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.
Reached skill level 20
Slipstreamed for 800 or more feet
Pulled a wheelie for 1,200 or more feet
Got a gold medal in every event
Collected all skulls around the world
Got at least 5 seconds of air time
Played any splitscreen match
Landed a trick worth 20,000 or more points
Finished 5 bike club tasks
Earned $100,000 in cash
Bought all bikes
Unlocked all Iceland race tracks
Got dressed in a matching outfit set
Scored a slingshot distance of 2000ft
Reached skill level 10
Unlocked all Australia race tracks
Got high score during Trick Session
Pulled a donut for at least 30 seconds
Unlocked all Egypt race tracks
Seized the top spot on one of the club walls
Finished your first club task
Drifted 400ft
Bought your first new bike
Scored first place in Rivals
Crashed while riding with at least 100 mph
Bought 1 new upgrade for any bike
Reached skill level 2
You've won your first gold medal
Completed 3 Avatar FameStar weekly challenges
Completed the tutorial
Unlocked all 10 Trick Jam skills
Unlocked 5 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
Won all the Stunt Race events