What I noticed about Formula Fours is that they are considerably softer than their Bones counterpart. The Spitfires that have 101a durometer are similar to, or even softer than Bones Easy Streets 99a. You can tell because the Spitfire F4 101a’s are considerably smoother over rough patches than your average 101a wheel or even your average 99a durometer wheel. And the spitfires, IMHO, seem to soften over time.