How to tighten a track cog without a chain whip - In my opinion, this works way better than with a whip.
How to wrap bullhorn handlebars - Note: its different than with brake levers.
How to build a cheap fixie - Almost any old road bike will work (70's frame anyone? mmmm)
Other useful tools:
Skid patch calculator
Gear inch/ ratio calculator - In my experience around a 70-75gi is good for flat city riding with few intersections. 70 is good for quicker acceleration. A much lower ratio would be easier for polo and freestyle. (use that flip flop fixed/fixed hub son!)
