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James R. Schmidt, Jr., BSME, Collision Reconstruction Engineering Analyst
Before departing a trucking facility, a loaded tractor-trailer parked in the “cars only” parking lot for office employees. The tractor-trailer operator went into the office. When he returned to his truck and pulled away, he struck the parked car of an office employee. Why did this happen? Besides driver error/inattention, the off-tracking of the trailer’s tires relative to the tractor’s tires led to the crash.
During a turning maneuver, a trailer’s tires will always track to the inside of the tractor’s tires. In other words, the trailer’s tires will follow a smaller radius than the path followed by the tractor’s tires while turning.






Having been provided only a surveillance video, DJS performed a “desktop-only analysis” of the occurrence. The make and year of the tractor was approximated from the surveillance video, and a search of our library of vehicle inspections was performed. Similar tractors were found, from which specs for the subject tractor were approximated. The same procedure was followed for approximating specs for the subject trailer.


Using an aerial photograph of the area of the occurrence, a basic computer simulation was setup and iterated to model the movements of the tractor-trailer as it accelerated into its turn from a stop. The tractor-trailer got up to a speed of close to 10 mph by the time the right rear tires of its trailer impacted the driver’s front fender of the parked car. The impact was like an elephant striking a fly. Minimal effect on the elephant (i.e. the tractor-trailer), but a huge effect on the fly (i.e. the parked car).




The tractor-trailer operator gave a false statement to his insurance company, wherein he indicated that the occurrence took place at the loading dock, with the employee’s car being parked improperly at that location. Office management pulled the surveillance video, showing exactly what happened, and where it happened. DJS analysis re-created how it happened and explained why it happened.
James R. Schmidt, Jr., BSME, Collision Reconstruction Engineering Analyst with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.