How to install a derailleur

Question
I recently pulled a mt. bike out of the garbage.  Judging by the markings on it, it looks like it had been some sort of promotional bike that was given to a person who bought a Jeep Cherokee.  My guess is, the bike fell [hard] and the fall broke the derailleur.  Rather than fix it, the owner tossed it since the bike really isn't worth too much.  I would love to get it working, but I'm just a casual biker and don't want to put much money into it.  Point being, it is not worth it to me to take it to a bike shop and have a professional install and adjust a new derailleur.  I can buy a derailleur, but don't know how to put it on. (I've checked a few new derailleurs but they did not come with any instructions.)  Can you tell me how to properly install and adjust a derailleur (or, point me to somewhere that I can get instructions)?  Thanks.

Answer
Good for you for saving a bike that is stiff functional! Actually, I should modify that statement and say "maybe functional."

There is a reason why bike shops, to some, are expensive... they have the tools (and knowledge) to do the job quickly and right. But if you are willing to try it you are going to need some tools... a set of metric wrenches and/or metric hex keys (Allen wrenches), a chain tool, and possibly a good cable cutter.

Without seeing the bike, although I have seen those bikes around, I am not sure if the derailleur is attached to the frame or if there is a mounting plate/hanger attached to the derailleur. If the later you will need to buy a derailleur with the mounting plate/hanger, if the former then , obviously, without the mounting plate/hanger.

Either way here is the procedure: You will need to break the chain with the chain tool, take the shift cable off of the derailleur, and then the derailleur off of the bike. Installation is just the reverse.

Simple, right? Well here are the caveats: The chain will have to be put back together with a special pin (Hyperglide if it is a Shimano chain) or a special link (like a master link on a single speed bike, but not quite the same). The frame should be checked to see if it is bent where the derailleur attaches, if it is bent the bike will shift poorly, or, worse, put the derailleur into the rear wheel and cause a crash. The travel of the derailleur will have to be adjusted so it doesn't go too far, and off, the gear cluster at the top and the bottom. Or, conversely, that it goes far enough. The shifting will have to be adjusted so each click equals one gear, this may mean a new cable and housing for which you will need a special cable housing cutter. The cable housing that is used for index (click) shifting is linear and doesn't cut well with a side cutter.

I don't think a bike shop will charge that much to put on a new derailleur, compared to the tools you will need and the time you will spend. Plus a shop can check the bike over and tell you if any damage was done, besides the rear derailleur, that may make the bike unsafe to ride.

If this is something you want to undertake check out a couple of bike repair books from your library, they will have pictures which help explain a lot. And the tools are an investment that, over time, will pay for themselves, if you keep riding.

The repair is not that hard, but there are a few things that can make it tricky. I will be happy to take you through a step by step if you still want to do the repair. But I will have to ask a few specific questions before you start.

Tad