A rocker assembly for a vehicle comprising a rocker panel and a side sill that enclose an injection molded reinforcement. The reinforcement includes a plurality of intersecting fins that may be provided in horizontal and vertical planes. An intermediate wall formed by frames within four sided cells that define windows. The rocker assembly may include two different types of reinforcements, one with intersecting fins, or walls, extending laterally and one formed by a plurality of longitudinally extending tubular members.