Attention! This achievement is
missable!
Make sure earn the max possible kudos in each race and go to the next championship if you met the requirement. If you finish a race and you don't earn enough points to earn a gold medal you can simply quit out to the dashboard, because the game only saves after the result screen. You can restart during the race, but if you finish/lose a race and press restart from the result menu you can't change the racing result.
The good news: It's not hard to earn gold medals. This game is one of the easier racing games, but if you don't check out this achievement soon enough it's easy to screw you over. It's also important to only invest your credits in bikes you really need for new championships. Don't waste your money for other bikes and make sure to complete at least a few missions for extra credits.
I ignored some missions like "complete the same race 3 times", but i guess you can complete these missions in quick race without messing up this achievement. It's your decision. I completed 75% of the missions after this achievement and i never had trouble with credits.
There are 10 Championship and you have to unlock the last one in at most 70 races. I'll write down the minimum number of races you need to unlock the next championship. You can reach the last championship with a minimum of 67 races.
1. Beginner Championship
=> 300 Kudos (200 Kudos for each gold medal = 2 races) == 2 races total
2. Rookie Championship
=> 1100 Kudos (400 Kudos for each gold medal = 3 races) == 5 races total
3. Junior Championship
=> 2400 Kudos (600 Kudos for each gold medal = 4 races) == 9 races total
4. Standard Championship
=> 4200 Kudos (800 Kudos for each gold medal = 6 races) == 15 races total
5. Regulars' Championship
=> 6700 Kudos (1000 Kudos for each gold medal = 7 races) == 22 races total
6. Veterans' Championship
=> 9900 Kudos (1200 Kudos for each gold medal = 9 races) == 31 races total
7. Seniors' Championship
=> 14000 Kudos (1400 Kudos for each gold medal = 10 races) == 41 races total
8. Aces' Championship
=> 19000 Kudos (1600 Kudos for each gold medal = 12 races) == 53 races total
9. Champions' Championship
=> 25200 Kudos (1800 Kudos for each gold medal = 14 races) == 67 races total
>>>achievement unlocked<<<
10. Victors' Championship
If you still have questions feel free to ask! Good luck!