Cool, right up my street. It all depends what look you're after and how much motion you want to portray. One thing to take into consideration with motorbike racing is that apart from long straights the rider is constantly moving around and therefore you might not want to use as slow a shutter speed as you might shooting cars.
My go to settings for motorsport is shutter priority (sometimes manual sometimes with auto ISO) and start off around 1/125 shutter speed and work my way to slower speeds, that being said at 400mm (800mm eq) slower speeds will be more tricky. In terms of focussing I mostly use single point AF-C, but sometimes will shoot manual and pre-focus.
If shooting through a fence and you want to shoot at low shutter speeds it's a good idea to get some ND filters so that aperture is as wide as possible to minimise the risk of the fence spoiling the shot.
I need no excuse to post photos
This is one of my favourites, shot at 1/160 at the inside of Brooklands through a fence. This has had some processing to add a bit more motion but very little, maybe 5%
Marc Marquez by
TDG-77, on Flickr
This was 1/10 and all motion is as shot SOOC, no fence
P6302247-Edit-Edit by
TDG-77, on Flickr
If you're 'lucky' you can get slow shutter motorcycle shots without complete rider motion blur (1/15), no fence.
A9_07749-Edit-Edit by
TDG-77, on Flickr
And for head on shots I will use higher shutters such as 1/1000, through a fence.
A9_06545-Edit-2 by
TDG-77, on Flickr
Sorry the last 2 are shot with Sony