I have been running a 32:16 set up for a while now on my converted stumpjumper with a Rohloff chain tensioner and have never had the chain jump. Unfortunatley the 32:16 ratio is a little to low for my commute (70% of my riding) and a little to high for the trails I ride. So I decided to change the 16 for an 18 and put the 42 back up front so I can have a 2x1 where I cange from 42:18 to 32:18 by hand when I hit the trails. The only 18 rear cog I could find was an expensive surley one which looks beautiful but unfortunatly my chain has started jumping terribly every time I get out of the saddle. The chainline is straight between the 32:18 and has not changed from how it was set up before. There is good tension on the chain and the chain is not stretched.

The only thing I can think that could be causing the jumping is that the expensive, beautiful surly cog is not really as good as the cheapo BMX 16 I had on there before.

Can anyone suggest anything else that may be causing this that I might have missed? If not I may have to go back to my old setup cause this is really pissing me off.

Cheers and thanks