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I know it's an mtb forum but it actually gets traffic. But this is my first time dealing with internal road bike cabling. Mech gear cable snapped - inside the shifter (of course) so that's a job in itself to extract. But then of course it's internal routing and the manual is less than helpful.
Theres a port in the downtube. The manual tells you how to install it but not how to remove it....
Theres a port under the BB. Gear cable runs inside the red tube at this point.
Where do I actually start on this? I'm guessing I need
- a new pack of the thin liner (recommendations?)
- to replace both cables at once (so I can get the plug out of the downtube - it's not going to come out while the front mech cable is connected).
- but I'm still at a bit of loss as to how to do it. Full length outers in mtb are a bit more obvious!
I did my synapse recently. It was a pain in the backside. Hope yours is easier.
- a new pack of the thin liner (recommendations?)
- to replace both cables at once (so I can get the plug out of the downtube - it's not going to come out while the front mech cable is connected).
- but I'm still at a bit of loss as to how to do it. Full length outers in mtb are a bit more obvious!
Can't help on the liner. There wasn't any in mine
Yes
Loosen the gear cables and mine pulled out easily
My synapse is older than yours. My experience my not be the same as yours.
Took me hours. Not the min it would take on my externally routed synapse or any other externally routes bike I've ever worked on.
I've done it wrong inside my left hand shifter and cannot be bothered to redo it. I'll just suffer through slightly crappy shift
Cotton on old cable - tie to new - super glue - wait to dry. Pull old one out from MECH end and new cable comes through
You don't need the liners so don't worry about them. The port in the downtube comes out very easily now that the bolt is out, you just squeeze the small upper section towards the lower and lift.
It's a good time to replace both cables, but assuming you're only doing the rear mech you don't need to remove either the downtube port or the bb guide. Just send the cable through the downtube port and down to the bb guide, using the front mech cable tension to hold the latter in place. Use your fingers to find the new cable at the bottom of the downtube and feed it into the bb guide.
If you want to do a proper job, drill the downtube port and run full outer from the shifter to rear mech. While you're at it, run the outers from both shifters to the 'wrong' side of the downtube port and there's no longer any cable rub on the headtube. I did this on my old mechanical-shifting Synapse and it made future rear cable changes far easier.
thanks. Downtube port had been pushed too far at factory. But have levered it out. That revealed end of red inner. Tried cotton but it snapped. Fed new cable in but can’t get past the exit port on the chain stay.
can’t see any way of lining up inner and hole with a cable in either direction
drilling out chainstay and down tube port to run continuous outer sounds like the winning strategy 
Try nylon fishing line that's almost the same dia as the gear cable , push the nylon in at gear side to line up the inner sleeve and push the gear cable in. With any luck the nylon will line up the sleeve and gear cable will exit frame.
And it's done. How did the bike industry turn what used to be a pretty simple maybe half hour task into something that's taken me probably 6 hours now of swearing and cursing. A shifter that inevitably breaks the cable inside itself to start with. Unguided cables that clearly need both done at the same time. A manual that gives no advice at all on how to do the job.
This must be hellish for bike shops too - the mechanic's got to go through the same process of working out where the cables actually run on this particular bike and it's uniquely poor design. I can't believe a shop would have done it in less than a couple of hours either. And the customer's going to be pissed off at the cost.
When they bury the guy who first thought internal cable routing on bikes was a good idea dies they should blindfold the gravediggers and those doing the burying, make the hole no more than a mm bigger than the coffin and give the mourners no details of how to get to the service.
The port in the frame at the rear mech is just an alloy insert - no need to drill, it should just pull out unless corroded in.
The port in the frame at the rear mech is just an alloy insert - no need to drill, it should just pull out unless corroded in.
A little late. That became evident when it span as I drilled it, but it was pretty solidly in there - the drill had done most of the work before it span. Anyway, now has full length outer so solved.