The SkelEx exoskeleton is a non-linear spring-based passive upper body exoskeleton that has the goal to compensate the weight of the wearer's arms. Bending of three stacked non-linear springs transfers force from the hips of the wearer to the back of the upper arm. The amount of bending of the springs is a function of the geometry of the exoskeleton. The SkelEx exoskeleton was built using an iterative design method to evaluate all the length in the exoskeleton and tuned until the output force was balanced for a certain user. Now that this exoskeleton functions for certain user performing a certain task, the consequences of changing the geometry of the exoskeleton for a different users and applications is not known. The goal of this paper is to be able to predict the change in force curve when different geometric adjustments are made to the exoskeleton. The adjustments can be made into mechanisms that can be changed in the field of operation. The mechanisms that are reviewed are: the cable length, the lever arm length and the vertical position of a spring constraint. The force curve is defined as the amount of force that the exoskeleton provides at different arm rotations for in plane motion. Finite element models were developed in several steps, each of them supported by evaluation experiments in setups that were built for this purpose. The model is able to predict the force curve when the geometry of the exoskeleton is altered. The adjustment mechanisms can be used to smoothen the force curve and alter maximum output force.