Spitfires newest shape. Spitfire Formula 4 Tablet 99a Wheels. Formula Four Performance Urethane lasts longer, withstands more abuse and performs superior to all other wheels. Designed to combine all the desired properties of a performance wheel without sacrificing any one attribute at the expense of another. Manufactured using a 100% pure urethane compound, with higher rebound for faster speeds over rough and smooth surfaces with a hard durometer for a smoother, faster ride that won't mush out over time. Designed with an exclusive anti-stick compound that gives a fast/consistent, smooth slide all the way through.