Recommendations for Refinement of Simplified Momentum Solution Procedures
Posted: Fri Jun 09, 2023 11:08 am
We split this off from topic The Major Shortcoming of Simplified Momentum Simulation and Analysis
- Important areas of research that vendors of simplified momentum programs like pc-crash, virtual crash and others should consider including in their solution procedure.
- These recommendations are from research we did and presented in our 1997 paper on the momentum portion of the CRASH program we wrote about in our 1997 SAE paper
We split this off as we heard about yet another June 2023 pc-crash update (14.1!!) annouced and still NOT A SINGLE refinement of their simplified momentum solution procedure
- Pc-Crash and Virtual Crash and others still have the crude requirement that users estimate a subjective "point & angle" for instantaneous momentum exchange...
NO scientific improvements or enhancements to that crude simplistic limitation in ANY of their MANY updates...WHY?
- Some better objective guidelines for
OR - An objective automation of the procedure for
- Pc-Crash and Virtual Crash and others still have the crude requirement that users estimate a subjective "point & angle" for instantaneous momentum exchange...
- These recommendations are from research we did and presented in our 1997 paper on the momentum portion of the CRASH program we wrote about in our 1997 SAE paper
We split this off as we heard about yet another June 2023 pc-crash update (14.1!!) annouced and still NOT A SINGLE refinement of their simplified momentum solution procedure
- In response to a claim that my critique of simplified momentum exchange programs like pc-crash, virtual crash and other programs are recent:
- I refer you to our 1997 SAE paper: CRASH-97 - Refinement of the Trajectory Solution Procedure, Paper 970949
- Do the math, that was 25+ years ago!
- Our message has never changed! (see below for details on our suggestions for refinements...)
- The 'simplified momentum exchange' vendors have never refined their procedures
- Updates to pc-crash, virtual crash are all graphics and glitz NO changes/refinements to the simplified momentum calculation procedure
- Do the math, that was 25+ years ago!
The following are direct quotes from the paper:- "A secondary task required in order to further refine and enhance the trajectory solution procedure of the CRASH3 program was a reactivation and refinement of the angular momentum solution procedure. The original CRASH program included conservation of linear momentum in the trajectory based solution to determine the impact speeds based on the separation velocities. A contract performed on CRASH2 to implement an angular momentum solution achieved mixed results [26]. A major hurdle for any procedure which includes an angular momentum solution is the need to approximate movement of the vehicles during the collision. In the CRASH2 formulation the impact and separation positions and headings were assumed to be identical. The research in [see Ref 26 from 1979!] revealed that the accuracy of an angular momentum solution procedure for accident reconstruction which includes the assumption of no movement between impact and separation will produce unacceptable error levels (>>20%) in many cases."
- Red emphasis and underline added as all the current (2022) programs (pc-crash, virtual crash, etc.) do NOT include any assumption/approximation techniques for movement between impact and separation!
AND if you check their 'validation papers' you will see >20% errors in their validation comparisons!
The 1997 paper also included the following comments on the current (as of then 1997) simplified momentum programs like pc-crash, etc.:
- "Other analytical accident reconstruction techniques which include provision for an angular momentum solution procedure and/or which are based on conventional momentum analyses, include the somewhat subjective input requirement that either a vehicle-to-vehicle contact "point" [27 ], or a "point of maximum engagement" [28] or an "impact center" [29] be specified. The additional input is required to compensate for the cited solution procedure’s lack of an independent and objective approximation of the separation positions and orientations.
The requirement that the user specify either an arbitrary impact contact "point" or an arbitrary "point of maximum engagement" detracts from the objectivity of the reconstruction techniques.
Figure 5 and Figure 6 show representative changes in positions and orientations during the contact phase of collisions.The subjective choice of a “point” can produce a large variation in the predicted results. During “validation,” when the results are known, the user has some guidance in choice of the subjective “point.” In real-world applications, where the answer is not known, the determination and arbitrary specification of a “point” can and will produce a wide range of predicted results. The normal input requirements of accident reconstruction programs of damage dimensions and approximate impact configurations should provide more than adequate information for any accident reconstruction program to independently achieve the function of any contact “point” or “point of maximum engagement” without user intervention. The movement of the vehicles between impact and separation can be initially approximated, for example, by moving the vehicles in their initial directions of motion to positions where the damage regions match. The procedure to determine a separation position should be automated to prevent subjective variations between users in the positions of match and therefore the results.
Other assumptions of the cited techniques [27,28,29] which may detract from the validity of their impact models for objective application to accident reconstruction are:
- During the impact no consideration is given for tire-to-ground “external” forces
- TIRE-TO-GROUND "EXTERNAL" FORCES: The effects of tire-ground forces must be considered in a motor vehicle collision reconstruction. During the early development of the SMAC program [19, 20] tests were performed to determine the effects of external tire forces on the collision solution procedure. It was concluded that “The conventional assumptions that the effects of vehicle deformations and of tire forces can be neglected in analytical reconstructions of collisions can lead to significant errors. This is particularly true for intersection-type collisions at low to moderate vehicle speeds, in which prolonged or multiple contacts and significant movements of the involved vehicles occur” and that "therefore it is essential in a general procedure for reconstruction calculations that both the collision and tire forces be considered simultaneously."
We may add some comparisons we have done over the years, first performed by the late Dr. Charles Moffatt when first testing the SMAC computer program for NHTSA. He used the comparison for a simulated crash on ice and then on normal friction to determine first whether SMAC obeyed Newton's laws (on ice without external forces) and then how much the presence of tire friction/external forces affect the results.
