  We seek to explore the internal dynamics of the cluster Abell 1758N, which
has been shown to host a radio halo and two relics, and is known to be a
merging bimodal cluster. Our analysis is mainly based on new redshift data for
137 galaxies acquired at the Telescopio Nazionale Galileo, only four of which
have redshifts previously listed in the literature. We also used photometric
data from the Sloan Digital Sky Survey and from the Canada-France-Hawaii
Telescope archive. We combined galaxy velocities and positions to select 92
cluster galaxies and analyzed the internal cluster dynamics. We estimate a
cluster redshift of <z>=0.2782 and quite a high line-of-sight (LOS) velocity
dispersion of ~ 1300 km/s. Our 2D analysis confirms the presence of a bimodal
structure along the NW-SE direction. We add several pieces of information to
the previous merging scenario: the two subclusters (here A1758N(NW) and
A1758N(SE)) cannot be separated in the velocity analyses and we deduce a small
LOS velocity difference of ~300 km/s in the cluster rest-frame. The velocity
information successfully shows that A1758N is surrounded by two small groups
and active galaxies infalling onto, or escaping from, the cluster. Removing the
two groups, we estimate ~1000 km/s and ~800 km/s for the velocity dispertions
of A1758N(NW) and A1758N(SE), respectively. We find that Abell 1758N is a very
massive cluster with a range of M=2-3 10^15 solar masses, depending on the
adopted model. As expected for clusters that host powerful, extended, diffuse
radio emissions, Abell 1758N is a major cluster merger just forming a massive
system.
