Internal cable routing question - first time builder

Hello everyone,
I have been reading the forum for a long time now, and learning from all of you while working on my first build. This is a great site full of inspiration and ideas, thank you!

My first question to you, experienced builders is whether you would do / avoid what I'm planning:

I'm building a fillet brazed Columbus Zona tube cyclocross frame for myself, as a first time build. All mitering is now complete. It will be using a Rival 1x11 groupset with hydraulic brakes. I'm planning to run the rear brake line and the rear derailleur cable inside the downtube. But having done my practice drilling / shaping on a scrap tube I had, it is quite a lot of material that needs to be removed from the tube, and I wonder if it would be structurally safe to do it.
Routing-wise I'm thinking it would be best if the two cables entered the down tube on the left hand side of the frame near the headtube, about 8-10cms from the miter, one above the other, quite close to each other, and exit at the bottom of the downtube on either side, close to the BB (8-10cms from the miter again), so each exit leads on to the bottom of the chainstays on either side of the frame.
I have stainless 7mm OD tubes that I plan using for the full length of the routing, brazed in place.

So: Would the downtube be strong enough still with the 4 holes? Or would it compromise the strength, and you would advise against it? Or would you place the entry / exit holes elsewhere to help structurally?

Any advice is much appreciated!
Thank you!
Gergely

As both a life long shop wrench and having taught a few how to build their first frame I would suggest avoiding internal routing for a few reasons, especially gear wires. Andy

Quote:

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Originally Posted by Gergely

... Any advice is much appreciated....

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Don't take any of these as universal truths, just personal opinions:
-Best advice on a beginner frame would be not to do it.
-You might probably have asked this question not wanting to hear so but how to make it best possible way even if result might not be good enough
-So, if you HAVE TO use internal routing on your very first frame (whatever the reasons you might have which conflict with first advice), practice it 20 times in scrap tubes of the same thickness and same specifications so to control your heat on the area with different material/thickness involved, as much danger as holes are the excess of heat.
-You might certainly consider using reinforcement plates or the like on the areas so to make the damage as little as possible on the long term (fatigue)
-Keep very much in mind the butted sections of the tube as to be clear on what are the possible weak not to get much action (holes, flames, etc)
-Keep very much in mind the forces involved on a working frame and which are the areas of biggest stress on a tube, so avoid the very bottom line "6 o'clock position" at the DT close to HT, as many forces go around there. Think of less demanding areas as 10&11 1&2, etc.
-Any hydraulic system would not suffer from wiggled routing, corners or curves, therefore either hydro disc brakes or rotor Uno gears would work the same on both external or internal
-Any cable operated system will certainly lose a bit of easiness on each routing corner/curve/obstacle you make, so the external routing would give a better performance (unless you work hard on your internal routing which probably means quite bigger-longer section holes)
-Always use stainless tubing for the internal routing and a complete route (no just cable stops at each entrance/exit, discontinued housing inside) so to make the whole thing as corrosion friendly as possible and make the manteinances much easier (would not recommend as pleasant task to change cables/housing on a discontinued system)
-Remember that making it succesfully once does not mean you might be ready to do it straight forward to anyone else, sometimes the first prototypes are safer as take so much time and care with many triple checks and are usually thought to your own use (where demands are well known for yourself), and all this does not necessarily mean you know the how/why to make it consistently safe to any other frame which might have very different demands (heavier rider, bigger size, tube diameters, thickness, etc)

Try to learn during the process, understand it

Thank you both for your quick responses. Very useful points. It all makes perfect sense. I will take your advice and go with external routing for now.
Thanks a lot
Gergely