Following...I have wondered the same thing. I have a 2012 bike that has original cabling except the rear derailer. This cable snapped after about 2 years. I do all my training indoors, try to cover everything well with towels and have a big box fan. I only recall 2 races in 6 years with light rain. Now that I have said that, every race this year will be wet.

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Blog: https://swimbikerunrinserepeat.wordpress.com

I think it largely depends on how much you sweat on them. I replace mine when they no long run through the housing smoothly. Probably every 3-4 years. I have a friend who replaces his every year and still has problems because he drenches everything in sweat every time he rides.

If they are working fine, I wouldn't worry about it.

I used to do shift and brake cables yearly. Now that I only have brake cables I do them every other.

Pretty much this, sweat rate and chemistry makes a big difference. I sweat a lot so my intervals are a little quicker.

I'll do mine when the FD starts to stick, usually once a year. I will try to make the rear last a bit longer, maybe doing it every other FD change if I can make it that long. Brakes only when needed, maybe 2-3 years.

Would internal cables me a difference here? I wouldn’t go as far as to say they never see any sweat or moisture but being protected like that has to do something.

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I still lapped everyone on the couch!

Might do -something-, but as you said, sweat and moisture/rain will find a way. It did with my Trek SC9. Not horribly, but noticeable. And once you've done the cabling once on an internally routed frame, its pretty easy to do it the next time since you have the old housing as a guide to cut the exact right length for the new one. My LBS sells me the shimano cable housing for something like $3/foot I think, and I buy plain ol $5 cables. Pretty cheap.

Winter bike (7800 dura ace ) is into its 7th year on the same cables,due to it not braking or changing gear as fabulous as normal it'll get done this year,good summer road bike (campag centaur) into its 8th year,still perfect,do normally do the race bike every year though,just bought a new giant trinity advanced pro,I'll leave that as long as I can,looks a pia to do!

I change my housing twice a year, and cables about 4x a year. This is for a road bike with external cable routing, about 9-11k miles a year, almost all outdoors and sometimes rainy weather.

Cables are so cheap and easy to swap out(at least for me with no internal routing). Replacing housing is a bit more of a chore, but I usually have to replace bar tape 2x a year so I may as well do it at that point as well. But when I do replace both, it makes an immediate difference.

I think for most TT bikes with internal cable routing, you save yourself from a lot of the exposure to dirt that can get on the cables and into the housing so the frequency you would need to replace them should be far less than what I do.