This is a pair of mirror-image rattleback models derived from jasay's original thing.

When the "cw" model is spun clockwise it will reverse direction to spin counterclockwise.

When the "ccw" model is spun counterclockwise it will reverse direction to spin clockwise.

Demo video at http://mysd300.blogspot.com/2010/12/rattleback-twins.html

These models differ from jasay's mesh in the following ways:

• The Z-axis height has been scaled down by 45%. (jasay has posted a comparable model)

• The base of the model has been aligned to Z=0 for the convenience of skeinforge users.

• The flat side faces down so Makerbot users won't need to reorient the model or use a raft.

(But if your build process and software can do it, I recommend building the part sideways because it'll produce smoother contours. That's how I built the rattlebacks in the video.)