Printable Linear Bearing For 8mm Rods

In the comments from MakerBlock's blog post on the MakerBot blog, tre3 had the idea of making a linear bearing from plastic. Since I really like how my ball bearings turned out using steel BBs, I thought I'd try designing a linear bearing using the same bearings.

The first step was to get the basic BB function working. I designed a "racetrack" so the bearings could make a loop and only contact the bar in a straight line. My original idea was to have three racetracks arranged so that the BB