[markgozz]

Are you holding the camera in one static position or are you panning to follow the bike ?
I would advise a static position and manual focus in the centre of the bikes flight path ( No AF ) , shutter priority and auto ISO so that the camera will sort out the aperture . Set shutter speed to something like 1/2000s and then check the results and adjust the shutter speed if you need to .
A nice bright day would help with a fast shutter and low ISO .

Mark
Thinking about it turn off auto ISO and just set it to 200 ( I think that's the base point on the D300 )
The two main things to get right is to manually focus on the flight path and a fast enough shutter to freeze the action , you cam then start to tweak the other settings depending on the results .