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Basically as the title says. Why won't a "roadbike" bike cable fit into an MTB, or vice-versa?

Consider this one. What would happen if this was mounted in an MTB (or any other kind of bike)?

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  • @clcto I avoid those cables wherever possible because the single-purpose ones have a smooth weld on the bare end, rather than cut edges. They're easier to insert and don't scratch up the insides of lined housings. On my tourer if I get the right length I don't even have to cut the back cable
    – Chris H
    Apr 1, 2021 at 6:19

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The difference is the lump on the end that engages with the brake lever.

(right) A flat bar lever generally looks like a small cylinder or barrel, with the wire coming out the side.

(left) A drop bar lever is more of a "droplet" shape with the wire coming out the axis. Campagnolo ones are different again.

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This wire comes with one on each end, and you cut off the end you don't need. Reduces the stock/inventory count and increases convenience, but it a little wasteful.

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    The disadvantage of cables which have both types of ends is that they can fray after you cut them off. Cables for a single type have a soldered end which is basically impossible to fray.
    – Michael
    Mar 31, 2021 at 18:54
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    @Michael true - I've soldered them myself before cutting, and some people use superglue.
    – Criggie ♦
    Mar 31, 2021 at 20:35
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    Or twist them clockwise as you thread them into the housing.
    – spikey_richie
    Apr 1, 2021 at 15:30
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If I remember correctly, the road bike kits are shorter because many road bikes run interrupted housing. They also include a special section of spiral wound shift housing for the rear derailleur final loop. The mountain bike kits are longer to accommodate full length housing on MTBs.

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    It might include less outer cable, but the inner cable still has to go the full distance. Curiously, they generally have 2 inner cables of length sufficient to do the rear brake twice, and not a shorter inner cable for the front brake. Ease of inventory perhaps ?
    – Criggie ♦
    Apr 1, 2021 at 10:06
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Why won't a "roadbike" bike cable fit into an MTB, or vice-versa?

Both the housing and the inner wire by any name brand are different on road bikes and MTBs. And this is not just only the shape of the cable end, which makes it physically impossible to attach a road bike cable to an MTB lever. It's also in the structure of the inner wire and the cable.

The reason for the difference is the different cable pull ratio. V brakes on MTBs have more than twice the mechanical advantage than cantilever and caliper brakes. Thus, more cable pull and less force is required on V brakes.

Thus, the structure of V brake cabling system is different from that of road bike cabling system. The outer cable is optimized to allow the inner wire to slide in it with as little friction as possible. The inner wire strands are mutilated by a "smoothening" process to make the inner wire surface as smooth as possible. This reduces friction but also reduces the amount of force the cable can withstand. The reduction in amount of force it can withstand doesn't matter because V brake cables due to their mechanical advantage never see large forces.

Road bikes, on the other hand, must have an outer cable that is dimensionally stable even when transmitting great forces to result in a brake feel that isn't "mushy". The inner wire is never mutilated in any manner by any "smoothening" process because it could result in an inner wire that might be more prone to fail.

You can obviously use road bike cables on V brakes but the cable friction is larger so the brake feel, although non-"mushy", is quite bad due to the large friction.

You should never use any V brake outer cables or inner wires on caliper or cantilever brakes.

For example, Shimano differentiates V brake and road bike inner wires through using different cable end, whereas the outer cables are named "M system" (for V brakes) and "SLR" (for caliper and cantilever brakes in road bikes).

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  • Interesting, thanks! That brings up a further question, though: what about "universal" cables that have both ends? I suppose those must be road bike cables, with one V-brake end? (Since you say that using V-brake cable in road bikes is the worst of the two options, because that would be more prone to fail.)
    – Attilio
    Apr 1, 2021 at 12:12
  • @Attilio There are off-brand manufacturers making the road bike cables using the "smoothening" process but as far as I know, the only Shimano universal cables are non-smoothened.
    – juhist
    Apr 1, 2021 at 15:09
  • So, with Shimano what you get is a road-bike cable with an extra V-brake type cap at one end. (And with those off-brand manufacturers you get a MTB-type cable with an extra cantilever cap at the end, if I understand correctly.)
    – Attilio
    Apr 1, 2021 at 16:09

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