Subject: SIS ® Cable Info
From: Jobst Brandt
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 0.5 mm diameter round spring steel wire wrapped in a 100 mm period helix around a 2.5 mm plastic tube. The assembly is held together by a 5 mm 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.
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