What's this thing on my rear derailleur? (See pics)
 

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not needed, that's what it is

That's a henweigh.

It's a cable housing seal..... and it's installed completely wrong making it functionally worthless.

+1

That pice on the right- with the long thin tube coming out of it- that is a housing ferrule. It should be capping the end of the housing at the housing stop and that long, thin tube should be coming out of the housing stop, and that piece on the left, the rubber piece should be covering the long thin tube.

So basically it's backwards and not in the right place. It's to keep muck and grime from getting in the cable housing.

For SOME reason I have ridden for many years without all these gee-gaws. Periodically clean the cables and housings, replace once or twice a year- cheap insurance.

does the cable housing to the right of this picture have a ferrule? if not, then maybe this piece isn't on backwards, but just slipped to the wrong side of the cable stop.

I take those things out or leave them off when I buy a new outer cable kit with those included. I worked on a mate's bike once, dry bike and when I took the cable off they were wet inside. They're meant to seal the cable to keep water and mud out. All it does (especially on water crossings) is let water in and "keep" it in. High quality inners are stainless-steel and high quality outers are plastic lined internally - so they really don't need to be sealed (in my opinion anyways).

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No, it's meant to be before it goes into the derailluer. Besides, it still wouldn't be installed right because it would still be on the wrong side of the housing stop. Those things can't slip, the plastic bit is the housing ferrule, it's jammed in the housing stop. Only the rubber part could slip off.

It's installed just like the Shimano version.

Check this .pdf --- at the end of step 3 there's a diagram as to how the end cap with the tongue and the rubber shield fits together.

http://techdocs.shimano.com/media/te...9830606059.pdf