Originally Posted by
cudak888
The chain may be worn, but if it is, that is not the cause of it sagging - the chain has too many links in it. The pulley cage of the RD isn't even under tension.
-Kurt
Not for nothing, Kurt, but if you look at the picture again you will see that the sagging of the chain occurs in parallel. This yields only one conclusion: Chain stretch.
If, as you mentioned, the chain was too long, then the sagging would only occur in the lower half of the chain, as the distance between rear cog and crank set would create proper tension.