surly old man
Join Date: Sep 2006
Location: Carlisle, PA
Bikes: IRO Mark V, Karate Monkey half fat, Trek 620 IGH, Cannondale 26/24 MTB, Amp Research B3, and more.
You probably can adapt the current spindle and crank to acheive this. Some older spindles were asymetrical, so you can bring the crank in towards the centerline by flipping the spindle around. If the rings are held on with bolts, you can take them off and put the desired ring on the inside of the spider arms. If you still have room between the ring and the chainstay, you could put spacers between the spider and the ring. Get the ring as close as you can to the chainstay. Then sight down the cog and ring from behind the bike and see if you are in-line. If not, then you might have to look into a shorter spindle (check first to see that you have a gap between the base of the crankarm and the BB cups; if you then get a new spindle that is slightly less short than that gap is wide). Also, you could re-dish and respace the rear wheel to bring it out a bit.
The name of the game in these conversions is primarily to bring the chainring in as tight as you can. And secondarily to bring the cog out as far as is needed to meet it.
The first problem can be solved neatly with money (track spindle and track crank), or more messily by getting the ring in as tight as possible and seeing where that leaves you for re-dishing the rear.
The messy solution is not that hard, but there are a number of variables to work through.