A: Intervehicle friction, as simulated by original SMAC, is in the form of simple Coulomb friction. It requires the existence of a force perpendicular to the contact region and it is proportional to that force. In actual collisions, tensile forces which are independent of a normal force, frequently act along the vehicle-to-vehicle interface. Also, significant resistance to rotations relative to the collision partner is sometimes generated by interlocking of the damaged structures.
In those cases where acceptable matches of the positions and orientations at rest cannot be achieved with the original SMAC and/or the EDSMAC program and, further, where there are no obvious obstacles and/or topographical features that have significantly affected the vehicle motions, it becomes obvious that the simulated vehicle interactions are overly simplified. The SNAG routine of the m-smac computer program provides a capability for achieving higher fidelity matches of the positions and headings at rest while maintaining the principles of conservation of linear and angular momentum.
The SNAG routine incorporates impulsive linear and angular constraints that momentarily resist relative motions of the two vehicles while continuing to conserve both the linear and the angular momentum of the two-vehicle system during the collision. By this means, it is possible to simultaneously refine the matches of the positions and headings of both vehicles at their rest positions with a series of iterative adjustments of the impulsive constraints. The rationale of the analytical approach is the concept that inappropriate inputs for the impulsive constraints cannot achieve acceptable responses of both vehicles.
Also see our presentation of SNAG in:
- 1997 "SMAC-97 - Refinement of the Collision Algorithm", McHenry & McHenry, SAE Technical Paper No. 970947
2008 McHenry Accident Reconstruction Book section:SNAG Option
Online smac manual section DISCUSSION: What is the SNAG Option? (login:mchenry password: thankyou!)
1987 "SMAC-87", McHenry & McHenry, SAE Technical Paper No. 880227,