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?