The 99 duro wheels from Spitfire are a little softer than the 101 duro formula. This means that the rollers don't slide quite as well, but have more grip and smoother handling. The conical shape ensures that the role reacts faster and weighs less. Spitfire Formula Four Performance Urethane is unlike any other reel. Even under the highest loads, the wheels retain their original shape and size longer than any other roller. The combination of high rebound and hard duro urethane results in a roller that is fast and that everywhere - on rough as well as on smooth surfaces. The Hard Duro mixture ensures that the role does not lose its weight over time. The exclusive non-stick coating of the Formula Four enables fast and even sliding without the rollers becoming edgy. The Formula 4 Wheels thus offer maximum control and a constant grip that you can rely on.