Vehicle Frontal Stiffness in a Front to Front Crash 2005-01-1375

In the effort to understand and solve the frontal crash compatibility problem, one needs to use values of frontal stiffness. Various definitions of stiffness have been used in other studies based on measurements from NHTSA's 35mph frontal NCAP test. Those definitions varied from assuming a linear stiffness based on static crush to more refined ones that vary with time dependent crush. A major consideration in selecting a method is the amount of vehicle damage that occurs in an incompatible crash. To partially address this issue, a method was introduced based on the energy absorbed in a front to front crash at 25mph approach speed. Four alternative definitions of stiffness were studied. The definitions were: (1) using static crush and assuming all energy associated with the 35mph approach velocity was dissipated, (2) using barrier force at 400mm of crush, (3) using the energy dissipated at 400mm of crush, and finally (4) using the displacement (crush) needed to dissipate energy associated with a 25mph approach speed and a frontal collision with a vehicle of average mass at same approach speed. The four methods were applied to a total of 16 NCAP tests; the results were compared and correlated to the fourth method. Most methods did not correlate well with the 25mph approach speed method and ranked vehicles differently. However, this latter method was considered to better represent the stiffness that would most influence compatibility.

Stiffness was then calculated for the available 1999-2004 NCAP tests and results were analyzed. The stiffness values varied from 548 kN/m for the 1999 Saturn SL1 to 3265 kN/m for the 2002 Chevrolet Trailblazer. A stiffness-per-unit-mass value was introduced to judge self protection needs. This value averaged 777.7 N/m/kg for all vehicles, 749.9 N/m/kg for cars, 854.1N/m/kg for SUVs, 800.5 for trucks and 654.2 N/m/kg for minivans. The stiffness-per-unit-mass values were compared with IIHS frontal overall and structure ratings as well as to NCAP driver and passenger ratings. Results showed no correlation between stiffness-per-unit- mass and safety scores in both NCAP and IIHS tests.