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I recently replaced the shifter cables on my cyclocross bike (Specialized Tricross). Both cables run exposed (not in cable housing) under the bottom bracket, through a little screwed-on plastic guide that keeps them in place.

When I removed the old cables, one of them also included a short, maybe 8-10" piece of what I would call protective plastic sheeth that kind of sat within the screwed-on plastic guide, extending for a few inches out either side. It was pretty shredded up and I decided to chuck it.

Now that I'm looking at the new cables (approx 200km riding), I'm wondering if I should have looked for a replacement for that protective sheath.

Is that protective sheath important and should I have replaced it? If so, what is it called?

The image below shows the cables right now, looking straight up at the BB, front of bike towards top of image.

I saw this similar question, but it seems like the OP there did not at any point have the protective sheath I originally had on my bike.

Exposed shift cables under bottom bracket

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  • I've never seen anything such as you describe. Unless it were pretty thick I don't see that it would offer any "protection", other than to keep the area marginally cleaner.
    – Daniel R Hicks
    Commented May 5, 2018 at 2:07
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    I have - it was just a thin lightweight piece of plastic tubing, part of the formed plastic shape under the BB housing. It literally did nothing, and the inner cables were still exposed at the BB turn other than a small retainer bar. You're not missing anything by not having this. Cables are consumables - replace them every year or two, or when you notice fraying anywhere, or when shifting gets hard, or when you notice corrosion.
    – Criggie ♦
    Commented May 5, 2018 at 4:52
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    The plastic block thing itself is absolutely critical. Without that your cables will wear grooves into the frame. So don't remove the entire block unless you replace it with a full run of housing.
    – Criggie ♦
    Commented May 5, 2018 at 4:53
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    I believe the little plastic tube is more to protect the guiding block than anything else.
    – gaurwraith
    Commented May 5, 2018 at 15:57
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    The more the cables are exposed and not hidden under some protection the easier it will be to keep a quick eye on them and to clean and maintain them if needed.
    – Carel
    Commented May 5, 2018 at 17:55

3 Answers 3

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The thinking with them is to protect the contact point from contamination and provide a slick surface, reducing friction. They do achieve that to an extent, especially early in their life.

The problem is when the liner itself starts junking up with grime that can't be easily removed, it will then cause friction problems itself. For this reason, I think going without is fine in many cases.

Some bikes use them to prevent cables from rubbing the frame on painted surfaces, raw carbon, etc. In that case I would not recommend going without.

You can get replacement pieces from inside shift housing. Take a long piece, cut the outer plastic down it longitudinally with a razor, then whip it hard against a hard surface. The wire strands will come apart and you can take the liner out.

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  • Is the reduction in friction meant to improve shifting performance or reduce wear (of course extra friction stresses the whole cable to some extent)?
    – Chris H
    Commented May 6, 2018 at 17:58
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    To improve shifting performance and feel.
    – Nathan Knutson
    Commented May 6, 2018 at 20:00
  • Yeah, my guess was that this was intended to maintain a low friction pathway around the bend of the bottom bracket, rather than having road grime and dust build up directly on the cable and increase the friction. After riding ~2,000km after installing the new cables, I did not notice any degradation in the shifting and haven't worried much about it since. Good tip on getting the liner out of shifter cable!
    – SSilk
    Commented Apr 12, 2019 at 12:22
  • Almost always used for issues with cables rubbing on the frame caused by larger diameter aluminum tubing (most of these cable routing approaches was originally designed when thinner steel tubing was the main construction material. You can also by the housing liner separately now (e.g., jagwire) which is handy as internally routed cabling schemes often use it inside the frames.
    – Rider_X
    Commented Apr 12, 2019 at 19:04
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I run my tourer over rough ground and don't clean it as much as I should. I recently had a cable fail at the shifter (after about 5000km). The section of cable under the BB was as good as new. This suggests that wear there isn't an issue. A bit of plastic isn't going to do a lot too protect against knocks, but the chainrings are much more likely to hit things than the BB.

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Returning to this issue after 5 years, I'm posting some follow up experience as an additional answer, although I stand by the accepted answer.

When I originally read the answers provided by others here, my takeaway was that the plastic sheaths on the cables where they pass through the cable guide are a "nice to have" but not critical, and may have some downsides.

However, cleaning up this bike again recently, I noticed that where the front derailleur shift cable exits the front end of the plastic cable guide under the bottom bracket, the cable is contacting the frame and eating away at the paint and now metal.

Below are a few views of the situation.

"Bottom" view (bike is upside down), cable under derailleur tension: enter image description here

Side view: enter image description here

Bottom view again, cable pulled to the side to show area being rubbed: enter image description here

Close up looking towards rear of bike with cable released at derailleur and pulled out of the way: enter image description here

You can see the cable is wearing down through the plastic guide.

My opinion now is that the plastic sheaths are indeed important, and I would view their main purpose as protecting the cable guide itself from cable wear since the guide block itself is presumably sized adequately to keep the cable clear of the frame metal.

I would advise others to replace the old sheath with fresh sheath each time the cable itself is replaced so that you're only ever wearing through sheath, not the guide block.

Kudos to Nathan and gaurwaith for hitting on this point in their answer/comment respectively. I wish I'd paid closer attention to those details at the time, but luckily have caught it before the cable sawed my BB shell in half.

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