Energy cannot be destroyed; only converted from one form to another. During a crash, the kinetic energy from the moving vehicle is converted into crush damage. When reconstructionists need to assess the severity of a crash, one of the tools at their disposal is the crush analysis method – whereby they measure the amount of crush damage (permanent deformation) to the vehicle, and scientifically integrate what is called “vehicle stiffness coefficients” into their calculations to be able to give more accurate answers regarding vehicle speed and speed change.
Stiffness Coefficients – What do they mean?
Vehicles of different size, weight, manufacturing year, and origin, vary in how much they deform or “crush” when involved in a crash because they differ in their stiffness. Cars crush like a spring in a can. The crush analysis method requires the crash reconstructionist to calculate two stiffness variables for the particular vehicle involved. The following values required for the crush analysis method can be calculated for specific vehicles using the data obtained from crash tests:
- The “A” crush stiffness coefficient represents the beginning of the damage threshold, i.e. the maximum force (per unit of width), that can be sustained without producing any permanent crush.
- The “B” crush stiffness coefficient represents the relatively linear relationship between the force and the amount of permanent crush – to put it another way, it is the ratio of the force per unit width (of the contact area) to the crush depth.
The A and B crush stiffness coefficients are then used along with the crush measurements of the vehicles to calculate the speed changes experienced by the vehicles (which is a measure of the severity of the collision). These speed changes are correspondingly used to compute the impact speeds of the vehicle(s).
Stiffness variables depend on the vehicle generation, Make and Model, but what are the general trends?
- The front end of vehicles in general have become stiffer over the years, however, the general trend also appears to be currently stabilizing.
- Stiffness coefficients did not significantly increase in the 1970’s or the early 1980’s; in fact they declined slightly.
- From 1985 to 2004 stiffness coefficients nearly doubled, corresponding well with the development and implementation of government regulations regarding vehicle safety, as well as advancements in crashworthiness and automotive engineering.
- After 2004, stiffness coefficients continued to increase, but at a lower rate manufacturers were seriously shifting their focus to other ways of improving occupant compartment crash worthiness and supplemental safety restraints (such as multiple airbags and rollover safety).
- In general, the stiffest vehicle type is SUVs, followed by vans, and then pickups and finally, sedans.
- European vehicles were found to be slightly stiffer than American and Japanese-Korean vehicles.
Stiffness Trends – Why do they matter?
While reconstructionists will generally calculate or otherwise attempt to obtain the stiffness coefficients for the exact vehicle year, make & model involved, sometimes there is no specific crash test data available for that vehicle, or, sometimes very little is known about one of the involved vehicles (such as in a hit and run). In these situations, it is useful for reconstructionists to have trends (and average A and B crush stiffness coefficients) to rely on in their calculations.
Note: The crash testing image above is from the National Highway Traffic Safety Administration (NHTSA).
Sam Kodsi, Sarah Selesnic, Shady Attalla, and Avery Chakravarty published “Vehicle Frontal Crush Stiffness Coefficients Trends” in Accident Reconstruction Journal. The following is an abstract of their paper:
Accurate information on vehicle stiffness is integral to reliable crash reconstructions, so one of our ongoing research endeavors has been to synthesize our own data set and trends based on NHTSA’s crash test database by vehicle type, vehicle origin, and vehicle weight (from 1974-2014). The comprehensive data set includes more than 2000 vehicle-to-barrier crash tests. We calculated the A and B frontal vehicle structure stiffness coefficients (which represent the threshold for the beginning of deformation and linear relationship of force-deformation) for each vehicle using crush-energy formulas. The process also involved validating the data (which included correcting data entry errors and negative values for average crush from the original raw data), and filtering the data using recognized statistical methods to remove outliers. We then categorized all the data into four vehicle types (pick-up, sedan, SUV, and van), three vehicle origins (European, American, and Japanese-Korean), and five vehicle weight classes (800-1250 kg, 1250-1500 kg, 1500-1750 kg, 1750-2000 kg, and greater than 2000 kg). Further, we analyzed changes in vehicle stiffness over time and found that “they don’t make ‘em like they used to”; they make them a lot stiffer. In fact, vehicles today are more than twice as stiff as the 1970s. The crush stiffness coefficient trends we uncovered are published in the Accident Reconstruction Journal, and the stiffness data will be automated in our Reconstructionator software.
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