A simple steric model of so-called rfr (rigid-flexible-rigid) molecules is developed. It consists of two rigid spherocylindrical fragments linked by a flexible alkyl chain. Molecular dynamics simulations of 128 model molecules are carried out in a periodic box, and the influence of density and alkyl chain length on the molecular order and the self-diffusion is studied. Different liquid crystalline phases are found which an stable within the time scale of the simulations. In agreement with experimental results the nematic order parameters and the stability of the phase, as indicated by the location of the isotropic/liquid crystalline phase transition, show an odd-even effect with respect to the number of methylene groups in the alkyl chain. A separation of rigid and flexible fragments is observed leading to smectic liquid crystalline phases.