Single Degree-of-Freedom (SDOF) mechanisms are used in huge range of applications. An affordable, simple and purely mechanic experimental method to evaluate their energy efficiency (e.g. energy loss due to friction), is the free-deceleration method. The studies that claim to apply this experimental method to SDOF systems, always consider the inertia of the entire system as a constant value. The novelty of this study is to model the effects of considering a variable inertia when applying the free-deceleration method to evaluate energy loss due to friction. The problem formulation is based in the work-energy theorem to obtain the equation of motion for a SDOF mechanism; in this case, a slider-crank mechanism. A test stand is presented to derive experimental values of the instantaneous total friction torque, the mean total friction torque per cycle and the energy loss per cycle.