The potential benefits of using advanced fiber-reinforced composites as an alternative to metallic alloys has been investigated for the design and fabrication of connecting rods in motion conversion mechanisms for internal combustion engines. Two types of mechanisms have been selected for this analysis: the common slider-crank mechanism and the new. Stiller-Smith Mechanism, in which the crankshaft is replaced by a floating gear system. An improved finite-element elastodynamic model, which includes the effects of longitudinal, bending and shear deformations, has been developed in order to quantify the relationships between the levels of bearing loads and vibrations of such mechanisms and the material design of their connecting-rods.An extensive parametric study has been conducted on the material system, the lay-up and the cross-sectional dimensions of elastic connecting rods, made of helically wound composite materials. The results indicate that the vibration levels in such motion conversion mechanisms may be reduced significantly by employing connecting-rods made of properly tailored composites.