mnichols wrote:BTW, my experience is similar to horizon. I replace my cables before each tour, but have still had occasions when they have broken, which is why I carry spares.
I am replacing my cables several times per year, but they don't seem to last. Mostly they fray at the mech, but I have had a couple fray and break inside the shifter, and some with the odd strand coming lose. My local bike shop say that I most be heavy on them, but not sure how I am doing that - surely a shift is a shift?
I agree that gear cables (and sometimes brake cables) are bound to break sooner or later. I think that some cables are substandard (and there have been recalls on cables, even shimano ones).
However if you read engineering texts which describe how to design machines using wire ropes (cables) so that the wire ropes won't fatigue in use, you will quickly notice that nearly every bicycle product on the market takes the most appalling liberties with the known design rules.
This gives some information on how rope design affects fatigue life
http://pythonrope.com/wireropes/wr_techinfo_6.shtml
note that bicycle cables have fewer strands again than these, arranged in helical winds, not bundles (strands). So they (relatively speaking) fatigue even faster.
You can see some more data here;
http://www.wwwrope.com/product_pdfs/EL_TB_09.pdf
Realistically a good quality gear cable might do ~100000 shifts or so on a typical shifter drum. This is borne out in practice; a friend of mine (who changes gear very often) has calculated that his cables last no more than 130000 shifts before they fail.
In the rear mech pinch bolt the fatigue is often worse; shifts (between the smaller sprockets especially in shimano mechs, check out the cable angles) will bend the cable near the pinch bolt quite abruptly. Corrosive conditions, lack of lubrication, high force (shifting under load) will all add to the fatigue damage. Front mechs can suffer too in a similar way.
In any event sharp edges on pinch bolt clamps can damage cables during installation. Sometimes a little filing can put a useful radius on the clamp. If you solder the cable ends to try and stop the damage, it just moves the fatigue someplace else because the soldered cable doesn't bend so well, and where it does bend, it fatigues.
Very many 'V' (and especially cantilever brakes whose arms articulate more) have pinch bolts that force the cable to articulate in a tight bend under load. You couldn't expect to design anything similar to break cables any better than that.... and if they are fitted with 'M' straddle wires, V boots or similar, the damage can be hidden from view.
So I agree that there are very many components that are basically designed to break cables in short order. At one time you could mitigate this by buying 'braided' brake cables but these seem to be no longer available.
No one manufacturer gets it right all the time, either; Avid's disc calipers have reasonable radii on them but their cantis are just like everyone else's; mostly with sharp flexures in the cable next to the pinch bolt.
cheers