Professional Driver's Handbook

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Curves and jackknifing

A vehicle travelling in a straight line develops a force called momentum. The higher the speed, the greater the force. When the vehicle enters a curve, it must overcome the force of momentum if it is to change its direction from the straight line in which it has been travelling. A vehicle travelling around a curve develops a force called centrifugal force. The greater the speed at which a vehicle travels around a curve, the greater the centrifugal force developed. This centrifugal force pushes outward from the centre of the curve and tries to keep the object on its original straight line.

The degree of control you are able to maintain over your vehicle is determined by the amount of traction your vehicle's tires have with the road surface. Entering a curve too fast can result in skidding or a rollover. Applying brakes in a curve can cause a skid or a jackknife.

To avoid rolling, skidding or jackknifing, reduce your speed before reaching the curve. Enter the curve at a speed that will not require braking. This will permit you to apply gradual power in the curve. The application of a small amount of power in a curve counteracts the centrifugal force.

For the safe negotiation of curves, here are some tips to follow:

  • Spot the curve in advance by heeding the curve ahead warning sign and suggested speed sign.
  • If speed adjustment is needed, slow down before the curve.
  • Accelerate when in the curve as conditions permit.
Safe negotiation of curves


Rev: 2017