Frayed cables!
This is a common problem with the modern shifters that route shift cables under the bar tape. There's a 90 degree curve inside the shifter body. (And some Sram rear derailleurs have a curved cable guide too, and this frayed on a local rider's bike.) The bending that happens while the cable moves when shifting gears eventually starts breaking strands.
My older 10 speed Campagnolo and 10,11 speed Shimano all do this. I broke official Campagnolo cables twice. I didn't learn my lesson the first time, I guess. I do my own cables, and carefully grind the ends of the housing flat -- there's no sharp edges. And the frayed part is always at the bend in the shifter.
It's the second most common "mechanical" after a flat tire!
Symptoms
For just a ride or two, the partly frayed cable starts getting noisy shifts, or missed shifts to a larger cog, or hesitant smaller cog shifts. Turn the adjuster barrel -- it's better for a brief time, then needs more adjustment. Now, usually in the same ride, the last couple of strands break. The derailleur goes to the smallest cog since there's no cable tension.
It's possible to inspect the bend on at least some shifter designs, but you have to pull back the hood to see. Checking weekly would be a way to extend the cable life quite a bit. Replace at the first broken strand?
Replacement:
For riders that shift quite often: Maybe 2500-3000 miles? The cables might last a whole year, it's somewhat unpredictable.
I usually just replaced the shifter wire, then replaced housing+wire every other time. Shift wires are cheap and it's easy to thread them through without needing to remove bar tape, etc. I rarely change the front cable, it's shifted once for every 500 or 1000 rear shifts, I think.
BUT!! if your cables run inside the frame, it's way more complicated. Essentially, you need to thread a thin guide tube onto the existing cable, then that tube sticks out at both ends of the frame to guide the new wire. Don't just pull it out without this, there's no guides installed inside the frame, and the fix will be annoying.
Last edited by rm -rf; 05-01-22 at 08:01 PM.