This paper provides a study of the mechanical energy distribution in dowel-type joints in timber structures when using expansive kits. The compression caused by the expansive kit increases the friction between the dowel and the timber’s hole, opposing the longitudinal sliding that occurs during the bending of the dowel. The ensuing rope effect increases the load capacity of the joint. The aim was to determine the advantages and disadvantages of using this kind of reinforcement. For this purpose, an ad hoc finite element model of the joint was prepared taking the contact between the different components of the joint into account and appropriately simulating the radial expansion of the dowel and the behavior of the timber. The model was checked for accuracy by comparing the results with those coming from a set of experimental tests. After that, the model was used to verify that the use of the expansive kit reinforcement leads to a slight improvement in the load capacity of the joint. This improvement is related to the frictional forces, whose effect is especially significant at low levels of joint displacement.