Originally Posted by
JohnDThompson
The experimental aircraft guys recommend that in order to minimize the risk of cable failure, control cables use pulleys with at least 50x the diameter of the cable. That's not going to fit inside an integrated brake lever/shifter mechanism, so cable failure in those things is entirely expected. At least, failure of a shift cable is unlikely to be as catastrophic as failure of a control cable in an aircraft.
I was looking for a number for Bowden cable, can't remember where I saw it, probably somewhere like Machinery's Handbook. 50x is higher than I recall, for some reason 38x sounds right, but with aviation it pays to add a bigger margin. I have some cables (that look like miniature aircraft cable) intended for MTB roller stems, which are high-stress and high-consequence parts. A wire rope manufacturer recommends minimum sheave diameters anywhere from 18 to 34 rope diameters, depending on the construction of the rope i.e how many wires in a strand and how many strands in the rope, those would be for cranes, elevators, excavators and other stuff that shouldn't break. Some wire ropes have a fibre core to aid flexibility, but that can also shorten the life of the rope.