Originally Posted by
JohnDThompson
Yes, it's due to metal fatigue from repeated winding on a small diameter drum. Lubrication won't prevent this. Regular inspection and replacement is needed.
N.B. downtube-mounted shift levers have the same problem, but at least there the drums are readily visible and when one strand breaks you can see it well before the whole cable fails (or, the broken strand pokes your finger when you go to shift gears).
If you mean
localized lubrication where the breakage occurs, that's pretty close to accurate.
But as for the balance of the cabling, especially where the rear-most length of housing has a tighter radius, any friction here (due to contaminated lubricant and thus failure to provide free cable movement) will greatly increase the cyclic tension variation, so may have a great effect on the cable's fatigue life up at the shifter end. Cable friction out at the "far" end of the cable (near the derailer) also imposes the biggest cyclic elastic
change in the cable's length, so any friction out back will have the greatest negative effect on shifting precision.
One last related issue occurs when older vintage bikes (having a cable guide at the bottom bracket
without polymer lining), and which no integrated shifter
system (with necessarily-long cables) was really designed to cope with. So in these cases, one may need to design some sort of
cable-path lining below or above the bottom bracket shell, to prevent the expected high level of friction whenever metal-on-metal contact might occur. There is no grease that will provide low friction at the guide the way that polymer lining will, so I always go to the trouble (even on friction-shifted bikes) because of the return on invested effort.
Having an assortment of spaghetti tubing on hand, sourced from either bikes or from old spray bottles, can allow one to fit lining to the bb guide such that it stays in place either by friction-fit or using fast-setting epoxy. I sometimes heat and stretch such tubing so as to modify it's diameter, making it fit and stay put before trimming away any visibly-protruding parts. Such tubing can even be slit along it's length, toward the
outside of the bend, so as to allow fitting it to the cable without having to first remove the cable (the resulting small bit of added cable tension will have to be slackened however). I've done this countless times by now.
My Bridgestone submariner has a pulley wheel guiding the cable above the bb, it's the smoothest of all, making for great shifting even with a lowly old Lark/RS derailer shifting across a 6s, 13-34t freewheel!
Lastly, rider technique counts. Slamming the lever to it's internal stop when shifting to the largest cog can't be good, and is why Shimano's 11s levers incorporated more-enforced lever position stops (with their subsequent front derailers having no hi-limit screw at all, only a reverse-acting "boost" hi screw, ...since the limit stop screw was no longer needed).