What is this thing?
 

I've never been able to figure out what this plastic thing, on the underside of the bracket (see pic), that the derailleur cables run through, is called:

http://bikeforums.net/attachment.php...hmentid=402407

So what is it called? And, can they be found easily in the event one needs to be replaced? Are they manufacturer specific?

Thanks.

It's a bolt-on bottom bracket cable guide. They're generic, and most fit any frame, though some larger diameter BB shells might want one with a larger radius of curvature (if they exist). The only thing that's not universal might be the bolt itself, because not all frames are tapped for the same thread. Some aren't tapped at all, and the guide can be installed with double sticky tape.

Thanks. I guess that's a pretty obvious name for something like that.

Interesting that you mention some are held on with double-sided tape. That seems so...not secure. I mean, what if the tape gets wet?

Actually the tape isn't essential either. The guide is held in place by the cable tension so it really doesn't need to be bolted or glued. The bolt/tape is useful mostly when you are changing cables.

In practice they tend to be kept in place pretty well just by cable tension - I had one that was pop-riveted in place on one of my bikes, and I drilled the rivet out because it got in the way when I tried to fit a cartridge BB. I was planning to tap the hole and use a screw, but I realised recently that I never got around to it, and the guide's still there.

Yes, cable tension keeps them in place, but in many or most cases, they're not necessary at all. The BB shell itself does the job, and the cable will find the shortest line and stay put without a guide. The guides main job is to keep the cables on a line symmetrical to the top tube for cosmetic reasons (so we don't get posts like "my RD wire is closer to the top tube than my FD wire, is this OK?). Also to prevent the wires from marring the paint under the BB shell.

Thanks for the input!

This has got me thinking. Let's assume a cable guide is screwed on, but the screw comes loose. Is there any possibility the guide could move side to side, fore or aft, even the slightest amount, when shifting, thus causing the shifting to be "off" or to cause "ghost shifting"? Or is this just an entirely unlikely scenario?

Thanks.

Yes, it's a possibility, but not to fret over. I doubt that most cable guides are mounted at the lowest length "natural" position. OTOH the added slack could easily be adjusted out.

Then again, consider the odds against the mounting screw coming loose. They could, as in any screw could, but in practice they don't. In fact the opposite is the case and the only reported issues with them inability to remove because of corrosion.