  The dynamical instability of rough hard-disk fluids in two dimensions is
characterized through the Lyapunov spectrum and the Kolmogorov-Sinai entropy,
$h_{KS}$, for a wide range of densities and moments of inertia $I$. For small
$I$ the spectrum separates into translation-dominated and rotation-dominated
parts. With increasing $I$ the rotation-dominated part is gradually filled in
at the expense of translation, until such a separation becomes meaningless. At
any density, the rate of phase-space mixing, given by $h_{KS}$, becomes less
and less effective the more the rotation affects the dynamics. However, the
degree of dynamical chaos, measured by the maximum Lyapunov exponent, is only
enhanced by the rotational degrees of freedom for high-density gases, but is
diminished for lower densities. Surprisingly, no traces of Lyapunov modes were
found in the spectrum for larger moments of inertia. The spatial localization
of the perturbation vector associated with the maximum exponent however
persists for any $I$.
