Can anyone help find some literature or some pictures of a 2 speed conversion which has worked? I ride a huge gear, but plan to bike cross country, and I want to put my smaller chain ring back on my crank to manually switch to for prolonged climbs in the Rockies, but none of the road rear derailleurs I've tried have successfully acted as a tensioner. The idea was to shift from chain ring to chain ring by hand, and the rear derailleur which I've fixed on the position I need for a good chain line, would take up the slack and apply pressure in both gears. It doesn't work. As the derailleur moves closer to its point of origin it loses tension and the chain skips horribly. Can anyone help me figure out how to make this work? Has anyone succeeded?
Here's a 2-speed using a chain tensioner & front derailleur, but without much of a gear difference: http://saddleupbike.blogspot.com/2010/11/two-speed-bicycle.html
If you want to use a rear derailleur, you probably want to make sure you're using a mountain or road triple derailleur with a long cage so you have sufficient 'tooth capacity', as mentioned above. And if you're using a vintage part, make sure the spring isn't worn out.
There are also several examples of retro-direct conversions on the internet, which are a 2-speed bicycle with 2 cogs, each having the freewheel clutch operating in the opposite direction, and the chain running around both. You engage one cog by pedaling forward and the other by pedaling backward. This allows for a pretty significant difference in gear ratios. Here's a bunch of pictures from a Czech dude: http://matej.boha.cz/retrodirect/ There are a bunch of other examples out there if you search for 'retrodirect'. I wouldn't put too big of a bet on the durability of such a cobbled setup, though.
Speaking as someone who's ridden across the US, you'll want a very low gear for cranking an ~80-ish lb. loaded touring bike up 4000'+ passes— probably close to, or even below, 1:1. You'll want a relatively low gear just for cruising all day at 12 or 13mph when you're lugging that weight. You'll probably want another gear for days when you're fighting a headwind all day, and another for when you're sailing the prairies on a mighty tailwind at 25mph+. And when you're lugging your touring load up and down rolling hills in, say, Kentucky, you'll want to change gears rather often. I reckon what I'm saying is that gears are really great for a cross-country touring bike, which sees incredibly varied conditions under very heavy load, even if gears are not so necessary around town. Even if you're on a really tight budget, something like the Windsor Tourist is pretty cheap, and you can find things even cheaper on the used market this time of year, especially considering you have 3 or 4 months until a summer and the likely start of a tour.
You could try something like this: http://www.bikecarson.com/2012/03/12/surly-dingle-speed/ http://urbanvelo.org/surly-dingle-cog/
With the right chainrings it gives you two different gears with no need for a chain tensioner, so can even be used for a fixed gear.
(But without more details of your current setup, I've no idea why your derailleur isn't taking up enough slack, or doesn't have enough tension.)