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Thanks to Nate W's answer here, I know that there are different cable housings for brake and shifter cables. Assuming the cables match the housing diameter...

Michael come lately
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2 Answers2

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spiraled brake housing would compress when shifting which would diminish the accuracy and using lengthwise cables for brakes can't handle the stress and split wherever the tension is too great.

this article explains as much: https://www.pinkbike.com/news/To-The-Point-Shift-Cables-2013.html

birthofearth
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  • Another point - the thinner gear inner will have more space around it when inside the bigger outer cable, so more space for ingress of water/dirt inside the outer. – Criggie Jul 16 '20 at 20:33
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    @Criggie I think your second point is what birthofearth meant with his/her first (half of a) sentence. – Michael come lately Jul 16 '20 at 21:50
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The opposite case, using lengthwise-braced shifter cable for a brake:

  1. The brake inner cable probably won't fit through the shifter outer housing, because its slightly thicker than a gear cable.
  2. Brakes might feel harsher because there's no shock absorbing from the housing's spiral.
  3. The gear outer cable would be a loose fit into levers and calipers, permitting the ingress of more water/dirt.
  4. Also, a smaller cable outer will move around inside a socket more than a good fitting outer, letting it mush the ends a bit more.
Criggie
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    Criggie, you are missing the most important point. Gear cable housings are not strong enough to hold under the considerable strain of brakes. The housing would break, the brake fail. Dangerous! – gschenk Jul 16 '20 at 22:19
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    I understand that lengthwise-braced cables don't bend as easily without kinking, but someone correct me if this is wrong. – James Bradbury Jul 17 '20 at 10:23