## What is a Sideslap?

General questions on the SMAC, msmac3D and other Collision Simulation programs
MSI
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Joined: Thu Jun 18, 2009 12:37 pm

### What is a Sideslap?

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A sideslap collision normally occurs in near 90 degree impacts where the vehicle rotate towards each other and create a slap of their sides after the initial impact.
For more detailed information see post on sidelaps here at
• Above is a snip of a simulation of a 90 degree collision (for more info, see 2007 NYSTARS sideslap crash) with the velocity vectors displayed in the second video.
We have also posted up a graphic and tabular summary of the results which includes the positions, velocities, momentum and kinetic energy calculations
Notice how the velocity vectors direction and magnitude changes during the sideslap.
Below are some pictures from the 2007 NYSTARS test:
NOTE the movement between initial and secondary 'sideslap' impacts!
OH sequential NYSTARS sideslap.jpg (301.78 KiB) Viewed 2013 times
Lumina Damage reduced.jpg (106.54 KiB) Viewed 2013 times
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MSI
Posts: 2110
Joined: Thu Jun 18, 2009 12:37 pm

### Re: What is a Sideslap?

On another forum in a discussion of the 'cone of departure' referred to in momentum crash reconstructions there was a reference to a full scale test documented in SAE paper 2000-01-0464.
The following is additional information on that paper, the test and the crash reconstruction with sideslap!
Here's a simulation of the crash:
SAE paper 2000-01-0464 An Analysis of a Staged Two-Vehicle Impact, Wirth, Marine,Thomas
• An Analysis of a Staged Two-Vehicle Impact 2000-01-0464
• ABSTRACT
• In this paper a detailed analysis of a staged two-vehicle impact is conducted. The staged impact consisted of two moving vehicles impacting in a left front corner-to-right front corner configuration. Both vehicles were outfitted with an array of triaxial accelerometers. An Anthropomorphic Test Device (ATD) was located in the driver position in one of the test vehicles. High-speed film cameras were installed on the vehicle, documenting the test dummy motion during the impact. The impact and subsequent vehicle motions were documented with offboard real-time video and high-speed film cameras. The accelerometer data from both vehicles are analyzed. This analysis demonstrates the effects of yaw motion on the determination of Delta-V and on occupant kinematics. The notion of a Principal Direction of Force (PDOF) in a yawing vehicle is also discussed. Common analytical accident reconstruction methods are applied to the post-impact information (rest positions, residual crush profiles, etc.) in an attempt to recreate the initial impact configuration. Popular accident simulation programs are also employed for the same purpose. The appropriateness and limitations of these methods are examined.
DISCUSSION OF THE MOMENTUM ANALYSIS IN THE PAPER
• A review of the 2000 SAE paper mentioned above
Available here:SAE paper 2000-01-0464,
reinforces our point that you MUST:
Be extremely careful when applying simplified momentum analysis,

Whether hand calculations or computer programs which use simplified momentum assumptions. Crash simulation programs like pc-crash or virtual crash or planar impact models.
Particularly if there is a sideslap or pre-impact swerve since a careful choice of the exit angles is important and essential.
• If you know the answer you want or need, and that is what they did in the paper, and so adjust exit angles accordingly to match, you get good correlation.
However as they said in the Momentum section of the paper:
• The angles from impact position directly to rest positions were -161° and -101° for the Subaru and LTD, respectively.
A mistaken use of these values can lead to impact velocities that are incorrect by over 20%.
NOTE: When there is a sideslap you are trying to approximate a collision as an 'instantaneous exchange of momentum' whereas there can be 300 to 400 milliseconds between the initial impact and the point of separation from the sideslap.
• That is far beyond the simplifying assumptions of an instantaneous exchange.
Obviously knowing the results you want and need you can tweak things to get a good result.
However as objective crash reconstructionists you need to test and refine and double check your work using either monte-carlo to see what variables are most sensitive or a simulation program like msmac3D.
The following it the section on momentum analysis from the 2000 SAE paper with important points underlined and in red bold:
• MOMENTUM ANALYSIS – The impact and rest positions of the two test vehicles were measured and are shown in Figures 1 and 2.
Using these positions a simple two dimensional momentum analysis was done to predict the initial velocities and the Delta-V’s of the vehicles. The velocities were adjusted to balance pre and post-impact momentum and to approximate the frictional energy lost during run-out to rest.
Additionally, an attempt was made to match a target damage energy loss of 200,000 footpounds. This was an approximation of the energy loss for both impacts. The impact angles were -90° and 180° for the Subaru and LTD, respectively. and although these angles would not normally be known with certainty, they were used in the analysis assuming a 90° intersection with no pre-impact steering. The departure angles of the test vehicles after the second impact, as derived from the data and overhead camera were -156° and -134° for the Subaru and LTD, respectively.

These angles must be estimated by the reconstructionist.

For example, if tire marks are present immediately after the point of impact they can be used to estimate the departure angles.

The angles from impact position directly to rest positions were -161° and -101° for the Subaru and LTD, respectively. A mistaken use of these values can lead to impact velocities that are incorrect by over 20%.

Using departure angles of -153° and -132° for the Subaru and LTD, which are estimates near the actual values, and matching the damage energy yields initial speeds estimates of 49.0 mph and 37.4 mph for the Subaru and LTD, respectively. These speeds are in error by less than 2 mph. The Delta-V values were calculated and represent the combined effect of both impacts. The values from the momentum analysis are 44.9 mph and 28.0 mph for the Subaru and LTD, respectively. These are within 1 mph of the values from the accelerometer date of 45.0 mph and 28.7 mph.

Here is the diagram from the full scale test:
2000tst veh and scene data.jpg (76.67 KiB) Viewed 1898 times
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MSI
Posts: 2110
Joined: Thu Jun 18, 2009 12:37 pm

### Re: What is a Sideslap?

Dug up some of the comparisons of Measured time histories v msmac3D
More refinement and discussion to follow
Subaru LTD Angular Velocty v msmac3D_reduced image.jpg (19.06 KiB) Viewed 1929 times
Subaru X,Y Accel_reduced image.jpg (21.86 KiB) Viewed 1929 times
LTD Accel v smac_reduced image.jpg (21.8 KiB) Viewed 1929 times
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MSI
Posts: 2110
Joined: Thu Jun 18, 2009 12:37 pm

### Re: What is a Sideslap?

Be sure to also see the topic which includes documentation of an impact between a Freightliner truck/trailer combination and a Chevrolet Silverado Pickup
and includes a sideslap.
The following is a video of that collision interaction:
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