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If I want to replace the front derailleur on an already set up bicycle (replace SRAM Apex with SRAM Force), do I have to unlink and relink the chain on order to thread it through the FD cage? Or can I open the cage instead? I notice that the FD cage has a screw near the bottom. Can I just unscrew it, open the cage, put it over the chain and replace the screw afterwards? It looks like a have to force the cage open for that to work, risking damage to the cage.

Virtually every FD replacement guide on the Net does it by threading the chain through the cage before the chain loop is closed, but most of them apparently refer to new setups. What about the situations when the chain is already installed?

A followup: Unscrewed the cage and installed the derailleur without unlinking the chain. The cage is actually slightly pre-tensioned to spread apart when the screw is removed. Granted, it is easier to perform the initial alignment and adjustment of the derailleur without the chain, but it is also doable with the chain in place.

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    Yes, you can unscrew the cage. – Daniel R Hicks Aug 7 '12 at 12:12
  • Also note that most bicycle chains, even without a "master" link, are quite easy to unlink and re-assemble. The appropriate tools are cheap or you can just use a hammer, a punch (or anything pointy!), some pliers and a socket or similar to hammer the pin into. Only takes a minute or two. Search online for instruction videos. – Abhi Beckert Aug 11 '12 at 21:28
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If your chain has a powerlink, you can disconnect it pretty easily. Otherwise, it is pretty straightforward to open up the cage and slip the chain out. It shouldn't damage it (as front derailleurs are pretty simple mechanisms).

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Yes, if you have a removable screw at the back of the front derailleur, you can remove it, spread the derailleur cage, and slip the chain through. Do it gently, and there is enough spring resistance in the steel to return to it s proper shape.

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10 speed SRAM Powerlinks require a special tool to undo and, unlike their 9speed counterparts, are not supposed to be reused. That said, you can use a pair of needle nosed pliers in a pinch, and the Powerlink can be reassembled after breaking it if you want to take the (relatively small) risk.

However, if both derailleurs have screws as opposed to rivets, just undo the screw. Not a bad idea to apply threadlocker to the screw upon reassembly as I have seen them fall out before.

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