This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
After Joe Gorin described the SIS "non-compressive" cable housing to
me I got myself a sample to understand what the difference is. I
believe "non-compressive" is a misnomer. This cable housing is NOT
non-compressive but rather a constant length housing. As far as I can
determine, and from reports from bike shops, this housing should not
be used for brakes because it is relatively weak in compression, the
principal stress for brake housing.
SIS housing is made of 18 strands of 0.5mm diameter round spring steel
wire wrapped in a 100mm period helix around a 2.5mm plastic tube. The
assembly is held together by a 5mm OD plastic housing to make a
relatively stiff cable housing. Because the structural wires lie in a
helix, the housing length remains constant when bent in a curve. Each
strand of the housing lies both on the inside and outside of the curve
so on the average the wire path length remains constant, as does the
housing centerline where the control cable resides. Hence, no length
change. A brake cable housing, in contrast, changes length with
curvature because only the inside of the curve remains at constant
length while the outside (and centerline) expands.
Shimano recommends this cable only for shift control but makes no
special effort to warn against the danger of its use for brakes. It
should not be used for anything other than shift cables because SIS
housing cannot safely withstand compression. Its wires stand on end
and have no compressive strength without the stiff plastic housing
that holds them together. They aren't even curved wires, so they
splay out when the outer shield is removed. Under continuous high
load of braking, the plastic outer housing can burst leaving no
support. Besides, in its current design it is only half as flexible
as brake cable because its outer shell is made of structurally stiff
plastic unlike the brake cable housing that uses a soft vinyl coating.
Because brake cables transmit force rather than position, SIS cable,
even if safe, would have no benefit. In contrast, with handlebar
controls to give precise shift positioning, SIS housing can offer some
advantage since the cable must move though steering angles. SIS
housing has no benefit for downtube attached shifters because the
cable bends do not change.