The Spitfire Formula Four Tablet Natural skateboard wheels have a flatter side to lock on ledges or copings. 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 99a wheels are generally decorated with a blue themed graphic. Unless of course it is one of the signature wheels. The signature wheels feature artwork custom designed for each rider. 99a is considered a softer wheel in the modern skateboarding scene. They are still very hard. The 99a formula will provide a little more grip on super smooth surfaces and is a little more forgiving to ride on rougher surfaces.