You could build a single speed belt drive bike without the ‘separable chain stays’ (and without using the elevated chain stays that b.dub suggested) by having the belt cog on the outside of the chain stays — just like the crank arms on a unicycle are on the outside of the unicycle forks. The four most obvious problems with this are:
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You will need to have a custom rear hub. This will be like a unicycle hub with a belt cog replacing one of the crank arms, and with no crank arm on the other side (note: a ‘flip-flop’ hub with different size belt cogs on each side won’t work unless you have a very wide range of adjustment for ‘belt-tension’ - see 3. below).
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You will need custom ‘dropouts’ to hold the rear hub. These dropouts will be like the ends of a unicycle fork that hold the unicycle hub/axle/cranks in place. These ‘drop outs’ can be held in place by two bolts on each side.
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Using those ‘dropouts’ will mean that you can’t easily tension the belt by moving the rear axle backward or forward. In order words, it’ll be like having vertical dropouts. In order to achieve correct ‘belt tension’ you will have to either:
(a) use a ‘belt tensioning device’ (i.e. like a belt-drive version of the Surly Singleator - see Parts | Singleator );
(b) use an eccentric bottom bracket; or
(c) mount the dropouts on sliding mounts (something like this: Greeny’s Sliding Dropouts ). -
This arrangement would put the belt cog ‘out-board’ of the chain stays and, without any other changes, would increase the chain-line (or ‘belt-line’) immensely. That would result in an unacceptably high ‘Q-factor’ (i.e. the see Sheldon Brown’s glossary Sheldon Brown’s Bicycle Glossary Q ) – even assuming that you could find a suitable crank-set. Reducing your belt-line could be your biggest problem (boom-tish!). The best answer is probably a very narrow rear hub. That will mean that the frame will have to have correspondingly narrow spacing between the ‘dropouts’. However, you will already be using a custom rear hub and a custom frame in order to address the other issues.
All of that custom engineering won’t be cheap. The ‘separable chain stay’ solution is much more affordable because it uses more stock components. The idea of actually building the bike’s chain stays around the belt is even less expensive. Indeed, that solution turn out to be the future of department store bikes — as those bikes are more likely to be junked before the belt wears out and the buyers of those bikes will probably be attracted by the lower maintenance needs of a belt drive drive-train.