I've always been a fan of these motocross games.
Your implementation seems promising - the basic physics are solid and it runs decently.
However, there were a few issues:
- I found the combination of colored background hills and the track's solid black line quite confusing. The background already looks like a moto game track. This really doesn't work for me
- The game is a little "twitchy" - perhaps you could soften the physics somewhat?
- When moving at speed, I only see a very small distance in front of me - the player really needs to learn the tracks as they have no chance of reacting in time - perhaps the camera settings could be tweaked (e.g. to align the bike towards the left of the screen when moving quickly)