The PHASE4 simulation program is a general purpose mathematical model for simulating the three-dimensional dynamic responses of trucks, tractor/trailers and triples combinations. The PHASE4 program was developed in 1980 by the Highway Safety Research Institute (HSRI) of the University of Michigan under the sponsorship of the Motor Vehicle Manufacturers Association (MVMA) and the Federal Highway Administration (fhwa)
Engineering Dynamics also offers a version of PHASE4 called EdVDS
From the EDCorp wesbite:
EDVDS is an HVE-compatible 3-D simulation of the dynamic response of a commercial vehicle to driver inputs and 3-D road conditions. A tow vehicle and up to three trailers may be simulated. EDVDS is an extended version of the Phase 4 program developed at the University of Michigan. Extensions include full 3-dimensional simulation capability, an updated suspension model with jounce/rebound stops, an updated tire model that supports 360° slip angles and 2-step radial tire stiffness, and the ability of each tire to respond to an arbitrary 3-D terrain. These advancements extend the capabilities of researchers to study real world highway safety issues involving commercial vehicles.
The PHASE4 program was created to model trucks, trucks w/trailers and triples. To simplify the coding the authors of PHASE4 used 'small angle" assumptions.
Small-angle approximations are used as a useful simplification of the basic trigonometric functions which is approximately true in the limit where the angle approaches zero. They are truncations of the Taylor series for the basic trigonometric functions to a second-order approximation. Therefore the PHASE4 program has program STOPS at +/-10 to +/-15 degrees of roll and/or pitch since that is the approximate range where the small angles assumptions as valid.
Engineering Dynamics/HVE published in 1999 that they modified the PHASE4 program for "large angles", in other words they extended the many equations for the PHASE4 program to INCLUDE the trigonometric functions instead of the simple angles.
Their only documentation of the changes they made to the code is in their publication:
This is the extent of their documentation of that MAJOR endeavor:
documentation of EDC changes to PHASE4.jpg (41.58 KiB) Viewed 5556 times
Their statement 'completely rewritten' of the code we'd bet is mostly transformation of the code from Fortran code to C++ code which is not required since all codes can communicate and link at the binary/machine level! Our bet is they simply took out the checks for program STOPS for angles greater than 10 degrees to 'extend beyond small angle assumptions' and they let it fly!!