I snapped a gear cable inner at Buller last time I went so ordered some of these as replacements: http://www.wiggle.co.uk/transfil-gear-cable-inner/. I replaced the cable housing too with housing of the same brand, but the rear derailleur did not shift smoothly. By a process of elimination I determined that it was the gear cable inner that was the problem. On replacing it with an XTR inner the previous smooth shifting returned. The XTR inners seem to not only stretch less but they have some sort of teflon or other coating that might reduce friction.
So I've got a few new spare inners left that will otherwise go into the bin if someone else doesn't want to take a chance with them. Customer reviews on Wiggle are positive but they didn't work well enough for my liking.
If you want 'em, be the first to post up saying so and send me a self-addressed envelope.
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Thread: Free Gear Cables