text: Many cyclists choose to convert freewheel bicycles to fixed gear. Frames with horizontal dropouts are straightforward to convert, frames with vertical dropouts less so. One method is to simply replace the rear wheel with a wheel that has a track/fixed hub. Another is to use a hub designed for use with a threaded multi-speed freewheel. Such a hub only has the normal right-handed threads for the sprocket and not the reverse threads for the lockrings used on track/fixed hubs. The sprocket on a hub without a lockring may unscrew while back pedalling. Even if a bottom bracket lockring is threaded onto the hub, along with a track sprocket, because the bottom-bracket lockring is not reverse threaded, the possibility still exists that both the sprocket and locknut can unscrew. Therefore, it is recommended to have both front and rear brakes on a fixed-gear bicycle using a converted freewheel hub in case the sprocket unscrews while backpedaling. It is also advisable to use a thread sealer for the sprocket and bottom bracket locking. The rotafix (or "frame whipping") method may be helpful to securely install the sprocket.
  Chain tension
  Bicycles with vertical dropouts and no derailleur require some way to adjust chain tension. Most bicycles with horizontal dropouts can be tensioned by moving the wheel forward or backward in the dropouts. Bicycles with vertical dropouts can also be converted with some additional hardware. Possibilities include:
  An eccentric hub or bottom bracket allows the off center axle or bottom bracket spindle to pivot and change the chain tension.
  A ghost or floating chainring is an additional chainring in the drive train between the driving chainring and sprocket. The top of the chain moves it forward at the same speed that the bottom of the chain moves it backwards, giving the appearance that it is floating in the chain.
  A magic gear—the right math can calculate a gearing ratio to fit a taut chain between the rear dropout and bottom bracket.  Also, using a chain half link and slightly filing the dropouts to increase the width of the slot increases the chances of finding a magic gear. It is worth noting that the magic gear setup is controversial, due to inevitable chain stretch and subsequent slippage that can lead to serious injury.