Been ages since I played some bike game demo's and its time to lighten up a little:
  Cnet have a demo version of that old Fave - Ducati Racing world:  
http://downloads.cnet.co.uk/0,39100207,39176004s,00.htm  Only one track and it never did quit properly, even in the version I had on CD but its still fun for a small game. 
 Cnet also have a demo of MotoGP but its 150Mb - I set it away last night - looks great but I gotta dig a controller out for that one. 
 Finally - great looking ad site for a game - but no media downloads 
 http://www.super-bikes.blackbeangames.com/
  http://www.super-bikes.blackbeangames.com/   Cheers - Frank