Designing the underlying mechanical structure of lower limb exoskeletons for assistance and rehabilitation is a demanding task that requires a good understanding of the interaction that takes place between the exoskeleton and the human user wearing it. Often the effects of a given mechanical design on the user are not straightforward or intuitive. One obstacle for research is that existing rehabilitation systems do not offer the flexibility that is necessary to investigate different designs and ideas. In this paper we present a passive experimental lower limb exoskeleton that is specifically built to evaluate exoskeleton design-elements and different characteristics. These are namely a joint misalignment compensation mechanism, a three DOF hip joint design, the lack of mechanical transparency as well as the placement of the interfacing cuffs. The motivation and mechanical design of the system is presented along with the results of pilot trials to validate the system as a suitable experimental platform for our investigations © 2015 IEEE.