Originally Posted by
FBinNY
Now that everybody has weighed in, let me toss a con wrench into the mix.
I suggest folks look at the photo, and read the question more carefully.
First of all the, at the risk of being called Goldilocks, the chain is neither too long, nor too short, and is just right as it is. Note that the RD has moor travel room in either direction, so there's no reason to make any change..
If the chain running from the top of the cassette to the chainwheels didn't have slack in it, you might have a valid point. However, there is no more room for the rear derailer to travel in back to take up the chain slack because the chain is slack.
Originally Posted by
FBinNY
Next, the chain sag isn't the result of chain length but is a normal artifact of changing spring tension as the cage rotated through it's arc. As the chain takes up slack with smaller sprockets, the spring gets more relaxed, and the chain tension. ALL derailleurs do this, and it's a progressive effect, with more tension with bigger sprockets and lower tension with smaller.
The amount of tension and how much it changes is a function of the derailleur's design, relating to the spring used, and the number of turns it has. Odds are that the chain always sagged this way, but the OP never noticed it because he never had a reason to. (and still doesn't).
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Yes, all springs loose tension as they get near the end of their arc. But that's something that you should adjust chain length to avoid. Once the chain is as slack as El Gato27 is in his original photo, there chances of the chain bouncing and catching on some part of the frame, derailer, tire, etc. is greatly increased. Entangled chains result in far more damage in my experience than a chain that is too short. The lower chain at the derailer is slack enough that it could bounce up, catch either the inside or outside of the derailer cage and rotate the derailer right off the hanger.