when ethan and wats-his-face (the main dude) are riding on the two bikes, if he put his feet down on the road like that he'd go flying in an interesting direction that'd probably involve some face squashing... AND
it's soooo obvious that when they go onto the sand that they have different bikes with ... miraculously appearing dirt tracks!
I know! The whole bike scene is soooo fake. I don't know, but I think they should have made it better or left it out of the movie. But I must say, it is darn good.
I saw this French guy actually "ski" behind his motorcycle on some stunt show before. He said it can only be done at a high rate of speed, which we all know they doing. So it is possible with killing yourself.
Tom's shoes are on the ground, BUT there are wheels on the bottom! I hope you all don't think that going even forty with rubber souls would hold up? But, if you have the video with behind the scenes, they explain that stunt.
About the scene where he is skiing on the pavement. The end of the movie shows behind the scenes stuff, and if you watch it they explain that this scen is real.
During the chase scene you can see the tread on the both bikes at the 1:47 mark and they are smooth. Then again when the black bike does a sliding turn, the back tire is clearly smooth. Once the bikes hit the dirt you can see the tread is now rough on both.