One of the interesting aspects of this case was that the data uncovered by the NHTSA indicated that there was a higher-than-normal rate of failure for these tires on all vehicles, but an especially large problem on Ford Explorers. In other words, the combination of those particular tires and the Ford Explorer seemed to make the problem with the tires worse. Indeed, Firestone tried to claim that the problem was really with the Explorer rather than with the tires: They claimed that the Explorer was poorly designed and was already susceptible to rollover accidents. This is an illustration of the synergistic effects that occur often in engineering: Sometimes two parts of a design that work well alone cause great problems when they are put together. It is important for engineers to keep synergistic effects in mind in performing new designs, even in modifying existing designs and in designing the test procedures for their designs.