Internal routing: do I need liner? If so, how use?

I'm cabling an old frame with internal routing that used to have Di2 on it.

Do I need to use the white plastic liner stuff (not cable housing) inside the frame? Is there any benefit from doing so or is it just extra faff?

If I use it, how far should it extend? eg all the way along the internal bit right to the front mech / chainstay exit hole, or just where the cable goes under the BB?

Is it for gear cables only or would I use it on the rear brake cable? I assume not as I think it is a pretty straight run through the top tube.

I have one bike with internal routing and only used liner round the BB shell guide, and for the front mech cable to pop through the grommet smoothly, without sawing the grommet itself. May be beneficial to run more if you cross the gear cables under the downtube, but otherwise I wouldn't think so.

If it does have stops don't need housing.

I just have a hole where the front mech cable comes out, about 7-8mm circle. So I could run the liner right to the front mech, from under the BB. But I thought the end would be exposed and would get dirt in it.

My Ridley was the same, and the bare cable would cheese wire the rubber bung. My solution (after some googling) was to run the cable with liner and clamp it in, then mould a grommet with Sugru

Ahh, I've found another little hole round the back where the cable goes! It has a conduit that goes down to the bottom of the BB, that looks nice and neat now. It was full of 8 years worth of gunk but I managed to force the cable through.

That big hole must be just for di2. I'll put a bit of tape over it.

Just discovered I could run housing up to the front mech. Don't think I will though as it would just be resting on the frame where the exit hole is, probably not designed for the force.

Unlike the little plastic insert where my cable goes into the down tube, it is taking the full force of the front shift. Maybe I'll end up running housing down the down tube...?

Shimano tech docs not quite kept up to date with developments in frames! This is from 11-speed 105 R7000.

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I’m not sure I understand this. If you run outer cable housing all the way through the frame to the mech how is it exerting force on the frame?
If you cut the outer you’ll need somewhere for the outer to stop against. There will be force there. If the frame wasn’t designed with a stop there I wouldn’t do it.

The big benefit of running a liner is much easier cable replacement in the future. It can also form a sealed system to help stop corrosion. So I'd run it right up to where the outer starts.

I can't run outer right through the frame as there isn't space in the cable guide under the BB, and the channel that goes through the frame from BB to the exit hole for front mech.
So I need to stop it somewhere. Only option really is that plastic insert where the cable enters the frame at the top of the down tube. I guess it is designed for the purpose, just seems to be asking a lot from a little piece of plastic.

The only alternative, which is starting to look quite appealing for an old bike, is to give up on internal routing and just cable tie outer all the way.

Yes, given I've bought it anyway I may as well use it wherever it will fit.

If you run outer cable housing all the way through the frame to the mech how is it exerting force on the frame?

Here I was referring to if I put a bit of outer between the mech and where the cable exits the frame behind the BB, like you do with a rear mech.

Totally depends on the frame. Many, many frames hide a whole bunch of crimes inside, and you might even find running Di2 is the only way it'll shift right.

If it's not a stupid bastard of a thing like the 1st generation Propel, you should be okay. Might take some head scratching and fettling, though. You need to minimise friction and anything that will add springiness to the system, especially for the rear.

If you can't run a full length of liner for whatever reason, the trick is to keep a length with your tools, and slip it over the cable and through the frame in future when replacing old cables.

Thanks everyone for thoughts. I've got it done, with maximum amount of liner being used, although I think it would have been fine if I hadn't used any. But it's definitely useful for re-cabling as I had to undo it a couple of times and was much easier just to whizz the cable through the liner. And I'll undo it again as I want to fit some inline adjusters.

It looks like it will shift ok, when I have enough cable tension, but I've not put a chain on it yet and tested properly. Main thing is that the cable runs smoothly through the liner and there doesn't seem to be anything too crazy going on in the hidden depths of the down tube!

If you grab the cable with some pliers in place of the RD's pinch bolt and pull on the end of the cable, and it feels reasonably solid rather than another mm or two coming if you pull really hard, you should be golden.