Single cogs are available in two widths:
- 1/8" - BMX & old-school track standard
- 3/32" - 'Road' standard (you want THIS width)
Where do you get this sort of cog...? Try Surly Bikes or your funky, esoteric local shop. Places like Bikesmith or Recycled Cycles or even Gregg's have been know to stock cogs.
What size do I get? I recommend getting one with 18 teeth (combined with a 42-tooth chainring). See my gearing page for more on this topic.
