Update: I recommend the stainless steel-plus-plastic version of this cog (parts here
) -- this all-plastic version wears pretty quickly.
This cog is intended to be printed in plastic (e.g., "White strong and flexible"). It fits standard hyperglide splines, such as those on the Brompton one- and two-speed hub. The teeth fit a Gates Carbon Drive Centertrack belt.
The flanges on each side of the cog teeth provide additional strength, which appears to be sufficient, given that I've now been using it for about 8 months. The three radial holes are actually threaded for M3x0.5 set screws, which engage the circumferential groove on the Brompton hub (which normally holds a retaining ring). The threads are a bit too fine for the resolution of the print; I also threaded the holes by hand with an M3x0.5 tap, but one probably could twist a set screw into the plastic without first tapping the holes! A 4-mm long set-screw works well.
This setup is a good fit on my Brompton with a 60-tooth beltwheel in front and a 113-tooth belt, although I did file out the dropout by about half a millimeter -- I needed a little bit more tension to keep the belt from skipping teeth under load. Also, it's best to use the smaller, 12-tooth idler
because this 14-tooth cog is pretty small -- there's not much clearance between the belt and the chainstay. See also http://dkprojects.net/belt_drive_brompton
This works very well, and is very light.