Single Speed for my FSR

[xizang]

The one in the picture is a Superstar tensioner, £25, in either short or long cage.

 

[superstu]

19 tooth rear cog would be good

Make sure the tensioner lines up exactly with your cog otherwise there will be probs, and adjust chain length so its pretty tight

If you buy a more expensive tensioner i.e. Rohloff or NC-17, which are both like half rear mechs, you can adjust the rear cogs by about 6 teeth without changing the chain length, or run a 2 x 2 set up..2 x chain rings and 2 x cogs

Off road single speed ration is 2:1..so 40 up front and 19 rear will be fine off road

^^^
What he says
2:1 ratio works best
When fitting I found that alignment is absolutely crucial, and you need to have a pretty tight chain so may have to take a few links out if its an old one you're chucking on.