The Spitfire Formula Four Classic 101a skateboard wheels have a narrower contact patch, which has become the standard shape for street skating. Spitfire released their new Formula Four wheels a few years ago now. The latest formula was designed for higher abrasion resistance. This means it is a little harder to flat spot. Formula Four 101a wheels are generally decorated with a red themed graphic. Unless of course it is one of the signature wheels. The signature wheels feature artwork custom designed for each rider. 101a is the hardest wheel in the modern skateboarding scene. They are super fast on smooth surfaces and the easiest to slide. Being super hard, if you find they let go on smooth surfaces or feel unpleasant on rougher terrain, switch back to the 99a models. There are no hard and fast rules, best to try out different wheels and select what works best for you.