To facilitate the rational design analysis of off-road bicycle frames, this paper reports a dynamic model of the bicycle/rider system which estimates frame loads for bicycles traveling over rough surfaces. To develop this model, the frame loads at rider contact points were first measured experimentally as a rider coasted over a test track with randomly spaced surface irregularities in both seated and standing positions. Following this measurement, a dynamic system model was developed with the aid of the commercial software package, Dynamic Analysis and Design Software (DADS). The development process consisted of two stages. The first stage involved developing component models (e.g., seat) which could reproduce the essentials of the measured data. The second stage involved identification of parameter values for those parameters shown to most profoundly affect simulated load results. These parameters were all associated with the dynamics of the body thus emphasizing the importance of reflecting the dynamic system nature of the body in the model. Following parameter identification, simulated loads were statistically tested and were not significantly different from experimental loads. Also, maximum magnitude dynamic loads were compared to corresponding static load magnitudes and were larger for eight of the ten load components measured. With the importance of dynamic loads demonstrated and the model verified, it can now be used advantageously to generate a loading database for design analysis purposes.