Shimano 105 Rear Derailleur Housing Frayed While adjusting my rear shifting cables I noticed that the small piece of housing, which emerges from where the barrel adjuster is threaded in, was broken. Is it necessary to replace this or is it fine to leave it as? If I should replace it, what is the part name I should be looking for.
It's there to reduce friction when the derailleur is in its extreme positions and also to prevent contamination of the housing. It's not totally necessary, but if it's broken in a way where it can't do that anymore, replacing it will improve performance.
It's unfortunate that the first generation Shimano road 11 rear derailleurs were built to need them to perform well, but they do.
Shimano sells them. I believe their part number Y5XX38000 is a generic replacement for all the derailleurs that have it.
You can also remove it completely when the housing is out and replace it with any sufficiently long nosed shift ferrule to accomplish the same thing. It doesn't have to poke out as much as the original, only enough to route the cable around the bend that gets created in the extreme positions.