Losses and trapping regions in real and velocity space have been studied in the TJ-II flexible heliac for collisionless ions of low and intermediate energy (0.1-1 keV, roughly). The complexity of the magnetic field structure is described with emphasis on the ripple, Fourier mode composition, angular dependences and drift velocity behaviour. The resulting strong asymmetries of the loss distributions are discussed and explained. The influence of pitch and position is analysed by means of different complementary diagrams and a strong sensitivity to the angular position of trapping regions is shown. Electric field effects have not been included, this will be done in a companion article, with particular stress on the influence of resonances.