- TIRE-TO-GROUND "EXTERNAL" FORCES: The effects of tire-ground forces must be considered in a motor vehicle collision reconstruction. During the early development of the SMAC program [19, 20] tests were performed to determine the effects of external tire forces on the collision solution procedure. It was concluded that “The conventional assumptions that the effects of vehicle deformations and of tire forces can be neglected in analytical reconstructions of collisions can lead to significant errors. This is particularly true for intersection-type collisions at low to moderate vehicle speeds, in which prolonged or multiple contacts and significant movements of the involved vehicles occur” and that "therefore it is essential in a general procedure for reconstruction calculations that both the collision and tire forces be considered simultaneously."
- The impact duration and time for exchange of momentum is assumed to be infinitesimally small.
- IMPACT DURATION: The duration of a motor vehicle collision cannot be assumed to be infinitesimally small. Normally the exchange of momentum requires 50 to 125 milliseconds. Significant changes in positions and orientations can occur during the collision which can produce changes in the collision moments acting on the collision partners. Any accident reconstruction solution procedure which contains the assumption of an instantaneous exchange of momentum should be carefully evaluated.
The importance of the inclusion of external forces in collision analysis and use of a finite time increment for the impact duration has also been reiterated more recently by Fonda [30]."
- IMPACT DURATION: The duration of a motor vehicle collision cannot be assumed to be infinitesimally small. Normally the exchange of momentum requires 50 to 125 milliseconds. Significant changes in positions and orientations can occur during the collision which can produce changes in the collision moments acting on the collision partners. Any accident reconstruction solution procedure which contains the assumption of an instantaneous exchange of momentum should be carefully evaluated.
- During the impact no consideration is given for tire-to-ground “external” forces
- "Other analytical accident reconstruction techniques which include provision for an angular momentum solution procedure and/or which are based on conventional momentum analyses, include the somewhat subjective input requirement that either a vehicle-to-vehicle contact "point" [27 ], or a "point of maximum engagement" [28] or an "impact center" [29] be specified. The additional input is required to compensate for the cited solution procedure’s lack of an independent and objective approximation of the separation positions and orientations.
- Red emphasis and underline added as all the current (2022) programs (pc-crash, virtual crash, etc.) do NOT include any assumption/approximation techniques for movement between impact and separation!
- I refer you to our 1997 SAE paper: CRASH-97 - Refinement of the Trajectory Solution Procedure, Paper 970949
- Here we are, 25 years later, and the widely popular programs like pc-crash and now virtual crash and others have updates and enhancements THAT ONLY INCLUDE graphical and convenience functions!
- There are no modifications or objective instructions on:
- Objectively placing the "point" and 'angle' or instantaneous momentum exchange
- Adding consideration and approximation techniques for movement of the vehicles during the collision
- see our 1997 paper for further discussion of the importance and requirement for approximating the movement during the collision and additional ideas on much more, which if implemented, might improve the fidelity and objectivity of these simplified momentum 'instantaneous momentum exchange' programs
- From the 1997 paper:
"A basic outline of the procedure used to determine the impactspeeds for a combination linear and angular momentum
solution was as follows:- The separation velocities are approximated on the basis of the vehicle travel from impact to rest.
- The separation velocities are used with an application of conservation of linear momentum for an initial approximation of the impact velocities.
- The separation positions and headings are approximated using the initial approximations of the impact and separation velocities.
- With approximation of the separation positions and headings, the following steps are repeated to converge on a solution:
- The separation velocities are refined based on the vehicle travel from separation to rest.
- The refined separation velocity is used with an application of conservation of both linear and angular momentum for a refined approximation
of the impact velocities."
however
What are their guidelines for unknown speeds?- Why are there are NO objective scientific steps for determination of the 'point' and 'angle' or momentum exchange such that 10 experienced crash reconstruction folks using pc-crash to reconstruct a crash will get similar and correct results?
- And of course in a blind study since, as mentioned, if you know the results you want/need/require...it is easier!
- From the 1997 paper:
- see our 1997 paper for further discussion of the importance and requirement for approximating the movement during the collision and additional ideas on much more, which if implemented, might improve the fidelity and objectivity of these simplified momentum 'instantaneous momentum exchange' programs
- There are no modifications or objective instructions on:
- See our 1997 SAE paper: CRASH-97 - Refinement of the Trajectory Solution Procedure, Paper 970949 for additional details and information.
- 19. McHenry, R.R., "Development of a Computer Program to Aid the Investigation of Highway Accidents”, Contract FH-11-7526, December 1971, Calspan Report VJ-2979-V-1,NTIS PB# 208537
- 20. McHenry, R.R.,"A Computer Program for Reconstruction of Highway Accidents", SAE Paper 73-0980, Proceedings of the 17th Stapp Car Conference, November 1973
- 26. McHenry, R.R., Lynch, J.P., “Revision of the CRASH2 Computer Program”, US DOT HS-805-209, September 1979
- 27. Limpert, R., Andrews, D.F., "Linear and Rotational Momentum for Computing Impact Speeds in Two-Car Collisions (LARM)" , SAE paper 91-0123
- 28. Steffan, H., Moser, A.,"The Collision and Trajectory Models of PC-CRASH", SAE Paper 96-0886
- 29. Ishikawa, H., "Impact Center and Restitution Coefficients for Accident Reconstruction", Japan Automobile Research Institute, SAE Paper 94-0564
- 30. Fonda, A.G., "Nonconservation of Momentum During Impact", SAE Paper 95-0355