The thermal modeling and design process of highly utilized electric machines is a complex task. The main heat paths within an electric drive are influenced by a variety of different thermal interfaces, that can mainly be described by thermal conduction and convection. The proper selection and estimation of these design parameters is challenging especially in the early design stage. This paper provides a systematic investigation evaluating inaccuracies and design influences of single thermal resistances, such as the convective heat transfer in the rotor-stator annulus, in the cooling jacket or interfaces between different machine components. Within the study, electromagnetic finite element models and thermal lumped parameter models of two traction drives are developed and validated using test bench measurements. The thermal design parameters are varied in a range resulting from intensive literature research and own laboratory experience. The influence of the design parameters in different operational points of the drives are studied in a sensitivity analysis.