best settings D-4

Like your panning technique and I would say your shutter speed is right. To me, one of the important details of any vehicle in motion is to have the spokes or centres of the wheels blurred along with the background.

High shutter speeds freeze everything which means the bike could be stationary and all sense of speed / movement is lost.

As another poster said, having some space for the bike to move into can make a "better" final picture as can having the image "level" rather than going downhill - unless he was going downhill..... These are my personal preferences and I am very sure that others will vary!

Dave