S = Silica....its whats makes yokos nice. the silica compound heats up and sticks but doesnt melt as fast when extremely hot. found that out with my paradas...i drove then really hard for 20k with a bad camber and still hooked up. and it makes it awesome in the rain as well. and for amish, ive been in tires for 7 years now, and we have more mich defects than yokohama...and i work for a national chain