Originally Posted by bostontrevor

I run a 17 and a 20 on my Bridgestone MB-2 conversion. It's got nice but not obscenely long dropouts. A conversion with something truly monsterous like Campy 1010's or nice long track ends could get away with more. The rule of thumb is 1/8" lateral distance per tooth (I'm pretty sure that this is just an estimate though and that it will really depend on the size of the cog as well but the math got to cumbersome to deal with) difference. So look at where your axle sits now and how much play you have and figure based on that.