Two theoretical models are described for simulating the handling characteristics of automobiles towing trailers. These models are similar except that one excludes roll dynamically and has only four degrees of freedom, whereas the other includes roll dynamically and has six degrees of freedom. Both models are extensively validated by comparing digital-computer results obtained from them with experimental data obtained by Systems Technology, Inc. Both models yielded excellent simulations of measures of handling characteristics, the model with six degrees of freedom being slightly superior. Nonetheless, the most important conclusion of this study is that excellent simulations of handling characteristics of car-trailer systems can be obtained with a model having only four degrees of freedom. The study includes a sensitivity analysis to indicate the importance of certain effects included in the models.