A precondition for a good rating in Euro-NCAP frontal impact is a low value for chest deflection. To achieve this, detailed knowledge of the mechanism of chest deflection is needed. Consequently, the objectives of this study were a detailed analysis of the belt induced chest deflection and a finding of solutions to reduce it.

Theoretical investigations as well as simulation (software Madymo 6.1) were used to study the mechanism of chest deflection. The simulation environment represented a typical middle class vehicle. A special simulation dummy which allowed a detailed analysis of the internal and external forces acting on the thorax was used. Finally, sled tests were carried out in order to confirm the theoretical and numerical results.

For the environment investigated, the belt force turned out to be the dominant factor for chest deflection. In fact, the value for chest deflection showed a good correlation to the arithmetic mean of the shoulder belt force (FB3) and the force of the inner diagonal belt (FB4). While FB3 is commonly limited by a load limiter in the retractor, FB4 is strongly influenced by the forces in the lap portion of the belt. These forces are usually 1kN to 2kN larger than the level of load limitation. Preventing the belt slippage through the buckle tongue proved to substantially reduce FB4. Along with this chest deflection in tests with Euro-NCAP deceleration pulse was reduced.

A prototype of a locking tongue was developed and sled tests were carried out in different vehicle environments. It was shown that depending on belt geometry a reduction in chest deflection of 10-25% could be achieved.