How to select cog for emergency single speed conversion with a broken derailleur I broke the rear derailleur on my surly karate monkey recently.  I had the bike setup 2x9. I decided to try and get home by converting to single speed via trail side repair (made easier by the horizontal dropouts on the bike). 

I put the chain on the big chainring in the front, figuring the front derailleur would hold it in place. I pulled the chain out of the derailleur, shortened the chain so it would not be routed through the derailleur, and I tried to find the straightest line in the rear, and selected the 20t cog to place the chain on, this was the 4th cog in from the end of the cassette. 

I pulled the rear wheel backwards to tension the chain as much as possible. I had already been using a chain tensioner (surly tuggnut) to keep it stationary, so I screwed that down to keep the rear wheel from slipping under pedaling. 

However, after a short time, the chain started to upshift on the cassette. It went up a cog and jammed, shifting the non-driveside forward, pushing my tire in to my chainstays. this was difficult to fix, because I had to remove the tuggnut and undo my very tight quick release.

What's the best practice for positioning the chain in the rear to avoid auto shifting and jamming like I'm experiencing?
The recommendation of Calvin Jones of Park Tool for a trail/road-side conversion to a single speed is:


If you have multiple chain rings, put the chain on the 2nd-to-largest chain ring.  I.e. if you have two chain rings, this will be your smallest.
For the cassette, put the chain in a straight line back from the chosen chain ring.  If in doubt, use the smaller of the two choices.
Join the shortened chain outside of the rear derailleur, and as tight as is reasonable. 


This setup will give you a versatile gearing that you can get going without blowing out your knees, and neither the chain ring nor the cassette will be likely jump down to a smaller gear.