  We study the effect of tidal torques on the collapse of density peaks through
the equations of motion of a shell of barionic matter falling into the central
regions of a cluster of galaxies. We calculate the time of collapse of the
perturbation taking into account the gravitational interaction of the
quadrupole moment of the system with the tidal field of the matter of the
neighbouring proto-clusters. We show that within high-density environments,
such as rich clusters of galaxies, tidal torques slow down the collapse of low
\nu peaks producing an observable variation in the time of collapse of the
shell and, as a consequence, a reduction in the mass bound to the collapsed
perturbation. Moreover, the delay of the collapse produces a tendency for less
dense regions to accrete less mass, with respect to a classical spherical
model, inducing a biasing of over-dense regions toward higher mass. Finally we
calculate the bias coefficient using a selection function properly defined
showing that for a Standard Cold Dark Matter (SCDM) model this bias can account
for a substantial part of the total bias required by observations on cluster
scales.
