Well i think the Bobs can only do a certain amount of races before they get to old, but that doesnt seem to have anything to do with the level, just with the experience/career bar.
Lets say bobs have 500 races before the career bar is full.
If you do American Indy grind all day long you fill up your career bar quicker then you would do with 24h races, because you get more xp/race than with indy grind.
My Bobs are 37/37/36/35/35/36 and the career bars arent even half full.
I think i can lvl them up to 40 without any concern.
Lets say bobs have 500 races before the career bar is full.
If you do American Indy grind all day long you fill up your career bar quicker then you would do with 24h races, because you get more xp/race than with indy grind.
My Bobs are 37/37/36/35/35/36 and the career bars arent even half full.
I think i can lvl them up to 40 without any concern.