Multi-speed Fixie?
 

how hard would it be to modify a Shimano or SRAM internal-shifting hub to fixed? I think it could be as simple as reversing some of the pawls so it can't freewheel, but I've not played with one in person...
Aaron

there are some old multi-speed internally-geared hubs that don't allow coasting or backpedaling. Sheldon's got a couple of them in his fleet of fixed-gear bikes, one with the bike's drivetrain on the left side of the bike. But I have no idea about the mechanics of modifying an internal-gear hub designed for freewheeling.
I doubt that reversing some of the pawls would do the trick, because I'm sure that the gears are designed to exert force in both possible directions of pedaling.

Very, very, very difficult. There are schematics online for making this kind of modification to a Sturmey-Archer AW (getting two fixed gears instead of three freewheeling gears), but you'd be completely on your own here. Furthermore, why on earth would you WANT to do this to a more modern Shimano or SRAM hub? These are nice hubs, they are very good at what they do, and you're likely to impair their function in some way even if you somehow succeed in modifying them. If you're going to take a chance on this kind of modification, why not use a hub that a) is dirt cheap, and b) gives you a snowball's chance in hell of getting it right on the first try? SA AWs are everywhere, and there are at least rudimentary instructions online. Do it with one of them. It seems a shame, not to mention really pointless and likely impossible, to undertake this kind of surgery on a modern hub.

Well, he thought it would be pretty cool to have a 7-speed fixed-gear bike. Which would be great - best of both worlds in my book. That said, I'm totally with you on the practical stuff here. It would be cool if some company designed a 7- or 8-speed fixed-gear hub. But it'd be low-volume demand.
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