the reason behind the firestone failure on the explorer was pretty complex, from what a professor who helped on the case said... ford tested the explorer and found that it was too heavy, so they had firestone take some weight off the tires, which came off the sidewall.. then they discovered with the lighter tires it had a tendancy to roll.. so to lower the center of gravity, they lowered the PSI recommended for the tires.. which resulted in more sidewall wear.. which lead to tread separation