FIBROFLEX® spring elements are made from highly elastic polyurethane elastomers. Shore hardness is the most significant feature of the various FIBROFLEX® elements. This feature is of fundamental importance when selecting the correct type for a particular application. Because of their physical properties, polyurethane elastomers have a tendency towards initial sagging. This is dependent on such factors as internal frictional heat, frequency and number of load changes, spring travel and Shore hardness. The sagging may amount to 4 to 7 % of the spring length Lo.