Diameter: 55.5mm Width: 34.8mm Riding Surface: 21.3mm Hardness: 99 duro Formula: Formula Four Shape: Radial Set of 4 In the Spitfire Labs and most importantly in the streets tested by Spitfire Team riders, Formula Four Performance Urethane lasted longer, withstood more abuse and performed superior to all other wheels. Spitfire Formula Four Urethane was developed to combine all the desired properties of a performance wheel without sacrificing any one attribute at the expense of another. Formula Four's True Lasting Performance Urethane comes not from a single benefit, but from a combination of qualities and features that together produce a truly great wheel. Longer lasting - holds size and shape longer Unmatched abrasion resistance with fewer flat spots Smooth anti-stick slide More controllable grip Lasting speed everywhere