How much difference can I get away with...
 

I am currently running 42x16 but I'd like a bail-out gear for when I hit the hills this summer. Is 42x20 too much to ask of my dropouts? Its my Centurion with longish horizontal dropouts. if I have to I'll run 42x19 but I'd like as much difference as possible.

what does this mean

are you saying because you'll have a flip/flop? otherwise you can just add and remove chainlinks as you need..

Another option is to use two master links and carry a spare piece of chain.

I should've been more clear. I meant with a flip/flop hub. Sorry. That's what I get for posting without my thinking cap on. Should've ate a piece of pie before posting or something....

The rule of thumb is usually a 2 tooth difference. That should be enough to get you up a hill or home thru a headwind. For more you might need additional chain which is possible thru the use of quick disconnect links like the wipperman or the sram.

Thanks for the info. I'll think about the extra chain option too. That just might work.

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.

Have you ever experimented with the Fuji track frame to see what difference is possible?

Thanks.

Nope, never have. I've only ever had 16t and 17t cogs out back on my Track and 48t and 47t rings up front.

Thanks anyway.