The Influence of Frame Structure on the Dynamics of Motorcycle Stability

Motorcycles are prone to two basic steering instabilities, a low-speed high-frequency flapping of the handlebars (‘wobble’ or ‘flutter’) and a high-speed low-frequency snaking of the whole machine (‘weave’), both potentially dangerous. This investigation replaced the standard production frame of a large capacity machine with two new frames of ‘duplex’ tubular type. One supported the engine mass from below - a so-called ‘cradle’ frame - and the other from above. The stiffness characteristics of both experimental frames were calculated and optimised by Finite Element techniques.

The resulting stability of the motorcycle was dramatically improved in each case, with the cradle frame showing superior flutter stability. Both frames also had excellent handling qualities. The frames were tested under a wide range of parameter variation and loading conditions.

The loss of flutter stability on fitting a currently-fashionable smaller front wheel was viewed with some concern.