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Factory bushings are made from a rubber compound containing natural products, which deteriorates with age. The bushings become softer and more pliable, therefore becoming less capable of resisting the forces being placed on them. Polyurethane bushings on the other hand are 25%-30% stiffer and do not deteriorate in performance over time. The benefit of the increased stiffness is superior control of the suspension components giving you, the driver, more control over your Miata.