An articulated vehicle is most at risk of jack-knifing during which condition?

• A. While it's manoeuvring slowly
• B. When it's under heavy braking
• C. When it's fully loaded
• D. In light drizzle with no load

Answer: (B)

Jackknifing happens when the trailer swings in relation to the tractor around the hitch as you slow down. In an articulated vehicle, the drawbar acts like a pivot, and the trailer’s inertia keeps it moving forward while the tractor decelerates. If you brake hard, the deceleration of the tractor becomes very large while the trailer may not slow as quickly (or its wheels may lock), creating a big mismatch in how both parts slow down. That mismatch makes the trailer yaw sharply and swing toward the cab, forming the characteristic “jackknife.”

So heavy braking is the condition that creates the largest risk because the forces involved in stopping are greatest, making any loss of coordinated deceleration between tractor and trailer most likely. Slower maneuvering reduces those forces, being fully loaded adds momentum but isn’t the primary factor by itself, and light drizzle with no load reduces both speed and inertia, lowering the chance of a jackknife.