Do inner-routed cables have segments without a housing?

Question

Bowden cables (whether for shifting or for brakes) running outside a frame save a little weight and expense by using the frame for compression in lieu of the housing in long stretches along the frame—using ferrules and cable stops brazed/soldered onto the frame.

Do inner-routed cables also sometimes have segments without a housing? This would significantly complicate replacing a cable, and the movement of the inner cable will eventually cause damage, by fraying the cable and by slowly cutting through something else, if it touches anything inside.

Background: a cross-section reveals a lot about the inner working of a device. A shoe shown sliced will tell me, as with a bike, how much mileage I can expect to get out of it. But since no one has yet sliced a bike frame to show us the details of inner-routed cables, I need to ask.

Answer 1

Mine does.

It's a Canondale Topstone, and the outers for the derailleurs stop where the cables enter the top of the downtube. They come out bare under the bottom bracket.

At least one model of Synapse routes the rear derailleur cable further inside (along the chainstay as well), in a light and flexible sleeve that serves mainly to guide a new cable into place, as it can't resist compression. Here the cable turns under the BB, guided under the BB shell. I've worked on this and it wasn't easy.

I've also had full length outers that went inside the downtube (a GT from 2010).

Don't forget that gear cables are always under tension once installed. There's a fairly tight limit on how much they can flex. And they're guided as necessary, by end stops for example. On my Topstone, the exit point under the bottom bracket is a plastic guide that ensures the cables run away from the tube walls.

Answer 2

It happens on occasion. My old 2019 Trek X-Caliber hardtail had this type of setup. Cables entered the top of the downtube and exited at the bottom. There were cable stops welded to the inside of the frame. There was no risk of frame damage because the downtube is straight.

It was indeed extremely difficult to route the cables because you needed to get the cable through the little holes in the cable stops, but it was all inside the frame and the only access/line of sight you had was the 5mm hole in the frame. Felt like I was like performing laparoscopic surgery, but without the camera.

Perhaps unsurprisingly, Trek moved to plain holes and full housing for the 2020 model frame (which I came to own after the 2019 cracked under warranty). I suppose the reduction in cable housing used per bike and the slight decrease in friction were not worth the hassle.

Answer 3

It pretty much depends on the segment. To take two examples:

• On an entry-level suspended hybrid: no housing inside the frame. Cables are only running in the downtube. They enter from the side of the downtube, and there's an opening in the downtube at the level of the bottom bracket where they exit from, From the bottom bracket, there's no housing and cables are mounted under the chainstays.
• On an upper-level e-hardtail, equipped for "utility" use, with fenders, rack, lights: full housing, but the cables are entering in the stem and totally internally routed. Rear brake and derailleur cables (and the speed sensor one) exit from the chainstays.

On your comment that "Someone on chat recently suggested that rattling of inner cables is a factor being used to distinguish between different market segments": the problem is not so much for derailleur cables (and brake cables for non-hydraulic setups), that remain under tension, but for hydraulic hoses that have no tension - on the example of the hybrid given above, I had to use zip ties to "fasten" the hose inside the downtube.

I don't think the right solutions are not well understood, they are understood, and depending on the segment, a solution is chosen or not. For example of the hybrid above, they probably expect the bikes to be used dominantly on pavement, where rattling problems are less of an issue. In the upper-segment example, the bike is advertised as all-rounder, more care is given because it's an expectation of the client - and also because cables are almost entirely internally routed, the path taken by the cables is much more complex.