This article will describe the simplest way to provide comfort in a vehicle and has been in use for over 300 years in some form! Prior to that, it could be argued that the bow weapon was the first implementation of the idea. In many respects it is a large spring to power the arrow over a battlefield and the English used the Longbow to great effect in the 14th and 15th centuries and I suspect that the idea was used long before that in other applications!
With respect to carriages and carts, the leaf spring as we now know it also came from England and was a simple arc of steel connecting two points of a carriage body. As the carriage bounced down a rough road, the spring absorbed the worst of the bumps. The “inventor” – and I use that loosely as the idea had been used in other applications before – was Obadiah Elliot. He was a coach builder based in London and he took strips of steel and used shackles to bind them together to provide enough strength to cope with the carriage weight and road surface.
An early form of suspension used leather straps to provide a better comfort for carts and then other materials were tried. It was the invention of spring steel that meant Elliot could take the next step in the evolution of this type of suspension. His design became known under several names: the cart spring, the semi-elliptic spring (from its shape) or the laminated spring thanks to the leaves fitting together in a similar fashion to laminated wood. The spring structure had “eyes” at each end to attach it to the bodywork and a centre bolt to connect the axle structure, thus allowing the wheel to move independently of the body – a technique still in use today. Elliot’s patent was based on the elliptic shape of his spring along with the use of spring steel.
A typical leaf spring system has a long leaf on top (called the “master” leaf) with ever shorter ones shackled below known as “graduate” leaves. This provides strength for the spring as well as flexibility at either end. These springs are typically fitted either side of a wheel, however they could also be used transversely to support a heavy component of the vehicle. The Summit Roadster used this idea to suspend the engine to provide a smoother operation.
Another development was the use of composite materials replacing the traditional steel. General Motors claimed that the composite leaf springs on the Corvette C4 from the early 1980s were up to five times more durable than a steel spring. They had fitted two glass-reinforced epoxy composite leaf springs to the car, replacing a ten-leaf spring and saved 15kg of unsprung weight. It was also transversely mounted across each axle replacing the coil springs found on earlier models. Doing this allowed for a lower, wider vehicle with better handling. This technology is now used by many European manufacturers for their light commercial vehicles.
My early memories of leaf springs were on my father’s Jeep CJ6 during the 1970s. He was typically the only person in the vehicle with a dog and heavy tools and equipment on board. The drivers side used to sag more than the passenger side and the body would start to tilt. Every now and again, he would remove a leaf or two from the passenger side to reduce the tilt. After a while he would run out of leaves to remove and would then get the whole lot reconditioned!
Traditional leaf springs are still used on heavy or utility vehicles however on passenger vehicles they have been superseded by more compact forms of suspension. They are cheaper to manufacture than other suspension units and are more suited to heavier vehicles. On the down side, the springs cannot provide as much progressive damping as coil-over shock absorbers can and as described above, they do sag over time which means more reconditioning work rather than simply replacing worn shocks.