84 Cornering properties of tyres

8.4.1 Static load and standard wheel height

A vertical load acting on a wheel will radially distort the tyre casing from a circular profile to a short flat one in the region of the tread to ground interface (Fig. 8.29). The area of the tyre contact with the ground is known as the tyre contact patch area; its plan view shape is roughly elliptical. The consequence of this tyre deflection is to reduce the

Fig. 8.29 Illustration of static tyre deflection

standard height of the wheel, that is the distance between the wheel axis and the ground. Generally tyre deflection will be proportional to the radial load imposed on the wheel; increasing the tyre inflation pressure reduces the tyre deflection for a given vertical load (Fig. 8.30). Note that there is an initial deflection (Fig. 8.30) due to the weight of the wheel and tyre alone. The steepness of the load deflection curve is useful in estimating the static stiffness of the tyres which can be interpreted as a measure of its vibration and ride qualities.

8.4.2 Tyre contact patch (Figs 8.29 and 8.31)

The downward radial load imposed on a road wheel causes the circular profile of the tyre in contact with the ground to flatten and spread towards the front and rear of its natural rolling plane. When the wheel is stationary, the interface area between the tyre and ground known as the contact patch will take up an elliptical shape (Fig. 8.29), but if the wheel is now subjected to a side thrust the grip between the tread and ground will distort the patch into a semibanana configuration (Fig. 8.31 (a)). It is the ability of the tyre contact patch casing, and elements of the tread to comply and change shape due to the imposed reaction forces, which gives tyres their steering properties. Generally, radial ply tyres form longer and broader contact patches than their counterpart cross-ply tyres, hence their superior road holding.

Tyres are subjected not only to vertical forces but also to side (lateral) forces when the wheels are in

Tyre toad <kN)

Fig. 8.30 Effect of tyre vertical load on static deflection motion due to road camber, side winds, weight transfer and centrifugal force caused by travelling round bends and steering the vehicle on turns. When a side force, sometimes referred to as lateral force, is imposed on a road wheel and tyre, a reaction between the tyre tread contact patch and road surface will oppose any sideway motion. This resisting force generated at the tyre to road interface is known as the cornering force (Fig. 8.31(a and b)), its magnitude being equal to the lateral force but it acts in the opposite sense. The amount of

Fig. 8.31 Tyre tread contact patch distortion when subjected to a side force

cornering force developed increases roughly in proportion with the rise in lateral force until the grip between the tyre tread and ground diminishes. Beyond this point the cornering force cannot match further increases in lateral force with the result that tyre breakaway is likely to occur. Note that the greater the cornering force generated between tyre and ground, the greater the tyre's grip on the road.

The influencing factors which determine the amount of cornering force developed between the tyre and road are as follows:

Slip angle Initially the cornering force increases linearly with increased slip angle, but beyond about four degrees slip angle the rise in cornering force is non-linear and increases at a much reduced rate (Fig. 8.32), depending to a greater extent on tyre design.

Vertical tyre load As the vertical or radial load on the tyre is increased for a given slip angle, the cornering force rises very modestly for small slip angles but at a far greater rate with larger slip angles (Fig. 8.33).

Tyre inflation pressure Raising the tyre inflation pressure linearly increases the cornering force for a given slip angle (Fig. 8.34). These graphs also show that increasing the tyre slip angle considerably raises the cornering forces generated.

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