By the results of computer simulation with orthogonal polynomial expansion of quality engineering performed on drivetrain bending vibration with propeller shaft, the followings have been clarified. Using an additivity of factorial effects of first-order coefficient of orthogonal polynomial expansion and selecting signal factors separated into the case of short length and long length of propeller shafts, we propose a conditional inequality of design parameters that meet the objective values. Then, we propose a sliding approach of control factors based on the conditional inequality, and a sliding approach of control factors based on S/N ratio and first-order coefficient, in order to obtain optimal solutions. By applying the aforementioned conditional inequality and the sliding approaches to drivetrain bending vibration of heavy duty truck, we could tune the vibration levels to objective values with the design parameters.