- "
**how many % may an investigator be wrong with pc-crash?**"

- Note that the crash outlined by desperate for answers was a 'who's on the wrong side of the road' question and so we included an example where pc-crash was used improperly as a basis for an incorrect opinion in a case.

- Where are you located? Are there not any local experts who can help?
- If you provide where you are located perhaps someone on this forum knows someone in that area.

- Pictures of exterior damage to the vehicles.
- Pictures of the vehicles at the scene.
- You mentioned drone pics? post those
- A police report? Any crash scene diagrams? post those
- The location of the crash? Google maps or Google Earth can be helpful in determining distances, etc.

- "
**how many % may an investigator be wrong with pc-crash?**"

- if so, first the answer to that question is
**'it depends'**.- pc-crash is a momentum based simulation solution and so subject to momentum limitations and simplifying assumptions

In several cases we have been able to make a singular change in inputs (the point and angle of momentum exchange') and change the results of a pc-crash simulation used as the basis for an opinion by an expert in a case.

One example was a 'who's on the wrong side of the road' crash (see below)

- pc-crash is a momentum based simulation solution and so subject to momentum limitations and simplifying assumptions

You should also demand you get the input file in electronic form for ANY simulation/animation so you can have an expert review all inputs and rerun the simulation to verify that the inputs produce the results and then also to allow a test for sensitivities of inputs.

- A demonstration of sensitivities of momentum based solutions comes from How is Monte-Carlo method used in Accident reconstruction?:

Which Includes:**A classic example of problems with the sensitivity of a linear momentum solution**is when it is applied to a t-bone type collision.- When a lighter car crosses the path of a heavier car/truck and the lighter vehicle is struck in the side by the heavier vehicle. If the heavier striking car/truck happens to swerve before the impact, either to the left or the right, the degree or two of change of impact angle can result in dramatic changes in the results of a linear momentum solution.

Why?

The swerve by the heavier vehicle will produce a change in the separation angles.

The change in separation angles, if all attributed to the smaller vehicle speed (which it will be if the impact angle of the striking heavier vehicle is assumed to be 0 (zero) degrees) will dramatically change the linear momentum solution approximated speed of the smaller lighter vehicle.

The result is that depending on the direction of the swerve, the small vehicle will be 'reconstructed' as either going very fast in the forward or reverse direction. Depending on the difference in the weights of the vehicles the assumption for impact angle of the heavier vehicle can result in very large errors in the analysis.

- When a lighter car crosses the path of a heavier car/truck and the lighter vehicle is struck in the side by the heavier vehicle. If the heavier striking car/truck happens to swerve before the impact, either to the left or the right, the degree or two of change of impact angle can result in dramatic changes in the results of a linear momentum solution.

- In most courts in the USA the inputs for any simulation or animation must be provided in electronic form.

This allows the other side to rerun the simulation to be sure all inputs are produced so they can be checked- weights? specifications? tire properties? values for friction? friction zones? terrain slopes? source of terrain information?
- Proper dimensions for evidence location and ability to compare that to the simulation/animation?
- if animation, is the animation physically possible and does it obey Newton's Laws?
- the electronic inputs provide a list of the positions and orientations for each frame so the speeds, speed changes, and approx accelerations can be calculated

- And do the graphics/video created match the inputs provided?
- we have cases where inputs (in paper form) and graphics/video provided looked OK however once we obtained in electronic form we found that the printed paper inputs were different than the electronic inputs and did NOT produce the same results/video/graphics!

- HELP! If YOU or someone you know has been in a crash!
- What files should be produced for Simulation and Animation

**EXAMPLE:**Here is a comparison of two pc-crash runs in a case.

- An expert used pc-crash as the basis for his expert opinion of who was on the wrong side of the road.

Once we obtained the electronic pc-crash inputs, we moved the impact point of the vehicles and changed the arbitrary and subjective 'point of momentum exchange' and also 'matched the evidence' (see below)

We demonstrated that either scenario could be proven with pc-crash.

So what to do?

We then tested the experts 'proof' by setting up and running the crash with msmac which produced very different results for the two scenarios. Since with SMAC the solution includes the trajectory and damage produced by a given impact configuration and speeds.- msmac is like running a mathematical full scale test since it models the collision forces for each millisecond the vehicles structures interact (100-150 milliseconds) instead of trying to approximate the results of a collision interaction with an "instantaneous exchange of momentum" at an "arbitrary point of momentum exchange"
- With msmac you set up the mathematical full scale test, set the speeds and then hands off run (simulate) the vehicles into each other. It calculates the movements of the vehicles and displays the structural damage that occurs from the impact speed, relative locations, movements of the vehicles and relative crush stiffnesses (it calculates the vehicle trajectories and collision forces for each and every millisecond!).
- With pc-crash you must pick a 'point of maximum engagement'' and move the vehicles to that position with no feedback or verification as to whether the speeds, relative locations and collision interaction of the vehicles will produce damage which in any way matches the amount and location of the actual damage. The collision forces are not calculated.

- We say pc-crash as an arbitrary and subjective 'point of maximum engagement and angle of contact plane' because the pc-crash input guidelines and training videos require the user to subjectively position the 'point of maximum engagement"

As the pc-crash user moves the point and angle around the user can watch the speed results change until an arbitrary and subjective position and angle is found which produces the desired or known results. Note: there is an 'auto calc' option on their crash simulation dialog however there is no indication if it has been used. In every case we have been asked to evaluate a pc-crash reconstruction it has NOT been selected and selecting it and rerunning the collision dramatically changed the results.

- msmac is like running a mathematical full scale test since it models the collision forces for each millisecond the vehicles structures interact (100-150 milliseconds) instead of trying to approximate the results of a collision interaction with an "instantaneous exchange of momentum" at an "arbitrary point of momentum exchange"

**Pc-crash, and other momentum based collision programs, can be easily manipulated. So be sure to test and check any simulations which form the basis for conclusions.**

And of course test and check ANY reconstruction, simulation or animation program results.

And of course test and check ANY reconstruction, simulation or animation program results.

The experts pc-crash reconstruction which "proved" a vehicle on the wrong side of the road was as follows: However we then with pc-crash put the vehicle in its proper lane and made a minor change to the arbitrary and subjective pc-crash '

**point of momentum exchange**which then also "proved" that the vehicle was in its proper lane!!

**Pc-crash, and other momentum based collision programs, can be easily manipulated. So be sure to test and check and conclusions based on a momentum based program.**

And of course test and check ANY reconstruction or simulation program results.

And of course test and check ANY reconstruction or simulation program results.