Originally Posted by wilkap
Why don't you try tying a bit of string/twine/whatever to the rack for whenever you need it. For the groceries, just loop the string through the bag loops and tie the string to the rack. At home, you just untie the string. I'd suggest not using a thin string or you'll end up having to cut the string.
If the bags hang too low, just tie the string to the opposite side of the rack so that the bag's loops lay across the rack. I suppose for added bit of security, you could clamp the loops on the rack too. Not sure if any of this would work though. I'm also thinking the bags might want to slip to the other side with the latter method in which case, you might want to tie the loops to both sides of the rack. Does any of this make sense? Mind you I've never tried this. =)
I was just sharing something that works for me when I don't plan ahead. If I think ahead I bring the panniers. A friend has a rack that a U-lock fits on so she finds it easier to fit the bag handles through the U-lock which is like your rope idea. Tying to the opposite side of the rack also works to keep the bags up if you need to. The rack in the picture keeps the bags out of the spokes but on my other bike I sometimes need to tie to the opposite side to keep them up. All these little details of transportational cycling that we take in stride but newbies worry about.