In this study, we developed a simulation method for rough road running condition to reproduce the behaviors of a vehicle body and to precisely estimate the input loads to the frame.We designed the simulation method focusing on a front fork model and a rider model optimized for this type of analysis. In the suspension model development, we conducted detailed measurement of the suspension characteristics on a test bench. Based on the yielded results, the friction force, as well as the spring reaction force and the damping force, was reproduced in the suspension model. The friction of the suspension varies depending on the magnitude of the reaction force associated with bending and this effect was also implemented in the model.Regarding the rider model, the actual behavior of a rider was investigated through the recorded motion video data and used to define the necessary degrees of freedom. Based on this investigation, the degree of freedom of three, i.e. vertical, longitudinal and forward leaning motions, was adopted to the rider model. In addition, the model was designed assuming that the rider is connected to the vehicle body at the points of the handle bar, the foot pegs and the seat through springs and dampers.Using the multi-body dynamics model composed of the aforementioned model elements, the input load to the frame was accurately estimated within an error of 10% to the actual measurement result.