Champion ranking system question

I'm curious if someone from RIOT could be so kind as to inform us as to what determines what champion rank you get at the end of the game. As an example, I just got out of a game where I went 1/1/22 as Bard. 31 minute game, and only came out with an A+. I feel as if the algorithm treats every champ the same and it makes getting S rank harder on those support champions and the Rank 7 banner.