i had the same problem. i still have to adjust the chain length every time i switch between 18 and 20, even with a tensioner. the trick is to get a tensioner that pushes the chain upwards to make more contact with the rear cog. the skipping is most likely from not enough wrap around on the cog when running the smaller size, because the poorformance ghetto brand tensioner pushes the chain away. first i took the spring out and bent the shit out of it to reverse it, which did ok for a while before it fatigued, but i think i may just remove it all together and support it onto the chainstay with a zip tie. im too cheap to spend $60 for a surly, which comes with 2 different springs, one to push and one to pull
not enough wrap...
better... probably won't skip
"The Singleator is the only tensioner with a push up "mode" (it requires a second spring that needs to be switched out). If you have another Singleator type tensioner, you can still use the zip ties but there will be no way to tension the spring (again they only work in one direction). Also, with the exception of the Paul Melvin, most tensioners are not designed to work with different size cogs using the same chain length. The Singleator manual says to shorten the chain as much as possible when using the tensioner (I would recommend using a half link if necessary). These tentioners skip due to a lack of chain wrap and tension, so anything you can do to increase either will help. Also make sure you have a cone wrench so you can tighten the spring as tight as possible. "
