Fixed said:
Just got an email reply from Sacha, suggesting an eccentric bb with a vertical drop out rear (as others had suggested, as well). I'm wondering, though -- how do you chain the rear tire and/or adjust chain tension on the road? Can someone explain how the eccentric bb works? Thanks.
One of those Treks, the Soho, I think, has an eccentric bb to convert the multi-gear beast to a singlespeed vixen. The ecentric bb on the Soho works much like your erstwhile eccentric ENO hub, except that it's the bb that is eccentric in an oversized bb shell. I think that the larger diameter of the bb, in comparison to an eccentric hub, gives a much great range of horizontal travel than the ENO hub can give.
In some arrangements, adapted from tandems, I think, the bb shell is actually split all along the bottom and bolted back together to tighten the shell around the eccentric bb. I didn't want that version on my Vanilla, which I initially wanted to make fixed w/ discs, because I thought that the relatively frequent tightening/loosening of the bb shell for fixie use would eventually fatigue the shell. In contrast, the shell in tandem usage would only rarely be loosened/tightened when the capt/stocker driver trains were ganged together, so the split bb seems fine for tandems. I bailed on the discs for my Vanilla and went with cantis.
But that Trek Soho has another sort of eccetric bb, which doesn't require the bb shell to be split. There's a bolt on the side plate of the bb that somehow tightens the bb in the shell. I don't know what surface the tightening bears against, so I don't know how it would hold up to fixie usage, but it does seem to solve the problem of fatiguing a split bb shell every time you remove the rear wheel.
