I just bought a fixed gear bike with a flip-flop hub, but it has absolutely terrible gearing, something like 87 gear inches. I know how many teeth I need on the new cog, but I don't know what kind of cog I need. I've gone on Jenson USA and they have track cogs and BMX single speed cogs from Surly. I don't know what kind I should be getting and any links to the right cog with 21 teeth would be greatly appreciated.