Design and manufacturing issues are here studied about the production of hollow structures made of an outer high resistance skin and a metal foam filler. These structures can be very useful as reinforcements against lateral impacts of vehicles. In car bodies, at the waistline or door height, the available space is very limited. Compact bars with light weight, high energy absorption efficiency and limited maximum deflection (i.e. high maximum load) are required. The skin of the proposed structure provides strength and stiffness and can be assembled to the vehicle body. The foam core provides energy absorption properties. The combination of non conventional technologies (hydroforming and metal foams) allows for the production of lightweight, high performance components, particularly suited for flexural resistance in terms of amount of energy absorbed for a given maximum load. A performance indicator y is proposed with the aim of comparing the performance of side impact absorbers with different cross sections and made of different materials. The tube hydroforming process is investigated as a suitable way for performance improvement of metal foam filled structures in side impacts, particularly for non-constant section bars.