Have an Affinity greater than 50% in all car classes
Go to your driver network profile and look bottom middle. There will be number of races in each class and number of wins in each class. Make sure you have more than 51% wins in each class. 1 win from 1 race is fine. If you have done 100 races in a class I suspect you will need 51 or 52 wins. Single lap, single player quick races work to increase your win ratio.
The achievement says greater than 50%. I needed 54 wins from 104 races in karts before the achievement popped.