Spitfire's 99 duro Wheels are a little softer than the 101 duro formula. This means that the wheels don't slide quite as well, but they have more grip and a smoother ride. The Classic Shape is the number 1 wheel in the world - proven at high speed and control at the same time. 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 role. The combination of high rebound and hard duro urethane results in a roll that is fast everywhere - on rough as well as smooth surfaces. The Hard Duro mixture ensures that the roll does not weaken over time. The Formula Four's exclusive non-stick coating allows for quick and even sliding without causing the wheels to become edgy. The Formula 4 Wheels offer maximum control and a constant grip that you can rely on.