It's not meant to be realistic. They didn't make it with that intent. That's what I love about this game. You can have pure fun playing it and not have to worry about "physics".
You remember MotoRacer? IMO this was the best ever bike racing game. Well NFSU reminds me of that.
It's the best racing game i've ever played, and i've played a few. Completed it twice now, playing through Colin McRae 3, pretty good in it's own league.