In collisions why does Mass1DeltaV1 not always equal Mass2DeltaV2?

General questions on the SMAC, msmac3D and other Collision Simulation programs
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MSI
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In collisions why does Mass1DeltaV1 not always equal Mass2DeltaV2?

Post by MSI »

Q: Why sometimes does Mass1DeltaV1 not equal Mass2DeltaV2 with msmac3D collisions (and/or if you have and compare EDR deltaV data for two collision partners) ? This is particularly obvious in lower speed collisions. Why?

A: (Please note this is a developing response so please also check back again soon! Preliminary summary and additional details added 7/10/2020, Send any question/comments/ideas to forum@mchenrysoftware.com. Thank you)
Sir Isaac pound note.jpg
Sir Isaac pound note.jpg (13.13 KiB) Viewed 716 times
Let us recap Sir Isaac's Laws:
  1. Every object moves in a straight line unless acted upon by a force.
  2. The acceleration of an object is directly proportional to the net force exerted and inversely proportional to the object's mass.
  3. For every action, there is an equal and opposite reaction.
In A Hurry Executive Summary:
  • Q: So why do we not always get M1*DeltaV1 = M2*DeltaV2 with msmac3D (and/or comparing EDR data for two collision partners)?
    • A: Why? Because of External Forces!
      • aka tire forces and other external forces (structures scraping)!
      Many of you evaluating collision with one or both vehicles having an EDR may have noticed this phenomenon affecting each vehicle particularly when braking is indicated in one or both vehicles.
      • We have often said
        • "The collision between two vehicles is not the same as a collision between two pool balls. More like two crushable rectangular boxes with wheels", [/url]
          with wheels/tires/rims/undercarriage produce external forces with or without braking
          • April 2021: We also need to mention "Effective Mass" as another possible explanation for M1DeltaV1 Not Equal M2DeltaV2
        • See Some Momentum Misconceptions which includes that statement and more issues to consider
        • See our SMAC paper from 1973 where we note the importance of including consideration of external forces

