I do panning shots on virtually a daily basis, either powerboats, or bikes. Boats are working in three dimensions, so they aren't just going left to right and up and down, but also twisting side to side, with the movement of the water. The boat I am in is also pitching and rolling at the same speed. You have to stand, balanced, and shoot.
Bikes, the HD engine most certainly makes a big difference - I know it from doing it since 1985 as the chief photographer for the biggest custom bike magazine in Europe.
You are right, in that you need to blur the spokes. Wire wheels are easier to blur than cast three spoke ones - because each spoke is moving more with multispoke wheels. 3-spokes are a real pain to get blurred to the same extent. This was 100th at 100mm (70-200) VR off, because I really haven't worked out how to use it yet.
It is a balancing act, and the trick is to get the SAFE shot first. Using a slightly faster speed, then start to play with slower ones, because if it fails, you have the safe one to fall back on. These are just what I could find in short notice - everything gets chucked on an external hard drive once the client has received, nothing lives on my confuser.
This was for Riva and Fiat. When they launched the Fiat 500 Riva, I was there shooting the launch for both Riva and for Fiat.
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See how the boat is rolling as she runs over the photoboat wake, just visible as humps on the surface. The spray and water surface can blur, but the boat has to remain sharp. You don't take too many chances in this kind of situation.
Safe shot in the can, slow it down a bit. You still need enough shutter to stop the motion of the boat though.