Good shot, the rider behind, movement in the wheels and the panning all add to the action. I know how difficult it is to pick your spot at some of these meetings and my eye was drawn to the building between and behind the 2 riders. You haven't shown us any data and without knowing how close, or far away, you might have been, I wonder if using the lens toward the 400m end of the lens might have pushed the background out of focus, or a slower shutter speed while panning to blur (even more) the background that way. Both options also have their issues (depth of field might have pushed the second rider out of focus which you might not want and slower speeds can lead to more difficulty in getting your subject sharp.
Anyway, minor points and it gets my vote.