Bones Wheels XF Cousins Skateboarding V5 99A 54mmX Formula (XF) is another industry leading innovation from the Bones Wheels factory. X Formula rolls over rough terrain like softer wheels do, but slides much like a harder wheel will. X Formula rolls much smoother and faster on all terrains because it absorbs vibration and softens impacts much better than any other wheel in the market. The slide ability comes from its unique blend of high performance materials. X Formula will slide when you ask it