Skypig - why a slower shutter speed?
Too fast a shutter speed freezes the action. Can look ok, or like you are stopped, or falling out of the tree...
Slower speed: wheels blurred rotationally at the very least.
"Panning" (moving the camera/tracking the target) can have great results with the background blurred giving a real feeling of speed. Also a big risk of blurring the subject. (From movement relative to the camera.
Slower shutter (around 1/80 - 1/125th of a second, faster with a longer lens as movement is magnified) can result in deleting a fair few duds, but can produce some nice results.
Recent pic of me (2004 model - almost Vintage!)
If you ignore the fact the wheels are "in line", I almost look fast!