Cheat mode:
To enter the following codes, choose the "Options" selection at the main menu. 
Then, press L or R to cycle from the "Game Settings" option until the "Cheats" 
option appears. Then, press Down until the desired cheat option appears. Press B 
to display a keyboard that is used to enter the code to unlock that cheat option. 

Get all bonuses
Enter CLAPPEDOUT in the character completion field in cheat screen

Open Up Pro Physics
Enter SWAPPIN in the character completion field.

50cc Class
Enter SQUIRRELDOG in the character completion field.

500cc class:
Enter BIGDOGS as a code. 

All machines:
Enter MINIGAMES as a code. 

All Freestyle tracks:
Enter BUSTBIG as a code. 

All National tracks:
Enter ECONATION as a code. 

All Supercross tracks:
Enter STUPERCROSS as a code. 

Bowling mini-game:
Enter WRECKINGBALL as a code. Alternately, play the Ocotillo Wells track in 
freestyle mode. Get to the purple stunt competition marker then look for the 
nearby row of power lines. Follow the power-lines to the west to reach the bowling 
lane. 

Expert opponents:
Enter OBTGOFAST as a code. 

Reset vehicle:
To reset you vehicle (dirt bike, monster truck, trophy truck, etc.), press L + R 
during game play in freestyle mode. If you try to do this with the biplane or 
helicopter, you must be flying slow and close to or on the ground. If you have your 
propellers broken off, you can also do it there regardless of the speed. If you flip 
your trophy truck or monster truck, you can do this to get up faster without waiting 
for the game to do it for you.