US Patent 7735841 Independent suspension system for in-line skates

A polyurethane biasing mechanism, comprising a first surface contact area with a first mass having a durometer that provides a first resistance and a first rate of resistance responsive to application of forces. The biasing mechanism having a second surface contact area with a second mass having the same durometer that provides a second resistance and a second rate of resistance responsive to the forces. The first resistance and the first rate of resistance are different from the second resistance and second rate of resistance, a combination of which provides a rate of resistance that commensurately varies and is correspondingly responsive in relation to varying forces.