thanks for response.  I was looking with some interest at Thule Tour Rack - surprised to learn its weight limit is so low too.  no room on Santa Cruz Hightower for down tube bag - can barely fit a water bottle in there!  My fantasy is to be able to load it up when I choose with sleeping bag, tent, etc for multi-day trips, maybe product doesn't exist yet to support the load.  Maybe we'll need to invent it!