Bill Sornson wrote:
Mike Jacoubowsky wrote:
{snip}

One of the variables we've recently found responsible for shortening
cable life is the use of drink mixes in the downtube water bottle,
which will sometimes get on the gear cable and run down to the bottom
bracket cable guide, where it will cause a lot of increased friction
in the system. Another is worn shift cable housing; I'd recommend
replacing the housing at the same time as the cable. And if neither
of those are responsible for your cables breaking, it's possible you
have a lever that simply likes to eat cables. Shimano admits that
there are some out there, and they'll replace, on a case-by-case
basis, levers that are no longer functional because a cable has
broken off inside the lever and the head cannot be removed.

This true of both 9- and 10-speed systems? Sure seems to be the case with
/my/ 9-ers -- and always at the lowest altitude point of my rides!
(Literally -- breaks have occurred at bottom of steep climbs with long ways
to go after that.)

That happened a few times for me and is one of the reasons I switched to
"low-normal" rear derailers. On one occasion this occurred about 25 miles
from home. I was too lazy to stop and move the set screw to force a higher
gear or implement some other temporary fix. Limited to about 16mph maximum
speed (spinning at about 120 rpm). I found myself home only about 5-10
minutes later than I expected.

My Ultegra 9 rear shifter broke off a cable head at least once; and my
Dura-Ace rear brifter -- replaced once under warranty after breaking off a
head and then crapping out -- broke another one just last week. Got the
head out after some doing and it seems OK now, but that's at least 3 gear
cables that have lost heads (might be one more) on two bikes in 3-4 years.

I thought I heard something about defective cables, too, but these have been
various brands (Shimano, Aztec, Jagwire, etc.).

When I used SunTour's bar-end shifters and later Shimano Rapid-Fire shifters,
the cable would always break within 3cm of the shifter end barrel and appeared
to break at a location where the cable was making a sharp turn in the shifter
mechanism. I'd get about 4000-5000 miles on rear shift cables (QBP
"stainless") with these shifters, and I shift frequently.

Since I switched to using SRAM Grip Shifters I haven't broken a rear cable
yet, and one of my bikes has over 12k miles with the same rear shift cable. A
recent inspection showed no sharp bends in the cable and no broken strands.

--
Bill Bushnell
http://pobox.com/~bushnell/