Any question answered...

Most newer frames made for derailer use have vertical dropouts. This is convenient when you're actually using a derailer, but causes a complication when you wish to simplify your drive train, either to singlespeed, fixed-gear or an internal-geared hub. The simplest way around this is to install a chain-tensioning pulley of some sort. This is not an option for fixed gears or coaster brakes, but is the easiest solution for freewheeling bikes. Even for singlespeeds, though, it is nicer to be able to do without a klugy and inelegant chain tensioner.

Fortunately, most "vertical" dropouts are not exactly vertical: they usually have a bit of a slant. As a result, it is sometimes possible to use this type of frame. To make it work, you may have to play games with chainwheel sizes. One of my fixed-gear bicycles is based on a Cannondale touring frame. It happens that there is just enough adjustment to make it work with my preferred 42/15 combination. If the chainstays were a bit different in length, I could replace the 42 with a 41 or 43.

Adding or subtracting a link in the chain will move the axle 1/2". Changing either sprocket size by one tooth is the equivalent of moving the axle 1/8" (4 mm). Thus, if I wanted a 5.75 gain ratio (75" / 6 meter gear), I might first try a 42/15, this gives a gain ratio of 5.77 (75.6" / 6.05 m gear). If the chain was too loose, I could take up 1/8" (4 mm) of axle movement by replacing the 42 with a 43. This 43/15 combination would raise my gain ratio to 5.91 (77.4" / 6.19 m). Alternately, I could get the same axle position with a 42/16 --5.41 (70.9" / 5.67 m).

If I was not happy with these choices, I could add a link to the chain and switch to a 45/17 --5.45 (71.5" / 5.72 m) If I added two links to the chain, I could get the same axle position with a 48/18 --5.49 (72.0" / 5.76 m)

Another possibility would be to do a little bit of filing at the back of the dropout to let the axle move back just a bit.

It is also possible to grind or file a flat on each end of the axle to allow a bit more adjustment, like this:

This bit I guess.