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Shimano rear derailleur cage

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Shimano rear derailleur cage

Old 10-05-15, 10:29 AM
  #1  
Spookytooth22
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Shimano rear derailleur cage

An admitted bonehead mechanic here. I am in the process of changing the cage from short to long on my Di2
Shimano Ultegra RD but it not working out for me when trying to put the chain back on. I searched until I've developed a raging headache for what I'm doing wrong. From every picture I've seen it looks as though the cage is directed toward the rear without chain and then when chain is installed its pulled forward. But as you can see in my photos that without the chain the cage is directed forward so obviously the chain won't install correctly. I've scratched my head until I'm bald. What the hell am I doing wrong. Please don't suggest the obvious "take to LBS", I am trying to learn to do some of my own stuff so as to avoid the time and miles of visiting LBS every time.

Thank you in advance.
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Old 10-05-15, 01:32 PM
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Tim_Iowa
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There is a coil spring in that lower pivot of the derailleur (where the cage and body meet).

The spring is #4 in the Shimano tech doc, which is the first link if you google "shimano RD- 6870 site:si.shimano.com"

You need to install the cage so that the protruding tab of the coil spring goes into a hole or behind a stop on the derailleur body.
It appears that you installed the cage with the spring well forward of this hole or stop.
The spring needs to be installed with some tension, so that it pulls the cage back toward the rear of the bike, as you observed correctly in pictures. Rotate it about 180 degrees.
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Old 10-05-15, 01:58 PM
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If the cage is installed the outer plate will look like a backward S. The chain route through the cage follows the plate, winding behind the lower pulley abd forward of the upper, then back under and around the sprocket.

When installing a cage, you have to introduce roughly a half turn of counter-clockwise spring tension, so when released, the cage will try to rotate clockwise until it hits the stop which prevents unwinding completely.

RD designs vary, but many use a removable stop which makes assembly easier. Assemble the cage without the stop in place and with the spring relaxed, Wind back (counter-clockwise) about a half turn until the stop peg clears what it hits against, then attach the peg to prevent further unwinding. Once the chain is threaded, it will wind the cage pulling the peg off the stop so the spring is tensioning the chain.
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Old 10-05-15, 02:31 PM
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Originally Posted by Tim_Iowa View Post
There is a coil spring in that lower pivot of the derailleur (where the cage and body meet).

The spring is #4 in the Shimano tech doc, which is the first link if you google "shimano RD- 6870 site:si.shimano.com"

You need to install the cage so that the protruding tab of the coil spring goes into a hole or behind a stop on the derailleur body.
It appears that you installed the cage with the spring well forward of this hole or stop.
The spring needs to be installed with some tension, so that it pulls the cage back toward the rear of the bike, as you observed correctly in pictures. Rotate it about 180 degrees.
Thank you Tim. Never thought of having to re-tension the spring but I see that's the issue. Thanks again!
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Old 10-05-15, 02:35 PM
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Originally Posted by FBinNY View Post
If the cage is installed the outer plate will look like a backward S. The chain route through the cage follows the plate, winding behind the lower pulley abd forward of the upper, then back under and around the sprocket.

When installing a cage, you have to introduce roughly a half turn of counter-clockwise spring tension, so when released, the cage will try to rotate clockwise until it hits the stop which prevents unwinding completely.

RD designs vary, but many use a removable stop which makes assembly easier. Assemble the cage without the stop in place and with the spring relaxed, Wind back (counter-clockwise) about a half turn until the stop peg clears what it hits against, then attach the peg to prevent further unwinding. Once the chain is threaded, it will wind the cage pulling the peg off the stop so the spring is tensioning the chain.
Thank you FB for taking the time to answer. That's exactly the issue. Never thought of re-tensioning the spring and it was never brought up in any YouTube "how to" videos. Thank again!
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Old 09-08-16, 08:39 PM
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gamyot
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Same problem, different derailleur

Originally Posted by Tim_Iowa View Post
There is a coil spring in that lower pivot of the derailleur (where the cage and body meet).

The spring is #4 in the Shimano tech doc, which is the first link if you google "shimano RD- 6870 site:si.shimano.com"

You need to install the cage so that the protruding tab of the coil spring goes into a hole or behind a stop on the derailleur body.
It appears that you installed the cage with the spring well forward of this hole or stop.
The spring needs to be installed with some tension, so that it pulls the cage back toward the rear of the bike, as you observed correctly in pictures. Rotate it about 180 degrees.

Hi,
I have the same problem with my Shimano Sora rear derailleur.

In my case, the problem happened when I was putting my rear wheel back (I have not changed the derailleur cage).
Tim_Iowa's answer made me understand what is my problem, but I'm unable to fix it.
I don't know how to rotate the cage 180 degrees.

In the Shimano's tech doc for the Sora, there is no indication for the "Plate Stopper Pin".

Do someone know if it is possible to repair my Sora RD?
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