irc wrote:reohn2 wrote:I tend to leave the bike in low front high rear to release tension on cables when the it's stood for any length of time.
Thread drift but ............. can static tension in a cable, presumably well below max tension when bike is used have any effect on cable life?
unlikely, except under weird corrosion conditions. The static stresses are very small (in relation to the static breaking stress of the cable). Old hands operate on the principle that it might not do any good but it won't do any harm and (decades ago) the springs in mechs might not be well tempered, and might weaken over time. I don't think this is a legitimate concern these days, unless the springs are complete rubbish.
As for cable breakages I can't remember my last one. Assuming any obviously frayed or old worn cables are replaced it must be minimal. I'm not even carrying any spare cables on my tour this year where I'll be several days away from a bike shop at times.
It is thought that the repeated flexing is the thing that kills them; comparing the design of bicycle cables with those of wire rope in industrial machines with known (good) fatigue performance, the radius of curvature of the pulleys (sheaves) is way too small on bicycle systems, and there is something less than clever going on at most of the pinch bolts, too. Estimates of fatigue life of the cables is from tens to low hundreds of thousands of repetitions before failure.
One of my chums has a flattish commute and he makes lots of (needless) shifts; he has counted them and over a six month period it is in the region of 130000 shifts, at which point several brands of cable (stainless polished) are liable to failure. Weirdly (for him) this invariably happens inside the shifter, rather than at the pinch bolt, even though the cable is flexed in an evil way at the pinch bolt.
Thinking about this, it occurs to me that the cable near the rear mech pinch bolt is under almost constant tension (from the rear mech spring), whereas the cable inside an STI shifter goes to much higher tension (higher than the rear mech sees, due to cable friction...) eg when downshifting and then sees full slack conditions and sudden tension loads as the spool releases on its ratchet. It may be the combination of flexing and (suddenly) varying tension load that causes the (fatigue) damage to the cable inside the shifter.
This is different from the conditions that are seen in other shifters; flat bar STIs have much larger cable spools, thumbshifters, DT and bar end shifters see less violent variations in cable tension, and (old school) Campag ergos may see less violent variations in cable tension. With the possible exception of the Ergos (which have always had different shift ratio that is kinder to the cable, tension-variation-wise) the leverage is highest over the cable in an STI, by far.
My record for 'short cable life' for me is zero miles; on a STI-based system where there was no barrel adjuster for the front mech, I had to reclamp the cable at the pinch bolt several times. It didn't even survive that without starting to fray....
Brake cable setups are well-thought-out in most systems; the nipple is anchored in a freely-pivoting receptacle and the pinch bolt is free to swivel under load. Brake cable breakage is rare these days in such systems.
I didn't use to worry about cable failure until I realised that one broken strand could spoil the party so badly; a few times I've had to peel back one strand (and cut it in an exposed cable run) in order to keep a bike working, but gear cables have so few strands that every one is larger proportion of the whole. For the weight of the thing, carrying a spare gear cable on tour is (IMHO) a wise thing to do.
cheers
(I've taken to riding down to Newton-le-Willows station to get the train to Chester rather than doing a 2 hop trip from Wigan these days).
