I seem to be having frequent issues with my gear cables, I’ve bust about 3 outers in the last few months.
I put new cables and new outer runs in, goes smooth as silk, but then very quickly shifting goes stiff, I’m using the same routing as before, same size outer sections…
Any idea why I could be suddenly having these issues?

What do you mean by “failing”?

Getting holes in them?

I had the same problem on my Roubaix, the problem being that the rear derailleur cable exits from inside the chainstay in an upwards direction then enters the derailleur adjuster in a downwards direction, so that at both ends water would run in and rust the ends of the steel inner reinforcements, hold grit and muck and mess up the shifting. It doesn’t help that the cables run under the bar tape so there’s a lot of friction at the front end.

I found that I could extend the life of the outers by packing around the ends inside and outside the ferrules with vaseline and by washing the bike down less often.

Try also using proper Shimano inner cable, not the unbranded stuff that comes in bulk in a big box at the LBS. It’s finer and more flexible.

Road or MTB?

If MTB, can you run full outer instead of partial? If so, do that.

Use decent ferrules at the ends. Harder ones in my opinion that don’t tend to split, then water gets in the ends and it expands and frays.

Use SP41 outer, at least for MTB (is this stuff used for road also?). Smooth running and internally greased. Any old stainless steel gear cable. Don’t waste money on teflon coating etc, the inner is greased up enough and stainless steel will run smooth.

Use *good* cable cutters on the outers to help prevent fraying with a clean cut.

Shimano SP4 sealed end caps will help too, if cables stiffening up is your problem.

have you got your end-stop set right for 1st gear? if its too far in, you might be able to get 1st gear but the lever is fighting the end-point which tensions the cabe up too tight. This effectively ends up with the cable cutting down into the plastic tubing and causing the same sort of stiffness you describe. Basically turns a brand new outer into a knackered one in a few rides.

Its a fine-line between a perfect setup and the chain disapearing into the spokes and takes a bit of patience to get right I found.

Good way to tell if its set right is listening/feeling for differences in the “click” when you shift down. Shifting down from 1st should sound the same as shifting down mid cassette and it shouldnt be any harder to shift up to 1st than up mid cassette.

Just a thought

It’s not trapped in some pivot / swingarm or anything daft is it?

Thanks for the suggestions… I will have another look

By failing I mean a couple have split, near the shifter, and one has bent, lost all integrity again near the shifter

If it’s failing by the shifter it might mean that you are running too short a section of outer from the shifter to the 1st stop. Check by turning the bars and seeing if the cable is being bent.

check the outers are not moving when the cable is at least tension.
maybe its coming part out of the shifter and when you change its not aligned properly, crushing/splitting the end?