How to single speed a bike in the event of a broken derailleur. Useful when having a bike that can peddle in a single gear is better than no bike at all!

I do Adventure Racing and a broken bike means lots of lost time in the race. While single speeding a bike was in my mental list of break down solutions, I'd never tried it at home before hand. A mate recently broke his derailleur during a race and attempted to single speed the bike, but the chain kept dropping down the cluster due to insufficient chain tension and so was still effectively a broken bike.

Thanks to Eric from my local bike shop (Beerwah Cycles) who suggested I combine chain shortening along with bmx style chain tensioning by shifting the rear axle.

The basic premise is
1 - using a chain breaker take out a link so that you've got a single chain length and not a loop
2 - trying to keep the chain fairly straight (ie centre front to centre rear sprocket) set the chain and observe the link overlap
3 - move up/down a couple rear sprockets until the chain would be 'just' too short if you were to shorten it
4 - remove the excess links with the chain breaker and rejoin the loop
5 - undo the axle quick release, set the chain on the sprocket then push the axle back until the chain has some tension (axle should not have returned all the way to its usual spot)
6 - do up the quick release
7 - start peddling and be careful to avoid bumps/jumps/speed which could force the axle back and damage the bike.

My first instructable, so please be kind with your comments ;-)

_post publish edit_
ahh just poking around the Instructables site a bit more and found a near duplicate. Could have saved myself the time of wondering how to do this and the authoring of this guide....
https://www.instructables.com/id/Single-Speed-on-the-Cheap/

Step 1: Break the Chain

So you've broken the derailleur or have broken enough links that the chain is too short for the derailleur.

Get out the multi-tool from your under seat tool bag and break the chain.

Step 2: Find a Good Chain Length

Lay the length of chain over the front and rear middle sprockets (so that the chain is fairly straight) and observed the overlap of the links. You want the chain to be 'just' too short so that only a minimal axle shift forward would allow the links to overlap properly. Move the chain up/down a couple sprockets until you find a combination that gives you the optimal length (mine was one up off the centre rear sprocket).

The three pictures show
1 - too short (or too long depending on your perspective)
2 - nearly there
3 - pretty good, the 10mm'ish of length won't take much of an axle shift

Step 3: Rejoin the Chain

So you've found a gear ratio that gives you a good chain length.

Mark or remember the overlap point. Using a chain breaker remove the excess links. Using your chain breaker (or in my case a quick-link) rejoin the chain

Step 4: All Done

Undo the axle quick release and bring the wheel forward in the axle recess so that you can lay the chain over the sprockets.

Push the wheel back into its axle recess and you should find the chain gets some tension on it while the axle does not quite reach its usual position. Do up the quick release firmly.

Start peddling so that you can move forward again. Be very careful with jumps/bumps/etc because if the axle gets pushed back from a thump then you could snap the chain (or do some other more major structural damage) and end up pushing the bike anyway.