So the first thing I learnt when looking into building a singlespeed bike is that you ideally need a frame with horizontal dropouts rather than vertical ones.
The second lesson I had to learn was what ‘dropout’ actually are. For those of you like me who don’t know what they are the following picture should help to explain.
Frame with horizontal dropouts
The dropouts are the slots in the frame shown within the red circle in the above picture. It is these slots that the rear wheel goes into when inserted into the frame.
The dropouts on this frame are horizontal and the benefit for a fixie project is that these slots can be used to move the rear wheel back and forth to get the right chain tension.
Racing bikes with multiple gears have a rear derailleur that maintains chain tension whatever cogs are being used and so lots of racing frames have vertical dropouts. If my frame had vertical dropouts I wouldn’t be able to move the rear wheel to tension the chain and I would need to fit a special chain tensioner.