How do rankings work?

Can someone at RIOT explain how your algorithm works to determine what rank you get at the end of a game?

If I'm correct there is only one single algorithm that determines your rank regardless of your position. This leaves specific roles such as Jungle and Support in a bind because both roles tend to act more as a supporting role to get their team moving (at least thats how I play them). Low kills and high assists. I just feel like there should be an algorithm in place that bases your rank based off of the position you have selected.

I just finished a game where I went 3/0/21 as Bard. The game lasted 25 minutes and I had some pretty amazing plays and saves. I don't know how that warrants a S- and not an S. I have gotten so many S- ranks on Bard that it's becoming more apparent that I will have to resort to kill stealing and unconventional full AP builds on Bard in order to get the Rank 7 that I've been working on for a long time.