Gear Cable Stretch and Tension - Rocket Science or Bike Tech 101?

[AUbicycles]

I would love to hear from the experts on this, potentially a very very basic part of bike tech but I will admit that I have rarely taken the time to understand it better, so now is my chance.

We are talking gear cables - I understand that new cables are good and ongoing maintenance adjusting the cable tension is required during the lifetime - as the cable wears and stretches - to ensure smooth shifting.

My specific question is why does a stretched gear cable need to be replaced? The front and rear derailleurs have springs and in this context the cable as requires a fixed length and tension but elasticity is required. During wear, the tension is adjusted to counter the stretch and that should be it.

Why would a cable (excluding cable outers) have a limited lifespan meaning that replacing it leads to a significant improvement in shifting?

Two things come to mind, first is a coated cable so the coating is also subject to wear from rubbing, stretching and degradation over time. The second is that cable elasticity helps shifting so a worn cable has less internal 'spring' leading to less forgiving shifting (related to ability to tension).

Look forward to having your thoughts, wild claims and facts on this.

Christopher

[Derny Driver]

At the places where the cable bends, it fatigues and eventually breaks. Downtube shift cables do not need replacing as often as the modern ones where the cable end is wrapped around the inside of a ratchetted integrated brake / gear lever.

[bychosis]

I wouldn’t have though that a stretched cable needs replacing, but at some point (will depend on the particular steel type) the stretch is too much for the steel in the cable and it will fatigue and break he individual stands which get caught up in the mechanisms and housings.

[Thoglette]

I'm sure Duck will clear this up presently but my understanding and experience is that "stretch" happens early in the life of a cable. Basically new cables "settle in" and I always have to adjust some slack out of new cables after the first weeks of use.

Cables fail via fatigue, strand by strand, as bychosis notes. Typically wherever they have to bend: usually near fixed anchor points or in the shifter.

[Duck!]

Pretty much all of the above. Yes, cables stretch over their lifespan, but mostly when new; you'll tend to find after replacing cables that you'll need to retune the gears a fair bit soon after fitting the new cables, but that will become less frequent as they settle in.

There isn't really any need to replace inner cables purely because they've stretched. However stainless steel is prone to work hardening and breaking where it is stressed around the shifter mechanism.

Different coatings behave differently, some remain intact for the life of the cable, while other will peel off and clog the outers, causing poor shifting.

Outer casings wear, particularly in tighter curves along the handlebar, and they too need periodic replacement.

[NewStew]

I read somewhere that the majority of the early "stretch" in the cables is actually caused by the outers moving into the end caps and stops and becoming properly seated. From my understanding of stainless steel wire and also changing cable outers (not that much really) it sort of makes sense. Thoughts?

Not that it matters as the solution is still the same and as long as a persons internal working theory on what is happening leads to the correct action then that working theory is sound enough.

[AUbicycles]

Thanks you everyone for the input - all very logical and appreciate.

[10speedsemiracer]

I read somewhere that the majority of the early "stretch" in the cables is actually caused by the outers moving into the end caps and stops and becoming properly seated. From my understanding of stainless steel wire and also changing cable outers (not that much really) it sort of makes sense. Thoughts?

Not that it matters as the solution is still the same and as long as a persons internal working theory on what is happening leads to the correct action then that working theory is sound enough.

This is what I was taught as well, i.e. cables don't stretch until later in their life and the initial experience of shifting going off after the first few weeks is down to cable housing and ferrules compressing and seating after early repeated use. When cabling I get around this by giving everything a good squish with the end point held immobile, and then retuning shift/brake cable. So for a rear derailleur, the process is to drop the RD to the smallest cog-hold it in place-work the shifter thru a click or two and release and then check shifting. The better prepped the cable housing is, the smaller the amount of variation before/after. Same for the brakes. wedge something (icy pole stick sans icy pole) between brake pads and braking surface, moderate squish on brake lever and release, then readjust.

I honestly don't know if this is truly the case however seems to work for me and has for a long time.

[Duck!]

I disagree. Yes there would be some compression in the ferrules, but the majority is in the cable itself. The steel wire, regardless of whether it's stainless or not, is elastic, so it does stretch, and it loses its elasticity over time. Secondly, the cable is not made of straight strands of wire, it's helical winding, so under tension it will try to elongate & straighten out the winding, resulting in detuning of the gears.

If anyone else on here plays guitar, they'll be familiar with stretching in new strings; when freshly fitted they'll need frequent retuning until they stretch a bit & settle in, then they'll be quite stable, until they get really old & dead. Gear cables are the same, except they don't really go beyond that settled state until they break strands and/or the outers wear out.

[AUbicycles]

If anyone else on here plays guitar, they'll be familiar with stretching in new strings

I was going to use this as an example and was considering the dead strings which is a whole new topic. But one trick I was told was to boil the strings first but I don't know if just old rockers voodoo or actually useful.

With creaky knees, the tone and twang of the gear cable probably can't be heard anyway.