    Now here's the preliminary details and quantification of the % changes due to external forces (tire forces or other forces (structure scraping)):
    SUMMARY OF PRELIMINARY FINDINGS
    We ran a series of preliminary tests to quantify the amount of energy loss or gain from tire forces due to damage or braking.
    We had 3 separate vehicle setups with 6 separate variations on braking:
    A simple T-Bone impact into a stationary vehicle
    3 weight variations:
    • Test1 series: 4000 lb vehicle into a 2000 lb vehicle (2:1 weight ratio)
    • Test2 series: 2000 lb vehicle into a 4000 lb vehilce (1:2 weight ratio)
    • Test3 series: 3000 lb vehicle into a 3000 lb vehicle (1:1 weight ratio)
    For each series 6 variations were run:
    1. 1st Impact on ice, surface with NO friction so that Mass1*DeltaV1 = Mass2*DeltaV2
    2. 2nd Impact: No Braking, Ground Mu=0.80
    3. 3rd Impact: Full Braking throughout, Ground Mu=0.80
    4. 4th Impact: Full Braking is ramped on from initial contact so full within 100 ms, Ground Mu=0.80
    5. 5th impact: 50% Braking throughout, Ground Mu=0.80
    6. 6th impact: 50% Braking is ramped on from initial contact so 50% within 100 ms, Ground Mu=0.80
    • NOTE1: On ICE with msmac2D/3D simulation, just as with reality, you can not get a "perfect" impact so the vehicle may yaw(2D) or yaw/pitch/roll (3D) a little which may make a 1%-2% difference in the 'exact' Mass1*DeltaV1=Mass2*DeltaV2
    • NOTE2: These preliminary tests represent a limited range of Impact Speed Changes
      • For the 15 MPH Impact Speed tests the DeltaV's range from 5-12 MPH
      • For the 30 MPH Impact Speed tests the DeltaV's range from 11-23 MPH
      • Obviously a more thorough investigation should expand on the range or speeds
    A SUMMARY OF PRELIMINARY FINDINGS IS AS FOLLOWS:
    For a T-Bone Impact into a stationary vehicle:
    • Striking Vehicle Results:
      • At 15 MPH Impact Speed the Striking Vehicle DeltaV may INCREASE by 2% to 13% over the theoretical value
      • At 30 MPH Impact Speed the Striking Vehicle DeltaV may INCREASE by 2% to 9 % over the theoretical value
      Struck Vehicle Results:
      • At 15 MPH Impact Speed the Struck vehicle DeltaV may DECREASE by 8% to 28% under the theoretical value
      • At 30 MPH Impact Speed the Struck Vehicle DeltaV may DECREASE by 4% to 14% under the theoretical value
    • These preliminary results raise some questions about how Simplified Momentum simulation programs which ignore all external forces during their assumed "instantaneous exchange" of momentum deal with physical phenomena. Full scale test results use accelerometers which have measurements that INCLUDE the effects of tire forces, friction and braking DURING the collision interaction so inherent in Simplified Momentum solutions is a potential error per the above ranges for DeltaV comparisons.
      • Note: We have been told that "well since the momentum is exchanged instantaneously and since the tire forces are simulated UP TO the collision and FROM the collision onward, the tire forces ARE included".
        That is true, however realize and consider also:
        1. The measurement of deltaV from full scale tests is based on accelerometers (or EDRs) which INCLUDE the change in accelerations due to the influence of external forces. The numbers from this preliminary investigation indicate that the influence of external forces can sometimes cause a significant change to the DeltaV
        2. During a collision the presence of tire forces also CHANGES THE MAGNITUDE AND DIRECTION of the resultant accelerations.
        We will investigate further.
    Summary Preliminary Charts of the Results
    and after are the Preliminary Test Series details:
    15 and 30  MPH Compare Ratio.jpg
    15 and 30 MPH Compare Ratio.jpg (81.21 KiB) Viewed 697 times
    • Summary Chart1 above is a side by side comparison of the Tests for the ratio of Mass1DeltaV1/Mass2DeltaV2.
      • For the 1st test on ICE the result is 1.0.
      • For all other tests the results vary due to the presence of tire forces and braking.
    15 and 30 MPH Striking Pct Compare.jpg
    15 and 30 MPH Striking Pct Compare.jpg (90.02 KiB) Viewed 697 times
    • Summary Chart2 above demonstrates the INCREASE in Impact Speed Change for the STRIKING VEHICLE in the T-Bone Crash over the 1st test on ice for the various Test conditions.
    15 and 30 MPH Struck Pct Change DeltaV.jpg
    15 and 30 MPH Struck Pct Change DeltaV.jpg (80.72 KiB) Viewed 697 times
    • Summary Chart3 above demonstrates the DECREASE in Impact Speed Change for the STRUCK VEHICLE in the T-Bone Crash over the 1st test on ice for the various Test conditions.
    THE FOLLOWING ARE DETAILS ON THE INDIVIDUAL TESTS:
    TEST1: 4000 lb vehicle T-Bone into Stationary 2000 lb Vehicle
    • In chart below on the left is scale for the DeltaV for each vehicle (Veh1 4000 lb, Veh2 2000 lb) and on right is scale for M1DeltaV1/M2DeltaV2 (1.0 if on ice!)
    • Note the differences are more pronounced at lower Impact Speed Changes since the speed loss (or gain) from frictional forces over the duration of the collision (75-150 ms) is a higher proportion of the collision crush forces impact speed change.
    • Note a friction of 0.80 may be conservative as many newer braking systems can generate friction values higher than 0.80.
    MOM Test1 15 and 30 MPH.jpg
    MOM Test1 15 and 30 MPH.jpg (87.97 KiB) Viewed 701 times
    The following is a summary of the effects of friction/tire forces/braking on DeltaV which can change results from the theoretical conservation of momentum as follows:

    For a T-Bone Crash, NOT on ice, on tires with frictional properties and varying amounts of braking:
    • For the 4000 lb striking vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~5.7 MPH may INCREASE by 2%-13%
      • at 30 MPH Impact speed, the theoretical DeltaV of ~10.2 MPH may INCREASE by 2% to 9%
    • For the 2000 lb struck vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~11. MPH may DECREASE by 8%-19%
      • at 30 MPH Impact speed, the DeltaVof 21.8 MPH may DECREASE by 4% to 7%
    DelV Chg Test1 15 and 30 MPH.jpg
    DelV Chg Test1 15 and 30 MPH.jpg (81.93 KiB) Viewed 701 times
    Next up, lets reverse the weights and see the results (same impact, 6 options, simply small 2000 lb veh into large 4000 lb veh)
    check back soon for results.
    TEST2: 2000 lb vehicle T-Bone into Stationary 4000 lb Vehicle
    • In chart below on the left is scale for the DeltaV for each vehicle (Veh1 2000 lb, Veh2 4000 lb) and on right is scale for M1DeltaV1/M2DeltaV2 (1.0 if on ice!)
    • Note the differences are more pronounced at lower Impact Speed Changes since the speed loss (or gain) from frictional forces over the duration of the collision (75-150 ms) is a higher proportion of the collision crush forces impact speed change.
    • Note a friction of 0.80 may be conservative as many newer braking systems can generate friction values higher than 0.80.
    MOM Test2 15 and 30 MPH.jpg
    MOM Test2 15 and 30 MPH.jpg (98.44 KiB) Viewed 701 times
    The following is a summary of the effects of friction/tire forces/braking on DeltaV which can change results from the theoretical conservation of momentum as follows:

