In looking at these two photos, notice how the motion of the rider coming in from the left is preferable to the top image, in which the rider has gone beyond the middle of the frame and is working his way toward the right edge. The second example allows the eye and brain to “fill up” the empty space with the implied and continued movement of the biker.

Both images: Nikon D3X with Nikkor 16–35mm lens at 16mm, f/11 for 1/400 sec., ISO 200