Cable Port Positioning Opinion

Hey together,

I’m into my second frame and just wondered if I just did a mistake in positioning the cable port outlet just in front of the bottom bracket (see before/after). Is this an okay position? I tried to lay as much bronze as possible (also internal) to reinforce the area. Any tips/opinions welcome before I continue to braze it onto the BB - otherwise I do a restart here.


I probably would not have gone there, especially with the bottle boss that close…but if you had reinforced with a doubler plate it woudl be ok. You have a decent amount of filler around which will help to spread the stresses plus the port is thicker walled than the tube so plenty stiff. My gut feel is it will crack at some point but you’ll probably get a fair few years of use out of it before it does.


I like the concept. it’s a good way of routing the cables internally in the downtube.

What is the butt thickness and tube diameter?

Without lab testing, it’s impossible to know if that design will fail or survive. There are bikes with that section completely open:

The minimum butt length Columbus specifies is 40mm, so that is a good rule of thumb for a no-go zone. If it were me, I would not risk it. A $30 tube is a cheap lesson to learn.


Thanks for the input.

I also thought about this semiopen attachments of the DT as your photo, I also had a frame like this - no problems. I would consider this as a greater stress on the tube.

It’s a 08/05/08, 31,8 Tube on a pretty tall, allroad/rando kind of frame. I want a little flex in there.

I could ride it and inspect it regulary for cracks.

I could do a little gusset Style plate around the lower part before brazing it to the BB.

Or if I redo it, what would you consider a better position in that area? More up somewhere sideways on height between the bottle bosses?

I also suspect that it’s going to crack eventually, but you won’t know unless you build it. If i was 180lbs/ 80k or less, this was my second frame, and I didn’t intend to do match sprints or dirt jump, I’d give it a go and see what happens. After my second frame i was eager to build the third anyway.

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I recall seeing a few photos of the Rodeo Flaanimals cracking at the ports. They seemed to have been scrubbed from the internet:

It just goes to show that bikes that have passed testing can also fail. So copying a design that someone has done does not necessarily mean it is a good idea. Also, many of the fancy custom bike designs you see are not ridden hard enough to induce a failure.

Maybe put it between the bottle bosses and space out the bottle cage? To me, design is always a compromise:

  • downtube bottle cage
  • internal routing
  • 31.8 tube for flex
  • durability

I always underestimate the strength difference when you go up a tube diameter. The photo you posted looks like a 34.9 tube? I would feel much more confident putting holes in a bigger tube.

Ultimately I think the decision just comes down to your risk tolerance. All frames will break at some point!


What cable/cables is this for? It seems like a big port.

For your second frame I would run everything external except for having a small entry into the seat tube for the dropper cable (assuming you have a dropper). As a hobbyist the only thing that I run internally is lighting wires, and those can be through very small holes.


True, maybe I should stick to the easier things that early, but those ports seemed to look pretty easy beforehand :confused:

It’s a double entry/exit port for a shifter cable and a brake hose - planning a mechanic 1x setup.

No dropper - lighting cables will go through the BB into the chainstay.


True…copying production frames doesn’t lead to 100% safety.

I won’t continue until the weekend anyhow, so I just keep deciding forth and back until then :smiley:

Right now I think I will go on having in mind that this won’t be the “hard ridden, forever” frame. I also talked to a bike frame engineer at work, he seemed to be pretty relaxed about it.

I combine it with a thick fillet connecting the BB that stretches all the way to the port.

If it rides like a noodle or I find a crack I will cut it up into tubes again for a smaller frame then​:man_shrugging: can’t wait for building more frames :smiley:


My vote would be to go for it (if the frame is for you). If it cracks then you learn a lesson, get to build another frame, which is about the best possible outcome!


It’s is a port design which has an outer ‘holder’ with bolt-in adapters which can act as a cable stop, hose/outer guide. Di2 port or blanked off.

All the best,
Dan Chambers

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Where is this from?


Those are from Ceeway but you can also find them stateside from most framebuilding part distributors.


@Carl_Snarl has something similar at Metal Guru


A lot of good discussion here. I may be wrong here, but my understanding is that a 34.9 tube is stiffer but not “stronger” than a 31.8 tube and is therefore just as vulnerable to fatigue failure - (or does the greater stiffness protect the longer tube?).


Depends on the wall thickness, length, internal stresses, etc. whether the 34.9 tube is “stronger” than the 31.8 tube. Stiffness doesn’t have anything to do with fatigue and cracking. My guess is most cracks at the cable ports are combination of fatigue and a stress riser at the acute end of the slotted hole.


A stiffer tube is a stronger tube and will resist fatigue failures. This is the theoretical explanation:

  • Stress is the “pain the material feels”.
  • When you have a bigger tube, it spreads the force across more material (less stress)
  • The lower the stress, the longer the fatigue life (described by S-N curve)

From a real-world perspective, I think Dave is correct. Typically it’s a material or manufacturing defect that causes a failure. That being said, if you reduce the stress (by having a larger tube), you reduce the risk of failure.

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I went on, this is how it looks now. Not yet ridden, probably in 2 weeks or so🤞


There is plenty of filler there supporting it. It will most likely be ok. Just send it!!!