    For a T-Bone Crash, NOT on ice, on tires with frictional properties and varying amounts of braking:
    • For the 2000 lb striking vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~11.4 MPH may INCREASE by +3% to +6%
      • at 30 MPH Impact speed, the theoretical DeltaV of ~22.1 MPH may INCREASE by +2% to +4%
    • For the 4000 lb struck vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~5.7 MPH may DECREASE by -17% to -28%
      • at 30 MPH Impact speed, the DeltaVof 10.9 MPH may DECREASE by -10% to -14%
    DelV Chg Test2 15 and 30 MPH.jpg
    DelV Chg Test2 15 and 30 MPH.jpg (83.3 KiB) Viewed 701 times
    TEST3: 3000 lb vehicle T-Bone into Stationary 3000 lb Vehicle
    Next up, lets make identical vehicles with SAME 3000 lb weights and see the results (same impact, 6 options, Both vehicles 3000 lb)
    • In chart below on the left is scale for the DeltaV for each vehicle (Veh1 3000 lb, Veh2 3000 lb) and on right is scale for M1DeltaV1/M2DeltaV2 (1.0 if on ice!)
    • Note the differences are more pronounced at lower Impact Speed Changes since the speed loss (or gain) from frictional forces over the duration of the collision (75-150 ms) is a higher proportion of the collision crush forces impact speed change.
    • Note a friction of 0.80 may be conservative as many newer braking systems can generate friction values higher than 0.80.
    MOM Test3 15 and 30 MPH.jpg
    MOM Test3 15 and 30 MPH.jpg (92.23 KiB) Viewed 701 times
    The following is a summary of the effects of friction/tire forces/braking on DeltaV which can change results from the theoretical conservation of momentum as follows:

    For a T-Bone Crash, NOT on ice, on tires with frictional properties and varying amounts of braking:
    • For the 3000 lb striking vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~8.6 MPH may INCREASE by +4% to +8%
      • at 30 MPH Impact speed, the theoretical DeltaV of ~16.7 MPH may INCREASE by +2% to +5%
    • For the 3000 lb struck vehicle in a T-Bone crash:
      • at 15 MPG Impact Speed, the DeltaV of ~8.5 MPH may DECREASE by -10% to -22%
      • at 30 MPH Impact speed, the DeltaVof 16.6 MPH may DECREASE by -5% to -10.5%
    DelV Chg Test3 15 and 30 MPH.jpg
    DelV Chg Test3 15 and 30 MPH.jpg (82.02 KiB) Viewed 701 times
    (Please note this is a developing response so please also check back again soon!
    Preliminary summary and additional details added 7/10/2020,
    Send any question/comments/ideas to mchenry@mchenrysoftware.com. Thank you)
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MSI
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Joined: Thu Jun 18, 2009 12:37 pm

Re: Why sometimes with msmac3D collisions does Mass1DeltaV1 not equal Mass2DeltaV2?

Post by MSI »

I was reminded that we have actually been emphasizing the importance of including consideration of external forces since the invention of the SMAC program in 1973 with particular emphasis of the obvious problems which can occur when the vehicles have secondary "side-slap" contact.
  • More recently a lot of lower speed collisions are being reconstructed to determine why/why not airbag deployment.
    This forum topic demonstrates that braking and/or tire forces during the collision can and do make a possibly significant differences in the acceleration time histories and so MUST be considered when determining Impact Speed Change (DeltaV)
From the original Stapp Conference paper by Ray
smac title.jpg
smac title.jpg (18.95 KiB) Viewed 664 times
smac selected analytical approach.jpg
smac selected analytical approach.jpg (105.55 KiB) Viewed 664 times
smac paper references.jpg
smac paper references.jpg (86.04 KiB) Viewed 664 times
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Re: In collisions why does Mass1DeltaV1 not always equal Mass2DeltaV2?

Post by MSI »

Re-titled this topic since applies to any and all crashes...not just simulations.
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Re: In collisions why does Mass1DeltaV1 not always equal Mass2DeltaV2?

Post by MSI »

Left out an essential and important item: Effective Mass!
Unless the crash is central all the mass may not participate in the collision.

Please see What is the What is the "Effective Mass" in the CRASH equations?
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