Chain sagging when no pressure

[bushbiker]

Hi everyone.
This is a little problem I have been having for some time now, it is an annoyance and a slight inconvenience rather than a show stopper. But it is annoying.
Every time I free wheel and the tension on the chain loosens, the chain sags jjst before it reaches the front chain rings. If I am on the largest front chain ring the sag will eventually cause the chain to drop down onto the next chain ring. The chain can actually sag so much it rubs the back tyre.
I have measured the chain length so it is exactly the right size - as per the Park Tool guide so I don't think chain length is the issue.
Any ideas?
Thanks

[binzer]

You can check the pivot point of your rear derailleur to see that it's moving freely, and if it isn't, try some light weight lube to see if that frees it up.

[DaveM]

Yes. Either your rear derailleur has gotten stuck or the spring in it has failed. Try lubing it up, but you might need a new derailleur.

[Aushiker]

G'day
Not really a solution but more of a preventative measure. I always apply a little chain lube to the pivots on the derailleurs and brake calipers when I do a chain clean..
Andrew