The very best way to boost the 'reach maximum affinity level' with all DLC bikes is:
Time Trial mode.
Daytona USA oval track.
Bike: required DLC bike for the achievement.
As soon as you start move towards the wall and use a rubber band or sellotape on the accelerator/right trigger. Each lap takes approximately 1 minute and 5 seconds and you gain 4 bike affinity experience points per lap. You require approximately 524 experience to max level 5 which equates to 131 laps. 131 laps will take about 2.5hrs so use a wired controller to save having to tap your wireless pad to stop it disconnecting every 10 minutes or so.
Cook a meal, watch a film or do some house work with the knowledge you're secretly achievement hunting 😉
*please note: I have auto tuck-in disabled and might be slower on the straights. I have never gained enough speed that my bike has crashed and reset which is good. If you want to boost some laps without rubber banding then hold down tuck-in on the straights to go 40mph faster and depending on the bike, will lap in 50 seconds. This is faster but requires a lot more than zero effort*