If you can return the rack great if not try this. Get the bike on it so it is clear of the car. Put something on the rack bars that the cradle slides on to act as a stop to halt the movement of the cradle. Maybe some zip ties or wrapping the tube with duct or electrical tape, possibly a worm screw clamp. The old socks rags etc to cover parts that hit works but is a PITA. When I had a trunk rack I also noticed the bikes moved around back there pretty good at 70MPH but never caused any trouble.