Q Could you explain the terms roll-steer and roll stiffness and state how these characteristics influence the handling of a vehicle?
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AIn the case of a conventional leaf spring
suspension fitted to the rear axle of a vehicle, the effective distance between the front spring eye and the axle increases with deflection of the spring under load and is reduced when the load is reduced. When the vehicle is travelling round a curve at speed, load is transferred from the inner to the outer spring because of centrifugal farce acting on the sprung mass and the outer end of the axle is moved back slightly while the inner end is moved forward. Displacement of the axle in this way tends to steer the rear of the vehicle outwards and this is known as roll steer. As a result, the vehicle will oversteer and may be unstable when cornering or swerving at high speed.
The example of rear-axle roll-steer is taken because roll-steer of the front steering axle is automatically corrected by the driver and is not a serious characteristic. Under-inflated tyres will increase the steering effect; even without roll steer deflection of the tyre treads will cause the back of the vehicle to move sideways at an angle to the plane of the wheel and this also produces oversteer.
If the design of the suspension is such that the distance between the front anchorage point and the axle is reduced when load is increased the axle will tend to steer inwards on a corner and the vehicle will understeer.
The resistance to roll of a vehicle is proportional to the roll stiffness of the suspension system. This depends on the stiffness of the springs and the distance between them, which is known as the spring base. In some types of air suspension the springs are outrigged from the chassis at the rear in line with the wheels and this gives a wide spring base together with increased roll resistance.
It is more important that roll resistance be high at the back of the vehicle than the front. Excessive roll at the rear can increase roll steer unpredictably and on an S-bend the resultant "kick back" at a critical moment may increase the tendency of the vehicle to overturn.