a lightweight , composite beams for reinforcing a vehicle door is provided . the reinforcing beam comprises a metal tube having a longitudinal cavity which is partially filled with a reinforcing polymeric core which in one aspect includes a thermoset or thermoplastic resin - based material . the core is friction fit or mechanically held within the metal tube at the mid - span of the tube . the composite beam includes end pieces which are adapted to be mounted to a vehicle door panel in a door cavity by spot - welds or the like . the horizontally mounted beam provides substantially increased resistance to side impacts to the vehicle door in the event of a collision .