This paper deals with the lateral running behaviour of flat belts which can be found in industrial production processes (such as steel, paper or plastics) as continuous running process bands as well as transport bands for conveying purposes. Without any guidance measures, flat belts supported and driven by cylindrical pulleys run in an unstable condition, therefore angled pulley systems are employed for flat belt guidance. This paper deals with a mechanical simulation model for the guiding effect of a three-pulley configuration with the focus on the geometrical-mechanical relations. The belt's lateral movement as well as its bending stress during the guiding process is calculated. The aim of this investigation is to select pulley positions and shift directions that have adequate steering features despite low belt strain. To avoid instable lateral running conditions when applying an automatic controlled guidance system, proper positions for the belt edge detector are discussed. This paper is a cooperation between Vienna University of Technology, Upper Austria University of Applied Sciences and industrial partners who employ angled pulleys for flat belt guidance. The presented simulation method has been successfully applied for the optimisation of the configuration of the pulleys for a flat steel process belt in the new development of a drying system.