Questions related to SAE paper 97-0949

The following are our responses to questions we have received on SAE paper 97-0949 "CRASH-97 - Refinement of the Trajectory Solution Procedure"

• Question#1: Could you discuss in more detail the techniques you employed to include the effects of external forces in the revised CRASH procedure?
  

Answer #1: During a collision, impulses act at each of the four wheel locations. The magnitudes of the impulses are dependent on a number of factors:

• The impact configuration
• The tire slip angles
• The tire normal forces
• Duration of the impact
  

To approximate the effects of the external impulsive forces on the momentum equations, a new routine was created and tested as a part of SAE 97-0949. The routine was used together with the procedure which calculates the approximate separation locations and orientations. The ranges of the impulsive factors tested were:

• The time from impact to separation ( 0.05 - 0.15 sec. tested),
• The angle at which the tire forces are fully saturated (10-40 degrees tested)
• Tire overload factor as a function of impact configuration (1.0-2.0 tested).
  

An iterative procedure was required for refinement of the separation positions and orientations, from which the impulsive factors could be approximated. The impulse terms calculated were then included in the linear and angular momentum equations.

• Question#2: The reported results in the paper include the correct values of the impact velocity of zero. Did you round off calculated values near zero and/or how did your solution procedure determine that the value was zero?

Answer #2: The primary evidence that one vehicle was stopped in the case of a longitudinal and/or co-linear case is the extent of damage. If the Delta-V from damage is approximately equal to the speed of separation, a stopped vehicle may be indicated. Another discriminator or clue that a vehicle may be stopped which exists in the momentum solution procedure is a comparison of the calculated common and separation velocities.
For the results reported in SAE 97-0949, two simple tests for each vehicle were incorporated:

• Damage Delta-V vs separation velocity comparison
• Separation vs. Common velocity test
  

In the reported results of SAE 97-0949 to maintain focus on the trajectory solution procedure, the damage solution was not used, so the program defaulted to use the input VDI to approximate a ballpark damage-based Delta-V. A preliminary 5 MPH threshold was established for the comparison tests. The implementation of the low speed and/or stationary vehicle test in m-crash is as follows:

• First, a secondary question appears when a negative or low speed vehicle is detected asking the user whether there is any evidence to support a stopped vehicle, low speed vehicle or vehicle in reverse.
• Next, the program can either determine the speed which best matches the evidence or accepts a manual input from the user of approximate impact speed of the vehicle. This option is to give the user the option of manually testing various low speeds in the solution procedure.

The next phase of the testing of the m-crash program will be to include the damage measurements. The refined values from the damage based solution procedure (outlined in SAE 97-0960) used in combination with the refinements to the trajectory solution procedure (from this paper, SAE 97-0949) should further improve the ability of the m-crash program to detect stationary and/or low speed vehicles.