Ever heard of a motorcycle with a leaf spring? It has been known – Indian, amongst many others, offered leaf-sprung forks in the 1920s and ‘30s, but now cutting edge motocross and enduro bikes could be doing the same….though not for the front end.

KTM is developing leaf spring rear suspension for dirt bikes, as revealed in a recent patent application. This shows the leaf spring mounted just behind the engine in a near vertical position. The top of the spring is clamped to a linkage which pivots on the bike’s main frame, while the lower end of the spring is held by a lower linkage, pivoting from a bracket underneath the swingarm. Pre-load is adjustable by changing the effective length of the leaf, via a screw thread and adjuster knob. The patent shows that changing the shape of the linkages also allows for a large measure of adjustability, enabling the spring to be rising, constant or falling rate.

One major advantage of the leaf spring in this set up is that it’s far more compact than a conventional coil spring, freeing space between the engine/gearbox and rear wheel for a larger airbox, inlet tract or silencer. It could also be suitable for future electric off-road bikes, giving more flexibility for packaging other components.

But KTM hasn’t been the only dirt bike manufacturer to try leaf springs in the modern era. In 1992-93 Yamaha’s YZM250 0WE4 used a single composite spring mounted horizontally under the engine/gearbox, and that bike was a race winner. Will KTM’s own leaf ever see the light of a motocross circuit? Watch this space.

The BMF’s Chair Jim Freeman said: “When it comes to suspension development, off-road competition bikes have always been at the forefront. Interesting regarding the compactness, as rear shocks and springs have been an issue since the 1970s, when the mass adoption of single shock and linkage systems started.”

Written by Peter Henshaw

Top image courtesy of Cycleworld