I have a Giant TCR carbon road bike and a pretty old Swagman, four bike, top tube clamping hitch rack. Not a lying on top of the four bike arm with nice rubber overstraps and a padded hanger but a clamp down with large thumbscrews C L A M P. I was not going to clamp my nice carbon bike in that thing so I bought a Thule 982XT Frame Adapter that goes between the stem and seatpost of the bike giving you a place to hang or clamp the bike to whatever.
Bought the Thule from my local bike shop though another not quite so chichi adaptor looked better to me but I went with the store recommendation. I'd used the adaptor with the same rack on the same car for three trips from Albuquerque (Los Q is what I call it) to Los Angeles (LA is what we all call it) and on two of three trips all was well.
I did not like that the Thule adaptor would always allow the stem to swing and did not lock along its length making the bike never quite solid on the hitch rack but it still kept the nasty clamp away from the carbon fiber.
Last trip, not quite the worst thing happened but a bad one anyway. The adaptor bar puts the bike quite a bit lower in relation to the hitch rack than if you were hanging the bike off the rack so the wheels are a lot closer to the ground. That was obvious from the first time we used the rack and adaptor but on this trip, for whatever reason, we must have hit the Interstate 40 "whoops" a lot faster and the rear wheel ended up trashed on arrival in California. Hard lesson learned but not quite sure how to solve it (other than not have the teenager drive while the adults both take their sleep shift).