Jimbolee is right, but there's another mechanism, and it applies to all multi-strand cables when new. When the cable (or rope, to which this also applies) is made the strands are twisted together in the pattern for the job the cable's made for. At the end of the process the cable is put under tension to pull the strands together. Friction between the strands means that they don't quite bed as closely as they could and this tensioning helps overcome that.
Once out in the real world, with real world patterns of tensioning and release that process of the strands bedding together is completed, with the result that the cable or rope is slightly longer than it was at the end of manufacturing. Because cables only work in tension not in compression this is normally a one-way ratchet type effect.
That's why there's a readjustment needed after a few weeks riding. the same applies to other things with cables but most now have self adjustment. The weight penalty would be too high on a bike.
I did a work placement at a cable and rope maker when at school (they closed years ago, another bit of British industry lost forever) and looked at this through a microscope. I can still remember my surprise at how big this effect was on the 1 7/8" (I think) crane cables they were making for the Port of London docks. The docks are long gone as well of course.
I am a total numbty when it comes to mechanical, DIY type things but I went on a "course" at Evan's cycles; it was free! The one I attended was @ the Chill Factor-e in Trafford but I think other branches do it and maybe other cycle stockists.
Over the course of a morning, they (patiently) showed us the very basics, even puncture repair.
In all, they covered:
- Repairing a puncture
- Adjusting (cable) brakes, both rim and disc brakes, though I had to specifically request covering disc brakes
- Adjusting gears
- Cleaning the chain
- Lubricating the chain
I guess it was a selling opportunity (I came home with a chain scrubber thingy) but there was no pressure and it was fun.
Sounds excellent.
When I bought my first 'real' bike at an LBS in the 1960s the owner invited you to go round during workshop time (Wednesday early closing or after 5 pm) so he could show you how to do everything you'd ever need to your particular bike.
He covered the lot - including stripping out BB and headset, gears, brakes, trueing and rebuilding wheels, the lot.
I thought that sort of thing had gone - evidently not.
I know its been worked on cos its different gears that are rattling (very noisy, not just a bit). Is it possible i've knocked something putting it back in the car or is it likely the workshop had an 'off day'.
