852 Directional stability on a curved track

True rolling of all four wheels can take place when projection lines drawn through the rear axle and each of the front wheel stub axles all meet at a common point somewhere along the rear axle projected line. This steering layout with the front wheels pivoted at the ends of an axle beam is known as the Ackermann principle, but strictly it can only be applied when solid tyres are used and when the vehicle travels at relatively slow speeds. With the advent of pneumatic tyres, the instantaneous centre somewhere along the extended projection from the rear axle now moves forwards relative to the rear axle. The reason for the positional change of the instantaneous centre is due to the centrifugal force produced by the vehicle negotiating a corner generating an opposing cornering force and slip angle under each tyre. Therefore projection lines drawn perpendicular to the direction each wheel tyre is moving due to the slip angles now converge somewhere ahead of the rear axle. This is essential if approximate true rolling conditions are to prevail with the vehicle travelling at speed.

Oversteer (Fig. 8.47) If the slip angles of the rear wheel tyres are made greater than on the front tyres when the vehicle is turning a corner (Fig. 8.47), the projected lines drawn perpendicular to the direction of motion of each tyre corresponding to its slip angle will all merge together at some common point (dynamic instantaneous centre) forward of the rear axle, further in and therefore at a shorter radius of turn than that produced for the Ackermann instantaneous centre for a given steering wheel angle of turn.

Under these driving conditions the vehicle will tend to steer towards the bend. Because the radius of the turn is reduced, the magnitude of the

Direction of motion

Direction of motion

Fig. 8.47 Oversteer on turns centrifugal force acting through the vehicle centre of gravity will be larger; it therefore raises the oversteer tendency of the vehicle. At higher vehicle speeds on a given circular path, the oversteer response will become more pronounced because the rise in centrifugal force will develop more tyre to ground reaction and correspondently increase the slip angles at each wheel. This is an unstable driving condition since the vehicle tends to turn more sharply into the bend as the speed rises unless the lock is reduced by the driver. For a rear wheel drive vehicle the application of tractive effort during a turn reduces the cornering stiffness and increases the slip angles of the rear wheels so that an oversteering effect is produced.

Understeer (Figs 8.48 and 8.49) If the slip angles generated on the front wheel tyres are larger than those on the rear tyre when the vehicle is turning a corner (Fig. 8.48) then projection lines drawn perpendicular to the direction of motion of each tyre, allowing for its slip angle, will now all intersect approximately at one point also forward of the rear axle, but further out at a greater radius of turn than that achieved with the Ackermann instantaneous centre.

With the larger slip angles generated on the front wheels the vehicle will tend to steer away from the bend. Because the radius of turn is larger, the magnitude of the centrifugal force produced at the centre of gravity of the vehicle will be less than for the oversteer situation. Thus the understeer tendency generally is less severe and can be corrected by turning the steering wheels more towards the bend. If tractive effort is applied when negotiat-

Fig. 8.48 Understeer on turns

ing a circular path with a front wheel drive vehicle, the cornering stiffness of the front tyres is reduced. As a result, the slip angles are increased at the front, thereby introducing an understeer effect.

A comparison between the steered angle of the front wheels or driver's steering wheel angle and vehicle speed for various steering tendencies is shown in Fig. 8.49. It can be seen that neutral steer maintains a constant steering angle throughout the vehicle's speed range, whereas both under-and oversteer tendencies increase with speed. An important difference between over- and understeer is that understeer is relatively progressive as the speed rises but oversteer increases rapidly with speed and can become dangerous.

8.6 Tyre marking identification (Tables 8.1 and 8.2)

To enable a manufacturer or customer to select the recommended original tyre or to match an equivalent tyre based on the vehicle's application

Fig. 8.49 Relationship of steer angle speed and vehicle speed of neutral steer, understeer and oversteer

requirement, wheel and tyre dimensions, tyre profile, maximum speed and load carrying capacity, a standard marking code has been devised.

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