Summary

Print some accessories to to attach to a rat trap and turn it into a spring-loaded car!

Note that I have not yet successfully gotten this to work. Much more detailed information is available on this blog post: http://netninja.com/2013/11/03/on-designing-and-building-the-rat-racer

Download ratracer-src.zip for the OpenSCAD source for all three components.

Instructions

Print two end caps, two axles, and two pairs of wheels.
You will have to provide your own rat trap. This is sized to the Victor brand rat trap. Alter the OpenSCAD source files if yours has other dimensions.
Use pliers to remove the "cheese platform" trigger and retaining bar from the base. You should just have the spring-loaded U bar remaining.
Attach the end caps.
Push an axle through each end cap.
Attach the wheels
Tie a string to the rat trap's U-bar

Pull back the U-bar and, while carefully holding it down and not releasing it onto your fingers, thread the string through the hole in the axle. Wind it around the axle. Put the car on the floor. Quickly release the snappy part. In theory, the string is pulled, spinning the axle and wheels, making the car go. In practice, I haven't gotten that far yet.

Rat Racer