Talk:PORSCHE 917K/@comment-28176349-20180619213835/@comment-31668816-20180620080945

Slick tyres only started to come in to track, as opposed to drag, racing in the early 70s. For example 1971 was the first full season of F1 where slicks were used in the dry.

Until that point the science of grip wasn't fully understood. It was thought that the tread blocks gripped the imperfections on the road surface to impart grip, as opposed to the coeficient of friction between rubber and track.

It was discovered that as treaded tyres wore down grip actually improved as more rubber was in contact with the road surface. From there slick tyres became part and parcel of track racing. Nothing to do with regulations as far as I know.