I think my big concern is that if I've been riding in large chain ring & larger cog, then drop to smaller chainring (I've got VO 30-46 chain rings), that the spring tension of the pulleys won't have enough to take up slack in the chain if the mounting at the bolt/frame interface is loose & can just move freely. Does that make sense. The Huret "Eco" has chain wrap capacity of 31 and my chainring plus cog differences equal 30, so this derailleur should have the capacity to handle it. I just don't know how, having the entire derailleur easily able to rotate around it's mounting bolt, would/could affect angle of the body due to pull on the chain plus spring tension of pulleys. Yesterday in the stand when I shifted from large to small chainring the chain fell off the front due to the fact, I think, that I had the body of the derailleur bolted super tight to the frame and it could not move at all. It was stationary at the particular angle I set it at, and the tension of the pulleys alone was not enough to take up the chain slack. I've been experimenting with it on the stand and think I've got it figured out, but wanted to discuss with someone who actually knows these derailleurs.

Thanks for helping out.