Reuleaux Triangle

UPDATE: You might want to use the new screwless version of this design I made: http://www.thingiverse.com/thing:12644

A curious shape with constant width. It rolls smoothly in a square but the centre of rotation is not constant.

http://en.wikipedia.org/wiki/Reuleaux_triangle

More math about the Reuleaux Triangle (and the site where the cool animated gif that shows the path of the center of rotation is from):
http://mathworld.wolfram.com/ReuleauxTriangle.html