motorcycles, like the one shown in Figure 19-8, are very challenging to model for a number of reasons-the most important one being the fact that they have only two wheels. In order to stand on its own, a model requires at least three wheels (or wide, flat tires). To enable a motorcycle model to stand on its own, we can add a small, unobtrusive wheel or a full sidecar. Side cars have the advantage of offering plenty of internal space near the bike rear wheel, so they can be used to house a propulsion motor. Another alternative is to build a trike or a quad, which is something like a four wheeled bike.