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This is a general question, regarding the routing of the cables of both brakes and derailleurs on a frame.

I find it highly practical to mount all cables along the bottom of the top tube. This offers the following advantages:

  • The wide down tube protects the cabling from mud and gravy, originating at the front tire
  • The top tube can support weight e.g. sitting* there or supporting luggage.

What are the reasons of the other cabling schemes? I am most puzzled the combined scheme "2 cables on bottom of the bottom tube + 1 cable on top of the top tube".

* one of my pleasant habits is, when there are no free seats in the park and the grass is wet, sitting on the front tube, with the bike tilted, one hand on the brake and the other holding a beer

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As to why there is no "optimum solution", that's just the way life is -- everything's a compromise of some sort. –  Daniel R Hicks Jan 8 '13 at 16:48
    
I have seen bikes with cables routed inside the frame –  Mark W Jan 8 '13 at 21:32
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I carry bike on my shoulder to 3. floor. I think if rear derailleur shifter cable would be routed below the top tube my shoulder would get it out of tune quite quickly. –  Davorin Ruševljan Jan 9 '13 at 9:03
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3 Answers 3

up vote 4 down vote accepted

In general, designers will seek to achieve the shortest possible cable routes while minimizing the number and severity of turns. And off-road bikes will be designed with consideration of problems due to running through underbrush, etc.

Assuming a standard diamond frame: Under the top tube is generally a good place for the rear brake and derailer cables (except, of course, for bikes with downtube shifters). It's usually best to run the front derailer cable along the down tube, though.

Where/how are your cables routed that it's bugging you?

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For my specific setup, I don't see how it would be possible to route the front derailleur cable along the top tube. As it currently stands, it goes through the down tube under the bottom bracket, and pulls down on the mechanism. I don't think it would be possible for the cable to pull down on the mechanism if it was routed along the top tube. –  Kibbee Jan 8 '13 at 13:28
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@Kibbee - There are top-pull front derailers. And it used to be common to have pulleys for the front derailer cable, so running across the top, down the seat tube, then back up to a pull-down derailer would not have been out of the question. But, as I suggested, usually the front derailer cable runs down the down tube, to minimize length and turns. –  Daniel R Hicks Jan 8 '13 at 16:45
    
The cables on my CX bike are routed as pointed in the question. I guess the rear cables are routed along the top of the frame to facilitate shouldering, as ** Glenn Gervais** points out. Also, the front derauler cable runs under the bottom tube, to minimize length. So am I correct that the designers at Specialized chose to avoid the sharp 90 degrees bend under the seatpost, even at the cost of all the mud of mountain biking, hitting directly the cable? –  Vorac Jan 9 '13 at 9:15
    
@Vorac - Given that one can't avoid mud on the front derailer, avoiding it on the cable hardly seems worth the effort. –  Daniel R Hicks Jan 9 '13 at 12:36
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What are the reasons of the other cabling schemes?

A combination of practicality and aesthetics. Which direction it leans depends on the frame and routing scheme.

I am most puzzled the combined scheme "2 cables on bottom of the bottom tube + 1 cable on top of the top tube".

This is for practicality. The cable across the top of the top tube is for the rear brake and puts less of a bend in the housing that leads to the brake caliper- you reap the most benefit from this cable routing scheme with rim brakes.

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What are the reasons of the other cabling schemes?

Most Cyclocross bikes will route all the rear cables along the top of the top tube. In a cyclocross race, you will get off and shoulder your bike, the cables would get in the way on the underside of the top tube. Many CX bikes have a flat bottom top tube as well for easier shouldering. Routing the cables along the top keep them cleaner too, cyclocross races get muddy and if they put the cables on the bottom of the downtube, they would get very wet, muddy and allow grime into the housing.

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Many Mountain bikes run the cables along the top of the top tube for the same reasons. (Although now days most ride groomed/manufactured trails not a lot different to streets. Portage is becoming a thing of the past) –  mattnz Jan 8 '13 at 23:29
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