Frog Legs lightest castor wheel to date made from a strong light weight carbon composite, making it the lightest toughest castor on the market. The highly resilient solid urethane tread will not go flat and is designed to minimize energy loss and absorb vibration. Arched tread design ensures the contact surface is spherical to improve pivotability. The extra width better distributes downward pressure allowing the wheel to maintain higher flotation and does not sink into carpet,gravel, etc. Designed for those who want the ultimate smooth ride.