Full list of all 36 Motocross Madness achievements worth 500 gamerscore. It takes around 8-10 hours to unlock all of the achievements in the base game.
The base game contains 30 achievements worth 400 Gamerscore, and there is 1 DLC pack containing 6 achievements worth 100 Gamerscore.
Got a gold medal in every event
Collected all skulls around the world
Pulled a wheelie for 1,200 or more feet
Reached skill level 20
Got at least 5 seconds of air time
Played any splitscreen match
Landed a trick worth 20,000 or more points
Slipstreamed for 800 or more feet
Scored a slingshot distance of 2000ft
Earned $100,000 in cash
Got high score during Trick Session
Bought all bikes
Seized the top spot on one of the club walls
Reached skill level 10
Unlocked all Iceland race tracks
Unlocked all Australia race tracks
Unlocked all Egypt race tracks
Scored first place in Rivals
Completed 3 Avatar FameStar weekly challenges
Drifted 400ft
Pulled a donut for at least 30 seconds
Bought your first new bike
Got dressed in a matching outfit set
You've won your first gold medal
Finished 5 bike club tasks
Reached skill level 2
Crashed while riding with at least 100 mph
Bought 1 new upgrade for any bike
Finished your first club task
Completed the tutorial
Won all the Stunt Race events
Unlocked all 10 Trick Jam skills
Won all the Trick Jams events
Performed an X, Y and B combo within one event
Drifted for 2,000 or more feet on any of the Trick Jam tracks
Unlocked 5 Trick Jam skills