, the trailer keeps moving, and the tractor begins to swing sideways. Reasoning: this is a tractor jackknife caused by locked drive wheels. Release the brake, steer in the direction you want the tractor to go, and regain rolling traction. Hard braking made it worse.
Traps
- Braking hard in a curve or on a bridge deck.
- Assuming the trailer follows the tractor’s exact path.
- Continuing to hold the brake during a skid.
- Turning too soon and clipping the curb.