Energy Equivalent Speed (EES)
Definition
The speed at which a vehicle would strike a rigid fixed barrier to absorb the same amount of energy as observed from the crush damage. EES represents the energy absorbed by one vehicle, not the closing speed of the collision.
- Represents
- Energy absorbed by one vehicle in the collision
- Not equal to
- Closing speed or impact speed of the crash
- Derived from
- Crush depth profile and vehicle stiffness coefficients
- Combined with
- Coefficient of restitution to estimate closing speed
Common questions
Why is EES not the same as the vehicle's impact speed?+
EES only captures the portion of kinetic energy converted into permanent crush deformation on that vehicle. The actual impact speed also includes the energy that goes into vehicle rotation, post-impact travel, and momentum exchange with the other vehicle, so the two numbers coincide only in a rigid-barrier, fully inelastic collision.
How do reconstructionists use EES to get a full picture of the crash?+
They calculate EES separately for each vehicle from its own crush profile, then combine those values with momentum conservation and an estimated coefficient of restitution to back-calculate each vehicle's actual speed and heading at impact.
What commonly makes an EES estimate unreliable?+
Stiffness coefficients borrowed from a different make or model, crush measurements taken before repair photos were shot, or crush that mixes structural deformation with pre-existing damage all skew the energy calculation, so the source data must be vehicle-specific and taken from an undamaged reference profile.
Related terms
- CRASH3
- Calspan Reconstruction of Accident Speeds on the Highway version 3: a mathematical model that relates permanent crush depth to absorbed energy through...
- Dynamic Crush
- The maximum deformation depth at the moment of peak force during impact, before any elastic recovery. Dynamic crush is larger than residual...
- Residual Crush
- The permanent, non-recoverable deformation depth remaining after the elastic springback of the vehicle structure. This is what is measured at the scene...
- SMAC
- Simulation Model of Automobile Collisions: a trajectory and damage simulation that uses CRASH3-type stiffness data to model full collision kinematics, including vehicle...
- Stiffness Coefficients a and B
- Vehicle-specific constants derived from staged barrier crash tests. A is the force threshold below which no permanent crush occurs (N/m width). B...