I would advise having them tied down somehow. I would further advise that you find a way to keep the bike vertically secure. You can't see it in my picture but I have a cargo liner that has movable "book-ends" which fit into slots laterally. I use these "bookends" to keep boxes from sliding around or groceries bags from tipping over or potted plants from falling over and spilling dirt all over the place. They're as high as the vertical members of that free-standing floor bikestand I posted a picture of and I use them to keep the rear end secure both laterally and vertically. Notice I also use bungie cords to secure the bike against the inside wall tiedowns.