  We propose a mechanism for fixing the velocity of relativistic soliton based
on the breaking of the Lorentz symmetry of the sine-Gordon (SG) model. The
proposal is first elaborated for a molecular chain model, as the simple
pendulum limit of a double pendulums chain. It is then generalized to a full
class of two-dimensional field theories of the sine-Gordon type. From a
phenomenological point of view, the mechanism allows one to select the speed of
a SG soliton just by tuning elastic couplings constants and kinematical
parameters. From a fundamental, field-theoretical point of view we show that
the characterizing features of relativistic SG solitons (existence of conserved
topological charges and stability) may be still preserved even if the Lorentz
symmetry is broken and a soliton of a given speed is selected.
