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Last night it became clear that a friend's rear derailleur cable was failing, and I had a spare, so we thought we'd fit it - an easy job after all. Unfortunately not - it's a Cannondale Synapse with internally routed cables. I'd heard about taping the new cable to the old, but even after degreasing, and probably made worse by the PTFE coating, we couldn't get any tape to stick, that was thin enough to fit through the outer.

There's access via a plug under the bottom bracket, and a port in the upper top of the downtube. Threading the new cable would be easy using those (though the exit port might still be an issue), except the cable is meant to pass through a flexible sheath. This is nothing like a cable outer - it's almost floppy, and a much looser fit. It also dropped down from the upper access port to just out of reach. The RD cable has a red sheath and the FD cable a blue one. The RD sheath appears to run down the downtube and along the chainstay in one piece though by the time we looked at it, having already tried to thread the cable, there was a kink near the BB.

What's the sheath there for?

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  • BTW, it was indeed going, hanging on by 2 strands, and wouldn't have lasted the next big ride. I'd like to understand it better because I'm used to being able to replace cables at the roadside on my tourer
    – Chris H
    Apr 13, 2022 at 10:14
  • Good question, I’ve been wondering the same. If the cable enters and exists neatly (through housing end caps) and runs in a straight line through the inside of the frame I see little point in having the liner/sheath. Especially if the liner/sheath isn’t attached at both ends. If it’s attached it could help with guiding/installing the cable. If the cables cross it could help to reduce friction.
    – Michael
    Apr 13, 2022 at 11:40
  • @Michael if it was attached at the top to start with, it wasn't attached very well. We tried pushing the new cable down against the cut end of the old one, and when they reached the upper port felt a change that turned out to be the sheath + old cable coming away from the port. I think it was just a light push fit
    – Chris H
    Apr 13, 2022 at 11:42
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    Yes, my experience on 3 bikes has been similar.
    – Michael
    Apr 13, 2022 at 12:10

2 Answers 2

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They can do a couple different things.

Sometimes they play an important functional role in helping create a smooth surface where the cable wraps around the bottom bracket cable guide. Most or all Synapses want a piece there. It's often the case that the friction you're worried about isn't so much in the middle of the cable guide where you'd expect it to rub hardest as it is where the cable first or last touches it. If those kinds of transition areas aren't very smooth you can get issues where the cable catches additional friction in one direction but not the other. It's possible to encounter a piece that was once playing this role but got worked out of place or broken.

When you find a floating piece that doesn't appear to be doing anything or have ever done anything in terms of preventing friction, one possible explanation is that it was there as a guide for installation or re-installation. I've observed that some bikes have such pieces floating inside that would seem to be too short to have been guides, since they can't run all the way through and poke out the other side. Sometimes these make noise as they rattle around. If you can be confident that they're not doing anything to prevent rub against a frame surface or hose/housing somewhere inside the downtube, these are safe to remove.

On some bikes the liner plays a role in keeping dirt or moisture from affecting the area where the cable rubs against a guide. Usually this will be on a bike that has internal routing with an open access port in the bottom bracket area where the cables would be exposed if not for the liner. Generally speaking most bikes with this kind of internal routing layout should have a liner in those areas if at all possible, because they are often necessary for low friction.

I replace cables on a lot of Synapses (which do vary a little in their cable layout) and as with most road bikes, the main tools to do it are two pieces of liner, one long enough for the chainstay run and the other for the down tube. You can get as much fresh liner as you need by making a longitudinal slit in a piece of shift housing with a razor and then whipping it against a hard surface until it all springs apart.

When creating the new piece that will go around the BB cable guide area, it's good to think about whether it will be of benefit to make it long enough on the chainstay end that it finds some kind of hard stop there such that gravity will never cause it to slip down further if the cable is slack. Sometimes I size them that way, but other times the liner is passing through some kind of tight spot on the guide that makes that worry unrealistic, in which case it's better to keep it more minimal.

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  • That's really interesting, because (not expecting to find it in the first place) I was going to reuse the existing sleeve. On externally-routed cables that go under the BB housing, the cable runs freely enough in the guide, and you'd think it would be cleaner on the inside so run nicer for longer. It sounds like we could have used just a little of the existing sleeve at the BB and got rid of the rest
    – Chris H
    Apr 13, 2022 at 19:27
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    I like to replace any pieces that are located in guide areas where the inside is being abraded under force. It's not strictly necessary but they wear through eventually and the extra work is minimal once you're already replacing the cable in the first place. I often like the results I get with liner taken from high quality housings more than the stuff that comes on bikes or is available standalone. Apr 13, 2022 at 19:48
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Besides protecting the cable, the housing also protects the frame from cable rub as it routes through the frame.

Technically, that internal sleeve isn't housing. Housing consists of the inner metal strand housing and outer plastic sleeve (and sometimes lubricated sheathing), so it creates a stiff shifting environment where the cable will not flex.

If weight wasn't an issue, I would route full length housing, assuming the frame fittings allow it.

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