I have a problem where I got a frame made I asked for internal cable routing and it has arrived but there is no provision for the cable to pass through the bottom bracket.
It is a titanium frame and I'm trying to pass the hydraulic cables through but there seems no way of doing so without going under the shell leaving the cable exposed.
No doubt it's a little disappointing to have this hydraulic hose outside the BB shell if you were expecting fully concealed routing but what you have is the norm in a titanium frame. It looks like a lovely bike, I'm sure this small detail won't stop you enjoying the bike to the full!
Looks to me like SRAM Force AXS chainset, with a threaded bottom bracket shell. The crank spindle has a pretty large diameter and basically fills the whole of the bottom bracket shell. See below bottom bracket image, the plastic sleeve is nearly the same diameter as the threading. There isn't any room in the BB shell for hydraulic lines or gear cable routing. This is how metal frames are built, the down tube, seat tube and chain stays are attached to the cylindrical bb shell which is a very standardised dimension, while BB and crank standards have developed to make maximal use of the available space.
If there is room, perhaps in a larger spec shell like T47 or some other modification, the task is still difficult and would add cost to the build with no real benefit. Some builders do offer fully internal, but they are in the minority. The hose would be forced into a tight space with very tight bends which isn't ideal and the routing would make maintenance even more of a chore.
When you have bikes with cables and hoses which pass around the crank spindle inside the frame with plenty of room, these are carbon fibre bikes where the designers have the option to create space in the frame for the lines to pass through and even gear cable routing with removable covers.
image sram.com
Not all metal frames may be built with the hydraulic brake hose routing exiting the frame at the bottom bracket. In principle, with an oversize BB, e.g. a T47 threaded or a PF30 press-fit, there could be sufficient room to route the hose through the bottom bracket shell around a 29-30mm spindle.
Per this post on the Weightweenies forum, some titanium frame builders prefer to enclose the hoses in an internal sleeve as they go through the frame tube. Recall that metal frames are built from individual tubes that are welded together. It may be too difficult to pull off an internal sleeve that goes all the way from the down tube's entry port all the way to the exit port on the seat stay.
However, not all metal framebuilders appear to adhere to this philosophy. I have no insight into how much of a problem the hydraulic hose will pose if it's not run through an internal sleeve.
Last, as Swifty observed, your BB shell looks relatively small. If it's an English BB shell and you have a SRAM AXS crankset, then that 30mm spindle takes up almost all the space inside the BB shell. I suspect there would be no room to route a disc hose through the space in any case.
In theory, having that small section of externally routed cable should affect the frame's aerodynamics, but the effect is likely to be infinitesimal. The section that's exposed to the wind along the downtube appears extremely short. Further, it's around the crank, and the air there is already dirty from you pedaling. Aesthetically, because that area is around the cranks, it may be hard to notice the external hose in that section. It is true that most performance carbon frames have full internal routing. Nevertheless, there are both objective and subjective reasons that this is unlikely to be a big deal.
