All Bikes and Tracks
At the main menu, press C-up, C-left, C-left, C-right, L, R, C-down, Z. A tone will confirm the code.
Beat "The Beat Down"
To easily beat the hardest level on the game "the beat down", simply drive in the grass. The cops won't follow you there and the other bikers will get busted.
To play on the other side of the law, enter in the following code at the game select screen: Z, Left-C, Down-C, Left-C, Z, L, R, Down-C. This will unlock Cop Mode on the Thrash mode level select.
At the main menu, press the following in order: C-Up, C-Left, C-Left, C-Right, L, R, C-Down, Z.
These must be done at the game select screen. If you do it right you will hear a sound.These codes will also unlock more levels and bikes in Multiplayer
R, C-right, Z, R, Z, C-up, C-left, C-up
R, C-right, C-right, C-right, R, C-left, C-down, Z
R, C-Left, C-Right, C-Down, C-Right, C-Right, C-Left, C-Left
Z, c-right, c-down, c-left, c-right, c-right, Z, L
To race on scooters, press C-Down, C-Right, C-Up, C-Left, Z, Z, L, C-Left at the menu screen.
To get a boost-start at the beginning of the race, hold the wheelie-button until you've taken off, then release it. Your bike's front wheel will hang in the air for a few seconds then settle down. This little trick gives you enough of a boost to put you up in the front of the pack from the start (or at least in the middle.)