Suspension  - Springing

The object of the springing is to transform the short sharp jolts from the road into soft damped oscillations, so that only quite small forces are transmitted to the vehicle body itself. Also, the spring must prevent the wheels being lifted off the road as a sudden jolt occurs and thus safeguard the ‘grip’ or ‘ground adhesion’ of the vehicle.

Leaf springs are the oldest - and still frequently applied - springing system. Such springs consist of a number of steel strips (spring leaves) which are stacked one upon another and are held together by spring clips. The longest leaf of the spring is formed with an eye at each end through which a mounting pin is inserted.

At one end the spring is mounted in a bearing bracket, while the other end is attached to a spring shackle which takes up the elongation of the spring when it deflects. Usually leaf springs of the semi-elliptic type are used as longitudinal springs with rigid axles or as transverse springs.

Longitudinal springs are fixed at their ends, the sprung mass being supported at the centre of the spring, whereas transverse springs are fixed at the centre, while the sprung mass is supported by the ends.

Coil springs are increasingly used in modern automobile engineering. They are lighter than leaf springs and require less maintenance, but they do not possess oscillation-damping properties. What is desired of springs is that they should be very yielding at low loads and become stiffer at higher loads. A spring is said to be stiffer than another if it requires a greater force than the other spring to shorten or lengthen it a certain amount. In the case of a coil spring the force applied is proportional to the shortening or elongation it produces, i.e., it is equally stiff for all values of the load.

A progressively acting spring fulfils the requirement as to increasing stiffness with increasing load. In this arrangement, a load increase first brings the main spring into action; next, the auxiliary spring and finally also the resilient rubber pads come into operation.

The functioning principle of a torsion bar suspension system: One end of a torsion bar is fixed in the frame of the vehicle, while the other end is connected to a lever arm (pull rod). The spring actions are transmitted to the wheel by the lever. They depend upon the thickness and length of the torsion bar; thick short bars are stiffer than thin long ones.

Torsion rods consisting of bundled strips of spring steel may be used instead of solid rods of circular section.