8412Lateral weight transfer

For a given slip angle the cornering force generally increases with the increase in vertical load. This increase in cornering force with respect to vertical load is relatively small with small slip angles, but as larger slip angles are developed between the tyre and ground increased vertical load enables much greater cornering forces to be generated (Fig. 8.42). Unfortunately the relationship between cornering force and vertical load is non-linear. This is because

Fig. 8.40 Camber steer producing toe-out

Fig. 8.41 (a and b) Principle of camber steer

Load tkN)

Fig. 8.42 Effect of transverse load transfer on the cornering force developed by a pair of tyres attached to axle

Load tkN)

Fig. 8.42 Effect of transverse load transfer on the cornering force developed by a pair of tyres attached to axle

Fig. 8.43 Load transfer with body roll

an initial increase in vertical wheel load where the curve rise is steep produces a relatively large increase in cornering force, but as the imposed loading on the wheel becomes much larger a similar rise in vertical load does not produce a corresponding proportional increase in cornering force.

Consider a pair of tyres on a beam axle (Fig. 8.43), each with a normal vertical load of 3 kN. The cornering force per tyre with this load will be 2 kN for a given slip angle of 6°. If the vehicle is subjected to body roll under steady state movement on a curved track, then there will be certain amount of lateral weight transfer. Thus if the normal load on the inside wheel is reduced by 1.5 kN, the load on the outer wheel will be increased by the same amount.

As a result the total cornering force of the two tyres subjected to body roll will be 1.3 + 2.3 = 3.6 kN (Fig. 8.42) which is less than the sum of both tyre cornering forces when they support their normal vertical load of 2 x 2 = 4 kN. The difference between the normal and body roll tyre loading thus reduces the cornering force capability for a given slip angle by 0.4 kN. This demonstrates that a pair of tyres on the front or rear axle to develop the required amount of cornering force to oppose a given centrifugal force and compensate for lateral weight transfer must increase the slip angles of both tyres. Thus minimizing body roll will reduce the slip angles necessary to sustain a vehicle at a given speed on a circular track.

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