Thoracic Injury Mechanisms Due to Front Seatback–Rear Seat Pan Interaction in Severe Rear-Impact Collisions

Severe rear-impact collisions can cause significant intrusion into the occupant compartment when the structural integrity of the rear survival space is insufficient. Intrusion patterns are influenced by impact configuration—underride, in-line, or override—with underride collisions channeling forces below the beltline through the rear wheels as a primary load path. This force concentration rapidly propels the rear seat-pan forward, contacting the rearward-rotating front seatback. The resulting bottoming-out phenomenon produces a forward impulse that amplifies loading on the front occupant’s upper torso, increasing the risk of thoracic injury even when the head is properly supported by the head restraint. This study analyzes a real-world rear-impact collision that resulted in fatal thoracic injuries to the driver, attributed to the interaction between the driver’s seatback and the forward-moving rear seat pan. A vehicle-to-vehicle crash test was conducted to replicate similar intrusion characteristics and assess the relative kinematics between the seatback and rear seat structure. Results demonstrate that seatback bottoming out under intrusion conditions significantly elevates thoracic loading. These findings highlight the need for improved rear structural design strategies to manage load paths in underride scenarios and to minimize front seatback rearward collapse and associated occupant loading.