Track frame around £250

I'm still undecided, its still on my list of possibilties. It's the vertical dropouts that im unsure about.

Yeah, I understand your concern.

I did a bit of experimenting, and as long as the teeth on the cogs and sprocket add up to 69, then the chain will be tight. Therefore the following ratios would work:

49x20 (which is what I'm running on the bike at the moment)

50x19
51x18
48x21

etc.

Basically, if you add one on the chainring, you have to take one off the sprocket, or vice versa.

Each tooth = 1/8th of an inch. therefore, with a half link chain, you could minus 8 teeth from the chainring/sprocket 69-8=61 and lose a link of the chain and maintain perfect tension.

ie, the following ratios would also work with a half link:

42x19
44x17
etc etc

With a regular chain, the same rules apply, except that you have 2 inches instead of one, so you have to remove or add 16 teeth for each full link of chain.

Sorry for the long post.