I have a Shimano Sora derailleur on the bike I just bought with a triple chainring in the front. Right up by the shifters there is a sort of barrel adjuster on the cable. It doesn't seem to do anything when twiddled. When I was fiddling with the adjustment at the other end, and disconnected the whole cable by accident, this adjuster was separated from the cable covers on each side. I couldn't figure out at all how it was supposed to work. There didn't seem to be any threads, or screw mechanism to make it grow or shrink when adjusted. Yet it clearly does something. Can anyone explain what, and how?
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Your basic barrel adjuster is a tube with a thread on the outside of one end. The threaded end fits into a female threaded piece, either the shifter itself or another tube. (If you have this second type, where the female end is another tube, it's necessary to hold that second tube stationary while turning the first tube.)
The cable passes through the tube, and the cable housing "bumps into" the end of the adjuster barrel (which generally has a sort of "cup" sized to accept the housing and keep it centered).
The adjuster works this way: As you UNSCREW the adjuster, it pushes the cable housing farther out, effectively tightening the cable. The cable is held in tension by the spring in the derailer, so this adjustment moves the position of the far end of the cable relative to the derailer.
[Note that some adjusters have a thin "jam nut" on the threaded portion of the barrel, and this is tightened against the female piece to keep the adjuster from turning. It's often necessary to unscrew this jam nut slightly before working the adjuster.]
Note that once you've unscrewed the adjuster beyond the end of the threads it won't tighten the cable anymore, but will just spin. If this occurs (before satisfactory adjustment is achieved) it's necessary to screw the adjuster all the way in, loosen the cable clamp at the derailer, and pull it a hair tighter.
Also, if some jackleg in the past decided to "tighten things" with a pair of pliers they may have broken off the threaded portion inside the female portion, rendering the adjuster completely ineffective.
Try to post some photos of the adjuster along with the brand and model of the shifters.If you have an adjuster at the deraileur end of the cable it is possible there is no adjuster at the shifter end.The piece at the end of the cable may just position the cable in the shifter.There is also an adjuster that has small steps or knotches molded in to the shortened barrel. With each 1/4 turn it slides out the cable housing a small amount,but after a complete turn it returns to the original position.It appears to work as a fine adjustment similar to the trim control on more expensive shifters