Cross Wind:

All Commercial vehicles are influenced by cross wind due to their size and load. The effect is noticeable in case of sudden cross wind or when the vehicle emerging out of the tunnel. The cross wind can even lead to shifting of the vehicle side ways at high speeds.

The surface force caused by the cross wind can be replaced by a single force exerted in the pressure point of the surfaces. Depending on the position of the pressure point and center of gravity in relation to one another, sideways shift or Yawing of the vehicle can occur. As the pressure point is normally located above the instantaneous center, there is additional rolling of the vehicle.

Lateral and Peripheral Forces

In the moving vehicles, lateral forces and peripheral forces ie. forward driving or braking forces are often transferred simultaneously from the tyres to the road surface. A free rolling wheel can built up the maximum cornering forces. However if the peripheral forces have to be transferred simultaneously via this wheel to the ground , the possible cornering forces are reduced. 

 The above illustration shows for the relationship between lateral and peripheral forces on cornering under the given tyre slip angle. The maximum transferable forces in any direction are indicated as envelope H.

In the event of braking or accelerating during cornering, the commercial vehicle can swerve because the maximum cornering forces are no longer sufficient to maintain the course.

Braking on cornering:

If the commercial vehicle moves in the curve with constant speed (Point A in fig) and low drive power, the required cornering forces are within the envelope H- a stable driving state. In the event of sudden braking the peripheral forces change direction and simultaneously rise drastically. However as the cornering has not yet been reduced, the necessary cornering forces remain at the same level, but can not be achieved with the assumed tyre slip angle.

Excessive hard braking shift the operating point from A to B (Point B in fig) and it is then located out side the envelope. The less braking pressure and greater turning (greater tyre slip angle) enables safe deceleration of the vehicle. Point C still within the envelope. More braking pressure can only be applied when the speed and thus the cornering forces have been reduced (Point D in fig).

  ……..Next session to follow: A WONDER OF TRACTION EFFICIENCY


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