Skatepark Formula (SPF) is very high quality urethane specifically formulated to resist flatspotting on slick or smooth surfaces while maintaining a higher rebound for a quicker response and a faster roll. Skatepark Formula has a high resistance to abrasion while not compromising hardness and slide ability. Simply put, Skatepark Formula wheels grip better on slick surfaces, are almost impossible to flatspot, and are the fastest, longest lasting wheel on the market today. These wheels measure 54mm x 31mm With a hardness of 84B (104A).