Originally Posted by
davidad
Compressionless housing is necessary for brakes, not index shifting.
You have it backwards, not as to the fact, but the terminology.
Brake housing is made of shallow steel coil spring that lays on itself in a fully compressed mode, while "compressionless" index housing has a high helix angle and depends on a ballistic outer cover to keep the strands from buckling.
The functional issue is the neutral axis as the housing is flexed. Brake housing cannot compress so when curved the neutral axis is in the inner wall with the outer end opening. This means that the center (where the wire runs) lengthens slightly. High helix index housing flexes with the neutral axis in the center so there's no change in the effective length, preserving index trim.
Unfortunately Shimano chose to call index housing compressionless when they introduced it, though a more apt name would have been expansionless,
since the low helix housing it replaced is the one that's truly compressionless and suited to the higher load of brake cables.
BTW- a reminder
. High helix Index housing is totally unsuited for use on brakes. It depends on the support of the plastic outer cover for strength which will degrade over time with UV and oxidation leading to potential catastrophic failure (burst) in an emergency braking situation.
Low helix (brake) housing has continuous steel on steel support and will function regardless the condition of the cover. It's failure mode is expansion when kinked, which will make brakes spongy, but it won't fail suddenly in an emergency.