Twitch - Drivetrain
Twitch is separated from the average four-wheeler by its unusual drivetrain. This system, detailed below, allows the vehicle to drift and to change the orientation of its wheels, both of which make for silly and fun driving.
We have four independently driven wheels, each mounted on pegs that allow them to rotate about the z-axis. I could have driven this rotation with a servo-motor for each wheel, but that seemed excessive. If all you want to do is swap the wheels between forward-and-back and left-and-right orientations, then you'll notice that each wheel needs to turn 90 degrees, but each wheel needs to turn in the opposite direction from its neighbours. By connecting wheels at opposite corners with a linkage, we can cut the number of servos required from four down to two.
I produced the linkage bars from 1/8" aluminium cut by waterjet. They're attached to the motor assemblies by a shoulder screw, really useful little pieces of hardware for making revolute joints. The product of all this is a four-bar parallelogram linkage, forcing the orientation of opposite wheel assemblies to always be equal.
The components I used for the drive and linkage assemblies are almost identical to Shane's, as he found a nice combination of products small enough to fit inside the sandwich and powerful enough to actually work.