I think 3, 5, & 6 are better, the slight blur on the wheels gives more impression of motion.
I'd try at an even slower shutter speed and pan with the motion to put a bit more sideways blur into the background.
How on earth can they see in that second shot? All I can make out is mud covered goggles!
I think that's better than the 4th, even though I love the mud splash, because in #4 I can't see the eyes under the peak of the helmet.
Hmm, when the sun came out I think you upped the shutter speed?
That's got very sharp tyres, little sense of motion when the dirt's flying.
And the eyes are either in shadow, or they're using reflective lenses in the goggles.
In a way, that makes your life harder!
I was shooting in aperture priority mode to keep a shallow depth of field. When the sun can out the camera choose the faster ss to get the exposer correct. I'm shooting another race this weekend, I will experiment with a slower ss and try to pan so I can create some motion blur. Thanks again for the input.