Cable model has a strong influence on the complexity of the kinematic analysis of cable-driven parallel robots (CDPR). The most complete elasto-static model relies on Irvine equation that takes into account both the elasticity and the deformation of the cable due to its own mass and has been shown to be very realistic. This model is complex, non algebraic and numerically ill-conditioned, thereby leading to difficulties when using it in a kinematic analysis involving several cables. We exhibit some properties of this model that may drastically improve the analysis computation time when used in kinematic studies.