Internal cable routing is better

Bike frames are built to withstand pedaling forces, cornering forces, and road shock. These all impact the structure in sort of broad, general ways. Cable forces put point loads on very small areas of the frame. They're not huge forces, but the strength required to hold up to them is quite different from the more distributed stress of being a bicycle. 

Internal cable routing may seem like a needlessly complicated answer to a relatively trivial problem of air drag -- because it is -- but it also strikes me as a better way to load the carbon fiber frame, compared to attaching external cable stops. Because I don't build my life around the latest technology, I may have missed a memo on this. I focus on the annoyance of working on things that I can't see and can't reach. Belatedly I realized that it's probably easier and stronger to make reinforced entry and exit holes than it is so attach external anchor points.

A few years ago, Specialized road bikes even came with instructions not to pull on the external cables to seat the housings as we would with metal frames with welded cable stops. The stops on these carbon frames could pop off if pulled outward. External stops on all carbon bikes displayed various ways to reinforce the bond, including little pop rivets.

It's still a pain in the ass. If it went away I would not miss it.

On metal frames, holes may be a liability. Back in the 1990s I found cracks in an aluminum Klein frame at the entry hole for an internally routed brake cable. Aluminum being aluminum, this represented a potentially terminal condition. There seemed to be no good way to stop the crack from spreading in the thin metal, at least not with the skills and equipment available in our shop. The owner seemed angry at us for finding it. But to know is to be responsible. We couldn't just let him ride on it without knowing his risk.

Not every shop agrees. A customer brought in a Dahon folding bike to get a flat tire fixed. He complained that he had taken the bike to a shop where he lives, to get tuned up and have the tubes replaced. The shop did the tuneup, but didn't do the tires. As he went on at some length, he mentioned that the other shop had told him there was a hairline crack in the head tube. Without explaining the dangers of abrupt catastrophic failure in aluminum, they took his money for the tuneup and told him to "keep an eye on it."

The bike had been fitted with a very tall stem riser, atop a very short head tube.

Leverage is an amazing thing. The "hairline" crack was not hard to find.

We refused to work on his bike and advised him to junk it. Considering that we left him with the flat tire, he wasn't likely to jump on it soon.

Next up was a brake bleed on a mountain bike. There are two kinds of bleed: the classic removal of air from the line, and the more medieval leeching of excess fluid when someone filled the system without resetting the pistons first, leading to hot-weather lockups. This bike only needed the latter, which was nice. But some idiot somewhere had cut the rear brake line so short that I don't know why the rider hasn't torn it loose just making a tight turn, or in a mild crash that yanks the bars around.

I could go on, but the morning is evaporating quickly, and a long queue of repairs is still piled up at work.