I am not sure of the answer here but i would reckon that there is a constant ratio between the two axes of the ellipse of a biopace ring so that the difference between the two would be greater on the larger ring and therefore the slack would be more. i run a 42 tooth biopace ring and i have not had any problem with it. however i am a wuss and use a brake so if you are an avid skipper or whatnot that YMMV.