Polyurethane has been commonly accepted as an industrial product for over 40 years, but it wasn't until 1974 that urethane was used as a replacement for rubber in automotive suspension bushing. During the intervening 25 years, the types of urethane used have changed very little. Until 1986 use of the hard grades available restricted urethane to motorsport and heavy-duty applications where ride comfort was a very low priority. After 1986 the second grade of urethane development occurred.- softer grades evolved that permitted urethanes to be used in general passenger applications. But the key suppliers in the urethane market simply utilized the new grades, and no further research and development took place.
Consequently the common complaints that urethane makes the vehicle ride harsh was never addressed., mainly due to the fact that it was presumed that grades softer than #75 durometer could not survive in a vehicle suspension environment. Further development in Urethane technology was made to overcome the hard ride, yet still maintain the characteristics associated with soft rubber bushings. These new grades of urethane permit the comfortable ride of rubber but with the crisp handing, geometry control and durability associated with the harder grades of urethane. This totally new technology has revolutionised the suspension industry. #75, #85 and #90 durometers have been replaced with #60, #75 and #85 - (based on the window that #60 is the softest and #90 is the hardest durometer for automotive applications)