I have a Thule hanging rack that holds 2 bikes. It came with some pieces that prevent the bikes from swinging; basically a set of brackets and straps that hang down and meet the downtube or seat-tube of each bike. It's been a wonderful thing to have, and it has never done any damage to carbon frame bikes. I'm not really sure where that stigma comes from, but I've never really seen any evidence to substantiate it. In fact, every piece on my Thule rack that touches the bikes is made of some type of rubberized compound that feels pretty soft; it would be impossible to damage a bike with that material.