851 Directional stability along a straight track

Neutral steer (Fig. 8.44) Consider a vehicle moving forward along a straight path and a side force due possibly to a gust of wind which acts through the vehicle's centre of gravity which for simplicity is assumed to be mid-way between the front and rear axles. If the side force produces equal steady state slip angles on the front and rear tyres, the vehicle will move on a new straight line path at an angle to the original in proportion to the slip angles generated (Fig. 8.44). This motion is without a yaw velocity; a rotation about a vertical axis passing through the centre of gravity, and therefore is known as neutral steer.

Note that if projection lines are drawn perpendicular to the tyre tread direction of motion when the front and rear tyres are generating equal amounts of slip angle, then these lines never meet and there cannot be any rotational turn of the vehicle.

Oversteer (Fig. 8.45) If, due possibly to the suspension design, tyre construction and inflation pressure or weight distribution, the mean steady static slip angles of the rear wheels are greater than at the front when a disturbing side force acts through the vehicle centre of gravity, then the path

Fig. 8.44 Neutral steer on straight track

Direction of motion

Direction of motion

Fig. 8.45 Oversteer on straight track

of the vehicle is in a curve towards the direction of the applied side force (Fig. 8.45). The reason for this directional instability can be better understood if projection lines are drawn perpendicular to the direction the tyres roll with the generated slip angles. It can be seen that these projection lines roughly intersect each other at some common point known as the instantaneous centre, and therefore a centrifugal force will be produced which acts in the same direction as the imposed side force. Thus the whole vehicle will tend to rotate about this centre so that it tends to swing towards the disturbing force. To correct this condition known as oversteer, the driver therefore has to turn the steering in the same direction as the side force away from the centre of rotation.

Fig. 8.46 Understeer on straight track

Understeer (Fig. 8.46) Now consider the situation of a vehicle initially moving along a straight path when a disturbing side force is imposed through the vehicle's centre of gravity. This time there is a larger slip angle on the front tyres than at the rear (Fig. 8.46). Again project lines perpendicularly to the tyre tread direction of motion when they are generating their slip angles but observe that these projections meet approximately at a common point on the opposite side to that of the side force. The vehicle's directional path is now a curve away from the applied side force so that a centrifugal force will be produced which acts in opposition to the disturbing side force. Thus the vehicle will be encouraged to rotate about the instantaneous centre so that it moves in the same direction as the disturbing force. Correction for this steering condition which is known as understeer is achieved by turning the steering in the opposite direction to the disturbing force away from the instantaneous centre of rotation. It is generally agreed that an oversteer condition is dangerous and undesirable, and that the slip angles generated on the front wheels should be slightly larger than at the rear to produce a small understeer tendency.

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