Internal cables... replacement questions

Ideally, you do need new outers as well as inners. As for fitting the internally routed cables, it's usually a piece of cake. The way I do it on my cabled bikes, is to strip the old cables off the bike first. I then cut the new outers at the cockpit end of the bike to size, using the old outers as templates. There's then usually some long enough lengths of outer cable left to enable the last bit of the procedure. I remove the cable guide under the bottom bracket so that there's a big hole with which to work. Then remove any grommets from the down tube. Shove the long pieces of cable outer inside the down tube so that you have a bit sticking out of where the grommets go, and all the way through the down tube so they're sticking out of the hole where the cable guide goes, as well. Thread the new cables through the brifters and slide the new bits of cut outer cable over them followed by the grommets for the down tube. Then feed the cables into the long pieces of outer in the down tube. As soon as they're out of the other end, remove the long pieces of outer and slide the cable guide into position. It's then a case of either free feeding the rear mech inner along the chain stay, or using a piece of outer to guide it, much like with the downtube, and the front mech cable is just pushed up to the mech. Then it's just a case of pulling everything tight and re fixing grommets and guides.

If I remember correctly from when I cabled up a mates brand new Argon Nitrogen frame a couple of years ago, it is one single continuous piece of outer from shifter to rear mech.

There are no cable stops near the headtube where it becomes just inner through the frame, and it doesn't emerge at the bottom bracket either.

My mate had already fitted the BB too, so it obscured the view of the outer coming down the inside of the downtube and going under the BB internal sleeve and into the chainstay up to the rear mech end.

Because of the length of outer needed, that makes it difficult to feed it onto any kind of "guide line" or sleeve. To feed a new outer through easily then somehow you will likely need to attach one end of the new outer to the end of the old outer and then pull them through as one. Securing the two ends of outer butted up to each other is a bit of a challenge to do securely also. You don't want them coming apart half way through.

It's with out doubt the hardest frame to cable up that I've ever come across. Made worse if you have the matching internally routed handlebars too.

I've only had to do internal brake cables, but the method I like is to cut the nipple off at the lever end, join the gear end of the new inner cable to the old cable with heat shrink, and then use the old to pull the new through.

Has saved me a lot of faffing.

100% liners is the easiest method undo rear derailleur cable and thread liner onto cable back up to the exit port on frame. The last bike I did a few months ago already had liners so this couldn't be done easily so it was a cranks off job. If your bar tape is rubbish might as well do that at the same time as it all at least partially have to be taken off.

internal routing uses various differetn systems. Look for the cable entry points on your frame. Are they removable? if you can loosen the port cover plate without disconnecting the cabling you should be able to tell if your outers are continuous (optimal for replacing cables) or if the cable travels free inside the frame (optimal for reducing friction while changing gear).

If the outers are not continuous I would look for videos on changing cables on your bike. They all have slightly different systems

Depends on the frame, but usually the gear cable housing stops at the frame entry point and the cable itself continues uncovered to the mech. when changing it's a good idea to slide a thin liner tube over the cable and leave it sticking out at all the entry/exit ports, so you can slide the new cable back through without any hassle. Your jagwire housing is probably fine, if the gear changes aren't as "snappy" as the once were its most likely because your cable is about to snap inside the Ultegra shifter - as is common with shimano

this is the sort of thing you slide through the frame over the cable, once the cable is through just pull it out and stick it in the tool box for future use 

https://www.amazon.co.uk/JAGWIRE-SLICK-LINER-ELITE-SHIFT/dp/B00UCMLVY0/r...