Parabolic leaf spring

A parabolic leaf spring does the same job as a conventional multi-leaf pack but with far fewer leaves, typically two or three instead of eight or ten. Each leaf is rolled so that it is thick at the centre, where bending stress is highest, and tapers towards the eyes following a parabolic curve. Because every leaf carries load along its full length, one tapered leaf can do the work of several flat ones. The leaves touch only at the centre clamp and at the ends, where small spacers keep an air gap between them. Tata Motors, Ashok Leyland and Eicher fit parabolic springs to the front axles of many of their medium and heavy trucks, and to both axles on many light commercial vehicles.

• The axle pushes up and the tapered leaves bend as beams, each along its whole length.
• With a gap between the leaves there is almost no rubbing, so the spring moves freely and the damper, not friction, controls the motion.
• The spring flattens and lengthens, and the shackle at one end swings to allow it.
• With the load removed the leaves return to their arch with no sticking.

The driver notices a markedly smoother ride than a multi-leaf truck, especially when empty, and less steering kick on a leaf-sprung front axle. Fleet owners like the weight saving, which adds to payload, and the lack of leaf-to-leaf wear. The leaves must be made with precise rolling and heat treatment, so they cost more, and because each leaf carries so much of the load the spring is less tolerant of gross overloading than a thick multi-leaf stack.