Rotafix
The Rotafix is a method of tightening or loosening a sprocket on a track bike or fixed gear bicycle.
Method
The Rotafix enables the securing of a sprocket ("cog") to a track hub without the use of a chainwhip.
The chain is removed from the chainring and rests around the bottom bracket shell and on the sprocket with the rear wheel in the horizontal track ends. Wrap the chain around the sprocket so that it secures itself, shown here. To secure the sprocket onto the hub, when facing the drive side of the bicycle, rotate the wheel counter-clockwise until it is not possible to turn any more.
Reverse Rotafix: to remove the sprocket, reverse the direction of the chain wrap and wheel turning direction.