The Cooper S is heavier than the Cooper. I'm not surprised the Cooper S (with standard sports plus suspension and run-flat tyres) rides better than a Cooper with the optional sports plus suspension and the optional run-flat tyres.
According to the press reports I read, the S's suspension is stiffer only to compensate for its extra weight. They were aiming to keep the handling/ride similar to the stock Cooper. The road tests report that the ride is not as good as the stock Cooper, but presumably they mean the Cooper with the 15" wheels and without the sports plus suspension. |