It is challenging for vehicle engineers to meet structural requirements while avoiding vibration and panel deformation due to the unique coupling of structural performance with aerodynamic loading. Typically, testing for panel deformation occurs very late in the design process, using fully detailed prototypes. Wind tunnel tests can show static loading conditions, but cannot replicate the transient onset conditions found on the road. Only road tests can detect some panel vibration failures.
Early in the design process, structural requirements are usually met by overdesigning parts, retaining more part weight, and leading to greater parts cost. Door mounts and seals are designed to retain seal contact with the frame. The cost of missing some vibration or door seal problems can be high, because correcting for structural stiffness problems late in the design process greatly increases the cost of retooling and parts.