The Traffic Accident Reconstruction Origin -ARnews-
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The Traffic Accident Reconstruction Origin -ARnews-
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The following is from an Appendix of SAE 97-0949 which contains a discussion of rotating vhicle drag factors, etc.
(The paper is (c) SAE, please visit SAE at
http://www.sae.org
to obtain a copy of the full paper).
APPENDIX 1: DISCUSSION OF SPIN2 from SAE 97-0949
The SPIN2 procedure of the original CRASH program uses as a starting point the relationships developed by Marquard [38]. The Marquard procedure takes into account the fact that the linear and angular (i.e., yaw rotation) displacements of a four-wheeled vehicle subsequent to a collision each occur under conditions of intermittent deceleration when the wheels are free to rotate.
By approximating the linear and angular deceleration rates of a vehicle with either
(1) all wheels freely rotating or
(2) all wheels locked during different phases of spinout motion,
Marquard developed approximate relationships between the total linear and angular displacements during the travel from separation to rest and the corresponding linear and angular velocities of a vehicle at separation from its collision partner, for the two cited cases of rotational resistance.
In the CRASH program [1], the SPIN2 routine was developed to extend the relatively simple Marquard relationships to include the cases of partial braking and/or damage-locked individual wheels.
Evaluations of the resulting, modified relationships by means of trial applications to spinout trajectories generated with SMAC [20] revealed several shortcomings of the initial SPIN2 relationships.
First, a residual linear velocity frequently exists at the end of the rotational (i.e., yawing) motion.
Next, the general shapes of plots of linear and angular velocity vs. time changed substantially as functions of the ratio of linear and angular velocity at separation from the collision.
Finally, the transitions between the different deceleration rates of linear and angular
motions were found to occur gradually rather than abruptly. Slope changes in the plots of linear and angular velocity vs. time were found to generally occur in the form of rounded "corners" in the curves.
To improve the accuracy of approximations of separation velocities, provisions for the introduction of a residual linear velocity at the end of the rotational motion and the development of empirical coefficients, in the form of polynomial functions of the ratio of linear to angular velocity at separation, were incorporated in the SPIN2 analytical relationships of the CRASH program.
Since the separation velocity ratio is initially unknown, a solution procedure was developed whereby several trial values of the ratio, based on an approximate equation, were used to test multiple solutions.
The cited analytical developments, reported in [2], involved only limited efforts which were aimed primarily at demonstrating the feasibility of the CRASH concept. Polynomial functions to generate empirical coefficients were developed, on the basis of 18 single-vehicle SMAC runs with relatively high linear and angular velocities for starting (i.e., separation) conditions. In the more common, real-life accident case, a relatively small rotation (i.e., yawing) velocity may exist at separation. In such a case the initial direction of the velocity vector with respect to the longitudinal axis of the vehicle will obviously affect the sequence and the duration of the linear and angular deceleration rates
of the vehicle.
In consideration of known shortcomings of the SPIN2 aspect of the CRASH program, a subcontract to refine SPIN2 was undertaken in 1979 [24]. A representative sample of actual accident cases was selected from the NCSS [39] files for use in the study. A total of 50 cases were selected and then reconstructed with the SMAC computer program. For each of the SMAC reconstructions, separation information was used to formulate a basis for a refinement of the SPIN2 empirical coefficients. A careful examination of the time-history plots of linear and angular velocities for all of the cases in the sample revealed a significant number
of cases in which the SMAC-predicted behaviour deviated from the analytical assumptions upon which the SPIN2 routine is based. Attempts were undertaken within the research project to discriminate characteristics of separation conditions. Unfortunately, only partial success was achieved in the attempts to accommodate deviations by means of the use of logic and discriminators. As a result, a realistic appraisal of residual scatter in the empirical fits led to the conclusion in [24]:
"To achieve a general improvement in the reliability and accuracy of approximations of the angular and linear velocities at separation, a step-by-step time history form of trajectory solution should be implemented."
Subseqent work which has been performed on investigation and refinement of the SPIN empirical coefficients [30, 40,41] and the corresponding modifications to CRASH is subject to the effects of ‘scatter’. Any proposed refinements of the SPIN empirical coefficients and any reconstruction
techniques which are based on the refinements of the SPIN empirical approach will ultimately fail in some applications to individual case reconstructions due to the possibililty that the particular case being investigated may be characteristic of a "scatter" point.
The research cited in this paper strongly supports the conclusion from 1981 that implementation of a trajectory solution procedure should utilize an iterative time-history simulation.
