  In a fish-bone model for suspension bridges studied by us in a previous paper
we introduce linear aerodynamic forces. We numerically analyze the role of
these forces and we theoretically show that they do not influence the onset of
torsional oscillations. This suggests a new explanation for the origin of
instability in suspension bridges: it is a combined interaction between
structural nonlinearity and aerodynamics and it follows a precise pattern. This
gives an answer to a long-standing question about the origin of torsional
instability in suspension bridges.
