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. Usually adorned with a red coloured graphic, the Formula Four 101A wheels are the hardest Spitfire Wheels available. 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.