I ran a two speed set up on my Crosscheck for a while. I had a FSA double with cyclocross rings (48 and 38) up front, a Novatec single speed FREEWHEEL MTB hub with a 16 tooth sprocket in the rear, and a Paul's Melvin chain tensioner in between to take up the slack.

I stressed FREEWHEEL MTB hub, because I've never seen anyone run an effective fixed gear set-up with a derailleur, or any other kind of chain tensioner. As far as I can imagine, if you tried to back pedal on a drivetrain with that much slack in it things would get interesting in a hurry.

The Paul's Melvin tensioner I used resembles a rear derailleur, because it routes the chain the same way, but is built much lighter since it doesn't have to actually shift the chain between gears.

The two seed was fun for a while. I certainly never saw another one coming the other way. And I guess the novelty of the thing was it's main attraction. But I went back to a single speed after a while because even with a perfect chain line, the Melvin produced at least half the clatter of a bona fide rear derailleur - which is too much. I had also grown tired of shifting the chain between front rings with my finger and fitted a front derailleur... slipping over to the dark side... not really.

For those times when I simply must have a few gears I built an 8 speed rear wheel that I run with a single 42 tooth ring up front. It takes about a half an hour to install the geared wheel, a rear derailleur, grip shift and longer chain. The bike can be restored to single speed duty even quicker.

DanO