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rear derailleur

I want to replace my rear derailleur shown in the photo above. I don't know what kind it is and and I was hoping someone could tell me which cage it is?

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Welcome to Bicycles SE. I've made some edits to correct some grammar issues in your post. If I've misunderstood what you were trying to ask, please feel free to roll back the edits. – jimirings Aug 29 '13 at 18:21
Hard to tell for sure, but it looks like a fairly long cage. The thing to do is to find the total number of "teeth" the derailer must handle, as that, in combo with the number of rear cogs, will determine what derailer you need. – Daniel R Hicks Aug 29 '13 at 20:49
"Teeth" is calculated by adding the difference in tooth count of the large and small front cog to the difference of the large and small rear cog. This determines how much slack the derailer must "absorb", worst case, and that basically determines if you need a long or short cage. (A long cage can, obviously, "absorb" more chain slack than a short cage.) The manufacture should specify the max number of "teeth" for their different models. – Daniel R Hicks Aug 29 '13 at 20:53
(Having a long cage when you don't need one is not ideal, both because shifting is less "crisp" and you tend to get more "chain slap", and because the longer cage is more likely to hang on something and get damaged. Suntour used to make a very nice triple pulley derailer that worked like a long cage without the actual length, but Shimano basically drove them out of business.) – Daniel R Hicks Aug 29 '13 at 20:56

As @jimirings said its hard to tell from the photo.

To identify which derailleur you have there are a few options:

  1. If this is the original part on the bike use a site such as bikepedia to determine what came on the bike as new.

  2. Look at the rear shifter pod, if this is SRAM it is more than likely it is a SRAM derailleur, if it's Shimano its more than likely SHimano, they are not generally compatible.

  3. Count the sprockets on the rear cassette, this is a good starting piece of information to help working out what it is.

  4. Find any words on the cage, something is written there, google this.

  5. Find any other writing on the derailleur and google this.

Derailleur cage length is determined by largest chain ring and cassette cog. Once you know what derailleur will work you can calculate what cage length will suit.

From what I can see from here, I'm guessing you're probably running something similar to this derailleur

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I can't tell what type of derailleur you have from the picture. However, generally speaking if you match up the type of bike (e.g., road vs. mountain), the number of gears, and the brand, you'll be fine. There's some flexibility sometimes, but it's easier to just stick with something that definitely matches.

It's clearly a mountain bike derailleur, I'd guess that it's an 8 or 9-speed Shimano. If you can edit in a better picture, we could almost certainly figure out exactly what it is.

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Yeah, once you've determined the brand (Shimano or SRAM), the number of gears, and the cage length, you can find a model that's compatible. You don't have to replace it with the same one, just something compatible. Which one you choose will really depend on your budget and the quality of the derailleur you want. – Kibbee Aug 29 '13 at 19:51

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