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Convertable fixies

With the recent fad of fixies/single speeds, there are some bikes on the market now that are sold as a normal 21/etc-speed bike. However, they also include horizontal dropouts and/or an eccentric bottom bracket, to make it easier for the owner to later convert the bike to a SS/fixie if they decide to.

So... the obvious question then is: If one wants to experiment with both for a while, without committing to a fixie... what's the easiest way to convert a bike back and forth between fixie/SS and normal shifting? Obviously, you could remove both derailleurs, but that's a lot of work. Is there a quicker way?

Rear hub

Most obviously, just swap out the multi-sprocket rear tire for one that has a fixie/SS hub.

However, you can actually just use the same multi-sprocket hub, and just setup the chain to run on just one of the sprockets. The most important part of converting the rear end actually is disabling the rear derailleur... changing the hub is optional.

Disabling rear derailleur

I think you can just re-route the chain so that it doesn't go through the rear-derailleur anymore. (probably have to use a chain-tool to first remove the chain).

(TODO: I'm not 100% sure on this... is there anyplace that confirms?)

Crankset

It shouldn't be too hard to actually swap cranksets on/off. They're just connected to the bottom bracket axle with a small pin.

But it should also be possible to use just one of the existing chainrings while in fixie/SS mode.

Disabling front derailleur

This might be the hackiest part of a convertable.

The most trivial thing to do is just leave it enabled, but sternly warn the rider "don't touch this shifter!" (since the chain has no slack, moving the derailleur will probably cause it to instantly jump off) However, even the best rider might accidentally hit the shifter in hard riding, so other solutions might be preferred.

Another way is to set the derailleur position on the opposite side from the one-gear you want to use, break the chain, and re-route the chain outside the derailleur. Then (on a 3-cog crankset) you probably have enough slack to accidentally shift one gear (but shifting all the way over would still cause the chain to come off).

Another possibility is to put so much slack in the derailleur cable that no matter what position the handlebar shifter is put in, the derailleur won't move. However, this requires a lot of work to re-adjust it when you switch back.

So, possibly the best comprimise is just to cable-tie the derailleur to the far-inside position. (if that's the single sprocket you want to use, then route the chain through the derailleur; otherwise route the chain outside of it) Hey, if it works for Guantanamo detainees, it'll work for this.