I recently received a donor bike which had been through a period of neglect. My previous bike suffered major damage in an accident, so I was looking to build this free frame up with parts from my previous bike- this included the bottom bracket, cranks, rear wheel and handlebars.
My original bike was configured as a single speed mountain bike - luckily the distance between the freewheel and the chainset was perfect to keep the chain at the right tension and allow me to ride without a derailleur or one of those (relatively pricy) single-speed adaptors.
This new bike's frame isn't quite the same size and there's a very small amount of slack on the chain. The dropouts are vertical so the usual method of tensioning the chain won't work, leaving me with a chain that very occasionally will slip between the smallest rear gear and the frame.
As all that the rear derailleur does is guide the chain to keep it running where it should, I thought that either a flat piece of metal or something bolted through one of the holes on the rear of the bike would do the same job. There is a derailleur hanger still fitted to the bike, so I'm going to show how you can take a busted derailleur (Shimano in this case), cut it down, and fit it in reverse to keep your chain from slipping off.
To clarify; we're going to place a small piece of metal in the gap visible in the first picture, so that it would stop the chain from slipping down into that gap.
We'll be taking advantage of the hole on the bottom right of the second picture to do this. I tried with metric bolts but they didn't appear to be threaded quite right.
Take your donor derailleur. In my case, this one was rusted beyond reasonable repair, so I didn't feel guilty destroying it just for a bolt.
The bolt is secured in place with a clip (picture three) which will need to be removed by either slipping a screwdriver into the gap between the inside-top of the clip and the bolt, or pushing from both sides at the bottom of the clip. The first of these is easier, if you can do it.
For me the bolt basically just fell free and is pictured for reference.
Step 3: Hidden Bonus Step
At this point, with the hex head still on the bolt, try fitting it to the inside of the derailleur hanger - this will sort out any minor issues of threading (as the whole may not be tapped the whole was through, or may be caked with dirt) while you can easily apply force.
In the next step we'll be cutting the hex part off and then you'll just have to grip the cylinder, which is significantly harder.
I did not do this step, but immediately wished that I had.
With your final bolt, which should look like the picture, you can remove your wheel if you haven't already, and try finger tightening it. If that doesn't work, gets stuck or isn't tight enough for your liking, you could use a set of pliers to grip it, the marks in the metal won't matter.
The final position may need some fine tuning - the bolt shouldn't touch the chain as it's passing by normally, but should just be enough to save the chain from slipping off. Some trial and error may be needed. I'd say you want it adjusted to *just* the point that you don't hear a the chain clicking against it as you pedal.
And there you have it. This was my first Instructable and I'm keen to hear any feedback in the comments. To head off some easy ones though:
* I really need to learn how to use a rotary tool better, and more safely.
* This probably does add wear to your chain, part of the goal was to build the bike for £0 and better solutions probably exist.
* A hacksaw would have been quicker than using the rotary tool.
* Measure twice, cut once!