Specific Criteria For Getting an S based on Champion class.

I have noticed over years of playing league, that certain champion classes are easier to get S's on than others, Supports and Tanks being the easiest. I would love to hear from a riot team member what is the specific criteria that the automated grading system grades on? Please if you are not a member of the riot team keep your input to a minimum, for I have followed the advice given on other posts about cs and warding and the results are inconsistent at best. I am looking to find out for all of us players the true grading formula for receiving an S grade.