Results of simulation research and of some experimental tests on the influence of additional unsprung mass of in-wheel motors on the comfort and safety of a passenger car have been presented. This research had its origins in research and development works aimed at developing an additional electric powertrain for any front-wheel-drive passenger car with an IC engine, undertaken to convert such a vehicle into one with two alternative driving systems powered by engine fuel and electric energy for the car usability to be enhanced and for the car operation costs to be lowered. An idea was adopted to install two electric motors in hubs of the wheels that are not driven in the original vehicle version (rear wheels in this case). Based on analyses of similar engineering solutions and on design assumptions made by the authors, the motor mass was estimated and added to the unsprung mass in the form of extra weights fixed to the stub axle, with modifying accordingly the mass values adopted in the mathematical quarter-car suspension model that was used for the computer simulations. Results of the experimental tests were analysed with taking into account an analysis of the RMS car body acceleration value vs. frequency curves; to examine the simulation test results, the frequency response functions were calculated and analysed for sprung mass accelerations and dynamic wheel loads and the EUSAMA indicator values (normally used at shock absorber testing) were determined.