Hello,
I've aquired an older Huffy Granite 26" 15 speed womens mountain bike. Its been sitting out in the Louisiana rain for over a year so the chain, derailleurs and cables are fairly rusted. When I look to Amazon for parts, there are so many different types of derailleurs and I dont want to waste money on the wrong one. How do I choose the correct parts?
I enjoy tinkering with mechanical things and figuring it all out which is why I grabbed up this bike and 18 others in pretty much the same condition.

Jeff we need more information but I suspect any derailleur would work and will cost more than the bike will ever be worth. I feel you would be better off and learn more just trying to clean up and fix what you have. Rust can be removed and oil can improve things. Take the bike apart and just start cleaning and oiling things. Possibly replace the cables. You can buy enough cables for that bike at Walmart for about $10. Beyond that I wouldn't put anything into the bike. Roger

I agree with Roger, any decent derailleur will cost more than that bike did new, never mind in it's current condition. Shimano's "Tourney" is their lowest line derailleur and available for under $20 but, again, even that cost is marginal for what you have.

As he recommended, try cleaning up and lubricating what's already there and, at most, fit new cables and housing.

Throw it away and don't even bother with it. This bike is absolute garbage all around.

I hear you on loving to tinker, but I've been working at a co-op and these bikes are a penny-a-dozen. The cheapest steel falcon shifting components and crank, heavy junk steel. Total garbage, you will never sell this for more then what you buy for replacement components, let alone time invested.

I love tinkering and rebuilding old bikes! My advice is stay away from any thing with the word Huffy, Next, or Pacific. Scrap out those bikes you have and spend a little to get something worth working on. Even $20 for an older Schwinn off Craigslist is worth it. Stay away from Pacific Schwinns as well.

Google Shimano Tourney rear derailleur $9.99 from Atr's Cyclery. You almost certainly need the one with a hanger.

You probably won't be able to replace your bike for that much but you'll probably also need a new shift cable because your old one is frayed.

This is the correct answer. I volunteer at a local place repairing their donated bikes and have gone through this exact bike in similar rusty condition. It's not worth it even when you've got the spare parts on hand at zero cost.

If you're determined to rebuild it the best option is buy another used MTB and move the parts over. But at that point hopefully it makes sense to throw out the Huffy. Maybe save the tires, grips and seat if they are useable.

Thanks to all for the tips.
If it doesn't need a specific derailleur, then I'll just get the cheapest thing that I can find.
I'm not an avid cyclist so the higher dollar name brand items don't really appeal to me. But I certainly believe that you get what you pay for.
The bicycles came off of an apartment complex where they have been abandoned and unclaimed for possibly 2 years. I'm going to patch together as many as I can and give them to some of our many homeless people within our inner city. For the most part, I'm only having to buy inner tubes. I've been able to work everything else loose with the ol' oil can.
Thanks again!
Jeff

Good for you for fixing up these abandoned bikes for a good cause.
Not to sound cold-hearted, but i'd spend a little as possible to get them safely rideable.
If it's a thing that you might be doing more of, tubes and brake pads can be bought in case quantity or 'dealer packs' to save per-unit cost.

One 'advantage' to working on those 15/18-speed MTB-ish BSO's is that they built them from the same pile of parts for a long time, so mixing and matching is easier than on modern high-end stuff.
If you get a bundle of 5 of these bike, even if they're 'basket cases' you probably can find enough good components to build 2 or 3 complete bikes; you don't have to feel obligated to save them all.

It is not a sure thing that the derailleurs on that bike are non functional. It is more likely that the problem rests with the cables and possibly the shifters. Don't buy any parts until you are sure what the problem is. It would be foolish to buy a new derailleur only to realize that the shifter is seized. It is easy to test whether a derailleur is working, either front or back. Disconnect the cable and move the derailleur by hand from its rest position to another cog or chainring. If it returns to its original position when you let it go, it works. Buying the cheapest thing you can find to replace an already cheap component that may work is a waste. Giving a crappy bike to poor people who cannot afford to maintain them is not a gift, you are not doing them a favour. You would be better offering them a monthly public transit pass