They look pretty good. Yep, fill teh frame unless you are showing some of the track/background etc..
Shutter speed can vary with the light and what your intent is.
1/1000 will bring things to a halt, 1/500 as youve noticed can keep the rider sharp but the wheels to blur. Works very well when you pan but not so well when the rider is coming at or away from you.
Keep tabs on your white balance
And keep shooting!
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