CGCookie - Rigging Train Wheels in Blender

https://cgcookie.com/courses/rigging-train-wheels-in-blender

In this course, we'll rig a set of locomotive wheels in Blender using constraints. We'll focus on converting rotating motion to reciprocal (back and forth) motion. This type of rigging can be very difficult, and is most often achieved with the use of Armatures and Inverse Kinetics (IK). In this course, however, we'll look at a way to do it using the Locked Track, Copy Location, and Shrinkwrap constraints. During the first video, we'll take a look at the final rig and briefly discuss the Shrinkwrap constraint. In the second video, we'll do the bulk of the rigging work. Here we'll look at some of the difficulties involved in this rig, and how the obvious solutions don't quite work. Then we'll set up a constraint-based rig that does work. In the last video, we'll do some additional rigging on the wheels. We'll also use a Transformation constraint so that the wheel automatically turn as we move the train forward. Geometry of train wheels explained in PDF document by Terry Hancock (re-used with permission) and included with the downloadable source files..