The Leaf Spring: A Beam That Carries the Load and Locates the Axle

Cut four strips of spring-steel strip: 300, 240, 180 and 120 mm long. Stack them centred, longest on top, and clamp them together at the middle with a bolt through a small hole (or a clamp). Bend a small eye at each end of the top leaf, or hook it over two dowels set 300 mm apart on a board. Hang weights from the centre in steps and measure the sag with the steel rule. The slope of load against sag is the rate. Compare with the notebook's formula for your strip's width and thickness (measure them with calipers). Now remove the two shortest leaves and repeat: the rate falls — and the top leaf now bends into a sharper curve at the centre, where it would crack first.

Laminated carriage springs developed through the eighteenth century; **Obadiah Elliott's elliptic spring of 1804** (a British patent) let coaches be built without the perch, and the semi-elliptic form went on to railway wagons and motor vehicles. No patent number is asserted here. **Honest limits.** A leaf spring is heavy for the energy it stores. The friction between leaves damps small bumps poorly and large ones harshly, and changes as the leaves rust. And because the spring also locates the axle, the axle can wind up under hard acceleration and braking.