In order to guarantee the new emission regulations and customer requirements, crank train components must be developed for engine operation boundary conditions. In the last years, analytical formulation is being replaced by flexible dynamic analysis. This paper shows how important it is to develop crankshafts considering engine actual cyclic loads. Moreover, stresses and fatigue safety factors are evaluated as vital outputs.The consideration of the elastic matrix in the movement equation enabled the achievement of more precise results according to the actual crankshaft behavior. The procedure described in this paper is strongly recommended for new crankshafts design regarding their actual structural behavior under engine operation condition.