Bicycle TutorBicycle Tutor

Show off your bike!

Post Reply 

Choosing the right rear derailleur replacement

Related video tutorials:
How to Adjust Your Front Derailleur How to Adjust Your Front Derailleur
* Now with 3D interactive instructions
Author Message
mykalgrows Offline
New Member

Boulder, Colorado
Posts: 1
Joined: May 2010
Post: #1
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.

-Mike
May 27, 2010 01:05 PM
Find all posts by this user Quote this message in a reply
JonB Offline
Senior Member

Wales, UK
Posts: 232
Joined: Nov 2008
Post: #2
RE: Choosing the right rear derailleur replacement
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.
May 28, 2010 06:28 AM
Find all posts by this user Quote this message in a reply
DaveM Offline
Veteran Member


Posts: 1,344
Joined: Aug 2008
Post: #3
RE: Choosing the right rear derailleur replacement
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.
May 28, 2010 10:16 AM
Visit this user's website Find all posts by this user Quote this message in a reply
cyclerUK Offline
Veteran Member


Posts: 753
Joined: Jan 2009
Post: #4
RE: Choosing the right rear derailleur replacement
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.

Ride hard or ride home alone!
May 28, 2010 12:06 PM
Find all posts by this user Quote this message in a reply
j87soccer Offline
New Member


Posts: 1
Joined: Jun 2012
Post: #5
RE: Choosing the right rear derailleur replacement
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.
Jun 8, 2012 11:41 AM
Find all posts by this user Quote this message in a reply
trevgbb Offline
Senior Member

West Midlands UK
Posts: 416
Joined: Apr 2010
Post: #6
RE: Choosing the right rear derailleur replacement
http://cycle.shimano-eu.com/
http://bike.shimano.com/
search for component specs and compatibilty issues, see tech docs.
Jun 8, 2012 11:55 AM
Find all posts by this user Quote this message in a reply
Post Reply 


Possibly Related Threads...
Thread: Author Replies: Views: Last Post
  Rear and front derailleurs won't shift tsila060 5 354 Jul 19, 2014 12:34 PM
Last Post: tsila060
  Frustrating issue with rear gears crot4lus 4 527 Jul 9, 2014 12:57 AM
Last Post: crot4lus
  Derailleur or pulley? skyemc85 3 329 Jul 6, 2014 02:18 AM
Last Post: skyemc85
  Need help fixing rear derailleur Madhu 9 667 Jul 2, 2014 11:51 AM
Last Post: Madhu
  Schwinn Jaguar Rear Derailleur chicomandell 16 17,528 Jun 23, 2014 04:17 AM
Last Post: cradom
  Front derailleur not shifting properly srixon 10 35,466 Jun 17, 2014 07:39 PM
Last Post: Palash
  Derailleur Houston96 1 503 Jun 13, 2014 06:28 AM
Last Post: painkiller
  Help installing rear derailer Bikin4Life 2 435 Jun 9, 2014 10:47 PM
Last Post: 1FJEF
  Rear derailleur and chain issues rev2 2 678 Jun 6, 2014 08:25 AM
Last Post: nfmisso
  Rear Lever Gear Stuck? rmarkey 2 656 May 22, 2014 08:32 PM
Last Post: cny-man

Forum Jump:


RSS Forum RSS Feeds / Powered by MyBB

ISSN 1918-3445 © Copyright 2007-2010 Bicycle Tutor / Privacy Policy

feed