The thesis deals with the optimization of the axle of an unbraked trailer. The main objective of the thesis is to design a new attachment for the trailing bar of the axle to the supporting frame of the trailer. The theoretical part focuses on an overview of the trailers, their development in terms of history, and their classification according to different criteria and legislation. The content also covers the axles of trailers together with the different types of suspension, special types of axles, and the trends of development in this area. The theoretical part concludes with the theoretical foundations applied to the topic, which include FEM analysis and Creo Parametric software. The introduction of the practical part describes the designed car trailer, the simulation of its original state before the damage, and the list of requirements needed to elaborate the optimization proposals. Based on this list, four design variants of the attachment of the trailing axle bar to the trailer frame are made, where for each of them the simulation results at a load of 600 kg are shown. The different variants were evaluated and the most suitable design variant was selected based on the two-point method and the simulation results, and further compared with the original car trailer. The practical part also includes a description of the design of the optimization variant, which is implemented using the Creo Parametric software, and also a description of the procedure of all FEM simulations in the thesis, which were made using the Ansys Workbench software. The technical documentation for the optimization variant has been developed and at the end of the thesis, it has been evaluated from a techno-economic point of view.