We reworked the shape of the core and found (finally) a new core material that is stable, strong, tough, dimensionally stable. It can be injected with great shape accuracy. First, a middle - soft layer of PU is applied to the core, which excellently dampens vibrations. The outer hard layer ensures low rolling resistance and low wear. Through the external - direct contact between the core and the harder PU layer, the energy of the reflection is effectively transferred to the forward movement. range of tens of micro seconds.