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I observed that the bowden of the rear brake calipers runs "naked" below the bike frame. (Also the gear shifting cables, but let's focus on brake cables for this question.)

(I.e. the section between the lever and the frame is covered with housing, then, below the frame, no housing, and then the section connecting the frame and the calipers, again has housing.)

The question is: what is the reason for this? And more importantly, if I replace the bowden for the rear brake, would it be a good idea to add housing below the frame as well (to protect the whole bowden from dirt and water, rather than leave that naked section)? Of course the housing would have to be interrupted at the suspension points, leaving a little bit of bowden "naked", but at least most of it would be protected?

I tried to come up with some reasons, for why that part is naked:

  • keep costst down (but that extra half meter of housing would not matter much, would it?)

  • physically not being possible to add housing, because of how the bowden is suspended: the housing for sure would not fit through the suspension points; however... can't you just cut the housing at that point? Sure a very small part of the bowden will still remain "naked", but maybe even that would be better than having the whole wire outside? (Or even better, couldn't they have just made the suspension points huge enough to fit the housing in the first place?)

  • if there was a continuous housing (like I proposed above, with the slightly huger suspension points), maybe the rear brake would not work, because the the wire would be too long, so it would easily bend, instead of the bowden sliding inside it: this seems plausible, but even in this case: wouldn't I be better off, by adding housing (albeit interrupted) below the frame? (and still: couldn't there be a mechanism at the suspension points, which presses the housing together just enough to keep it steady, and at the same time allow the bowden inside of it to slide?)

  • the interrupted housing would be worse than no housing at all, because it would allow dirt and water to enter, but it would collect, and be more difficult to remove: while this seems indeed plausible, don't we have the same problem where the housed section of the calipers and the levers starts?

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  • You mean "along the top-tube of the frame" or how shifters run below the downtube and often below the bottom bracket ?
    – Criggie
    Apr 11 at 8:05
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    @Criggie it's not unknown for brake cables to run under the downtube. My daughter's bike (V brakes) has that, while mine (disc brakes) runs on the side of the downtube, so "under the frame" could well mean "under the downtube"
    – Chris H
    Apr 11 at 20:22
  • As a side note, folding bikes like the Brompton actually have full length Bowden cables, because the folding mechanism makes it more complicated to route the cable in the traditional way. Apr 15 at 19:43
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Several reasons.

Firstly, you anyway need some holding points at the frame to keep the bowden cable at the correct location. If there was housing for the entire length, then you would probably need a bit different type of holding point (one designed for a zip tie) and there would probably need to be more holding points than just two at the same tube because a tensioned inner wire always runs straight but a cable housing may not run straight.

Secondly, every centimeter where the inner wire runs in housing adds friction. You want cable friction to be minimal for best brake feel. However, the only drawback with bare wire is that the points at which it goes inside the housing are prone for dirt intrusion that can increase friction. Typically the dirt intrusion risk is minimal and the friction the extra dirt would create is so small that it's better to have exposed inner wire than it's to have whole-length housing. With gear cables, the dirt intrusion makes shifting worse so there are end caps with o-rings that have a worse friction when new but better friction when dirty.

Thirdly, as you correctly note, every centimeter of housing has a cost. By minimizing the amount of housing needed, it's possible to shave one dollar or so from the bike cost. This effect is added to the effect of needing less holding points which might save another dollar.

Also housing has a non-zero weight and air resistance. Some weight weenies might object to having excessive housing, but this effect is minimal and probably the air resistance of these two is the main consideration.

Today, there are bikes that have whole-length housing for both the gear cables and brake cables (if they still use mechanicals; popularity of hydraulics seems to be increasing). Typically these bikes just put the housing for all cables inside the frame. It's a major pain in the ass to replace these, sometimes you can replace the inner wire with a tandem-length one and use the tandem-length inner wire to guide a new housing and then replace the tandem-length inner wire with a proper-length inner wire. But if you make the mistake of pulling the old housing out then you need a lot of effort with expensive magnetic cable pulling tools to guide the new housing. For steel frames such magnetic cable pulling tools won't work because the steel frame interrupts the magnetic field.

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    There is no technical need to put cables in housings inside tubes. It's perfectly possible to route brake/shift housings around the frame without even a single attachment, simply by wrapping them around the tubes in a slight helix. Works like a charm and removes all the headaches that usually come with replacing cables. It just won't look sleek... Apr 11 at 18:07
  • @cmaster-reinstatemonica: But continuous rear brake housing will be less stiff than using frame stops.
    – Michael
    Apr 12 at 5:31
  • @Michael I never put that into question. All I remarked upon was the original statement in this answer that whole-length housing bikes are basically required to be routed through the tubes (this has been edited out, thanks). And my comment itself only remarks upon the ease of repair. Apr 12 at 6:33
  • I don’t think magnetic internal routing tools are terribly expensive. The Shimano/Pro one, for example, retails for just 15 dollars more than their polymer coated shift cable/housing kits. Pretty handy for any home mechanic with more than one or two internally routed bikes, but not essential. Even with nothing in the frame, you can usually get by with just a regular shift cable and a speed/cadence magnet; it just involved some fishing. It’s not “a lot of effort with expensive magnetic cable pulling tools” IMO, the magnetic tools and the high effort are more of an either-or thing.
    – Pisco
    Apr 12 at 15:24
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You do not really need the housing functionally, the bowden runs straight between two fixed points. There is nothing that could compress or deform. It just works well as is. You only could add it as a mechanical protection but it is not customary and could add some additional unnecessary friction.

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Racier bikes do it above all because it's weight that can be taken away. That may sound trifling, but you get to a light bike by marginal gains in lots of little places.

On other bikes it's about the compromise between squish and contaminant ingress.

Interrupted housing gives more entry points to water and dirt. So it needs to be replaced more and can be subject more easily to friction and poor feel.

All housing, but particularly traditional spiral brake housing, compresses under force. The more of it you have, the more compression is happening. So in other words if you compare the same brake and lever set up with interrupted housing versus continuous, the continuous setup has more of its lever throw being used up just on the housing compressing.

Front brakes have less distance to go and so this effect is usually minimal. Front brakes usually feel firmer than the back. When you give the back continuous housing, this disparate feeling is maximized. Some riders don't care about that but it really bothers others.

A case in point is the Speedvagen bare cable routing. I got to play with one once at NAHBS and it was pretty enlightening. Very symmetrical lever feel. It only works on seatmast frames with cantis or centerpulls, and executing it to not add much friction is probably very time-consuming. But I think there's a strong argument that given those other things, it's how the routing "should" be done. enter image description here

Compressionless brake housing mostly obliterates the issue, but it's more mechanically finicky and winds up needing replacement more often, so it's not all upside.

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