The Spitfire Formula Four Radial skateboard wheel features a wide contact patch with a rounded edge design for control, speed, and a responsive slide on all terrain.97a Duro is formulated for a softer, faster ride. Grips or slides when you need it, the perfect in-between for Street or Park. Spitfire Formula Fours were developed to combine all the desired properties of a performance skateboarding wheel without sacrificing any one attribute at the expense of another.The Formula Four Performance Urethane comes not from a single benefit, but from a combination of qualities and features that together produce a truly great wheel. An unmatched abrasion resistant wheel, meaning fewer flat spots and a longer lasting wheel that holds size and shape longer. Spitfires are top of their class. One of, if not THE best skateboard wheel out there.