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Choosing the right rear derailleur replacement
I busted my rear derailleur (shimano sora) on my specialized sirrus. This is a good-enough-reason to upgrade both my shifters and my RD.

It appears that it's difficult to find a basic, straight-forward explanation of what to look for & things to consider when you're looking for a replacement. I've been slowly upgrading my bike as things wear down, and I'd like either a shimano LX or XT rear derailleur, but I'm not sure if it would work with my cassette/chainrings. Here's the info about my bike from the manufacturer:

cassette: HG-50, 8 speed, 12x25T
chainring: 48s/38s/28s

and from Sheldon Brown, I know you subtract 12 from 25 (13), and 28 from 48 (20), and add the results (33). And that a derailleur that lists 33 or more as its total capacity should work.

But that doesn't say much to me. Brown's info is from an old article too, so does this still apply? What about short cage/long cage? etc.

Someone on another thread here had said that as long as it's shimano, and you get the right kind (gs or sgs), you'll be fine. That seems too simple.

Both the LX and XT should work, and I think you can get away with short cage (but I'm not sure about super-short). The advantage of the short cage is that if you run it off-road it makes it much harder to catch it on rocks, stumps etc. and bend it. XT as I recall can be used on 9-spd, so will be fine.
I believe any shimano rear derailleur will work, except "rapid rise" which springs the alternate direction. Generally with a triple in the front, you can't use short cage. "GS" are mid-cage and have a max of 33 teeth in the back so it would work. SGS can handle up to 42 in the back. Shorter cages generally have slightly crisper shifting and shave a tiny bit of weight, but it really won't make a big difference.

Changing shifters could be a bigger issue. I think Sora is the highest level 8 speed. If you go to a 9 or 10 speed shifter you at least have to change cassettes and I'm not sure if you get in to an issue with the wheel or not.
Does your bike have "Sora" shifters. The "Sirrus" is shown to have flat bars so the shifters won't be Sora.

You say the rear sprockets are 12-25T. Then that is why you have a Sora RD. Obviously XT or LX are MTB mech's and are designed to be used with a MTB cassette ranges. ie: 11-28 upto 11-34.
Unless you are going to go much bigger than the 25T at the back then you are better off sticking with a road RD. XT/Lx will work but not as sweatly.
The reason for the long cage rear derailleurs is for triple front rings. Depending on the sizes will decide whether you need a mid or long cage version. Definitely not short cage as they are designed for double chainsets only.

Just to add - Front derailleurs are different. You can't swap between road and MTB front deraiileurs as they have a different pull.
Road = 13mm per click of changer.
MTB = 19mm per click.
[font=Trebuchet MS]Ride hard or ride home alone![/font]
I need to find a rear derailleur for my mongoose sabre 21 speed bike but dont know what i need or anything thanks for your help.
search for component specs and compatibilty issues, see tech docs.

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