I don't like rear racks, used to use one, stopped one day half way down the motorway once, as it didn't look right in the mirror, and found the straps had vibrated loose… so i went and bought an estate and not looked back.
Anyway that ain't helping you.. get some pipe lagging on the rack tube or your bikes to stop the rubbing if nothing else. I've got some blue shapped wrap around tubing/lagging some-one sent me, more than big enough to go round even large bike tubes, no idea where it came from (proper bike packaging maybe? Ask your LBS?)