REFERENCES
1. McHenry, R.R., "The CRASH Program - A simplified Collision Reconstruction Program" Proceedings of the Motor Vehicle Collision Investigation Symposium, Calspan, 1975
2. McHenry, R.R. Lynch, J.P., "User’s Manual for the Crash Computer Program" Calspan Report No. ZQ-5708-V-3, Contract No. DOT-HS-5-01124, Jan 1976
24. McHenry, R.R., McHenry, B.G., "National Crash Severity Study - Quality Contol, Task V: Analysis to Refine Spinout Aspects of CRASH", Calspan Field Services, Inc. ZP-6003-V-4; DOT-HS-6-01442, January 1981
30. Fonda, A.G., "Nonconservation of Momentum During Impact", SAE Paper 95-0355
39. Kahane, C.J, et al, "The National Crash Severity Study", Sixth International Technical Conference on Experimental Safety Vehicles (1976) 495-516
40. Fonda, A.G., Metz, L.D., "Post-Impact Spin, 1968-1993", SAE Paper 93-0653
41. Fonda, A.G., "Energy and Major Diversion in Accident Reconstruction", SAE Paper 96-0888
Brian McHenry
mchenry@interpath.com
For example, to continue this discussion look for a thread titled
Rotating Vehicles Drag Factor
If this thread does not exist in the current archive, you can begin another one by using that title.
s = 3.87 SQR(rf)
where
s = 2.74d/SQR(m (d+h))
where
v = d/t
where
New: Tells iCAR to reset all the numerical fields to 0 and clears all buffers. A fresh entry screen will then be presented.
Rerun: Retains all data which was entered during the most recent New or Rerun compilation and allows the user to make changes to any or all data items.
Showit: This option first prompts the user to determine if a printout is required, then generates a screen image showing vehicle damage, PDOF, and D. At present, the screen print utility is limited to use by Hewlett-Packard LaserJet products. A driver for dot-matrix printers is in the works.
Additional Notes: Whenever New is selected, the user will be given the option to create an ASCII file which will record the results from any number of runs. Also, during each run, the user is given the option to alter the vehicle stiffness constants. The decision to offer this option was based on the fact that the default constants, based primarily on the CRASH III model, have some age on them now. More up-to-date information is becoming available. If the user has this information, it should be used. When a larger body of this information becomes available to me, I will alter the internal constants to reflect this change.
Time - Known Distance, Known Velocity
t = d/v
where
t = (vo - vf) / a
where
t = (vf - vo) / a
where
Constant Speed
t = d / v
where
d = .5at^2
where
R = (cý / 8m) + (m / 2) where R = radius c = chord m = middle ordinate
| Vehicle Type | A(lbf/in) | B(lbf/in^2) | A(lbf/in) | B(lbf/in^2) |
|---|---|---|---|---|
| Stiffness Catagory | CRASH3 Users Guide | CRASH 3 Users Guide | Strother, et al | Strother, et al |
| 1) Subcompact | 302.0 | 47.0 | 237.9 | 58.9 |
| 2) Compact | 259.0 | 43.0 | 240.0 | 60.0 |
| 3) Intermediate | 317.0 | 56.0 | 247.5 | 58.95 |
| 4) Full-size | 356.0 | 34.0 | 236.7 | 51.5 |
| 5&6 Largest | 325.0 | 37.0 | 247.2 | 57.9 |
| 7) Vans | 383.0 | 126.0 | 349.7 | 99.8 |
| MPV's | 383.0 | 126.0 | 350.9 | 100.5 |
| 8) Pickups | 480.0 | 50.0 | 425.6 | 72.5 |
| 9) Front Wheel Drive | 373.0 | 38.0 | 240.4 | 58.2 |
SIZE WHEELBASE
---- -------------
1 80.9 - 94.8
2 94.8 - 101.6
3 101.6 - 110.4
4 110.4 - 117.5
5 117.5 - 123.2
6 123.2 - 150.0
7 109.0 - 130.0 VANS
8 PICKUPS [Select 1 to 6 based on wheelbase]
9 JEEPS [Select 1 to 6 based on wheelbase]
11 IMMOVABLE BARRIER
The CDC is a seven-character code used to describe a field damaged vehicle. Columns 1 and 2 (force direction), Column 3 (area of deformation), Column 4 (specific longitudinal or lateral area), Column 5 (specific vertical or lateral area), Column 6 (type of damage distribution), Column 7 (extent).
For the purposes of iCAR the main characters of interest are in Columns 1 through 3. iCAR needs to know the force direction (either through the improved PDOF or through the data in Columns 1 and 2). iCAR also needs to know the plane of contact (e.g., front, back, left, right, etc.).
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