This game looks like it will be THE physics motorcycle game. It looks so, so, SO incredibly good, that I am going to buy it....uh.....that's all I can say...but it looks REALLY good.
On another note, that is one of the best looking sites on your eyes I have ever seen! If you actually designed that site, you should become a web designer, not a game developer! (Make this game, first, though!
Also, Muntu. It's gravel and dirt. I'm pretty sure it's friction, but it could just be a glitch in the physics at this point.