Want to build a single speed without having to buy a dedicated frame? Try the Melvin. The Melvin bolts to the frame where the rear derailleur would normally go and allows the use of a single speed rear wheel on bikes with vertical dropouts.

The Melvin uses two pulleys, just like a rear derailleur, which gives it 20 teeth of chain wrap. This means that the Melvin will work with multiple front chain rings while maintaining proper chain tension. Of course this is no longer a true single speed, but can come in handy when two or three gears are all you need. Twenty teeth of chain wrap means that chain rings with a maximum difference of twenty teeth can be used up front.

Chain tension is optimized when the chain length is cut so that the pulleys are aligned vertically. For 1/8" cogs Paul recommend the silver SRAM PC-1 chain and any 8 speed chain for 3/32" cogs.