Aim: How do scientists analyze tire tracks? Tire track impressions are retained on surfaces in two- and three-dimensional forms. Almost all impressions, including partial impressions, have value for forensic comparisons. In this lesson, students learn how to analyze tire tracks.
Students will be able to:
1) understand which aspects of tire tracks can be analyzed.
2) explain how tire tracks can be individualized.
Motivation: Distribute an image of a tiretrack and discuss what aspects of the tire might have probative value.
Content of the Lesson: Show a PowerPoint presentation to review characteristics of tire track analysis.
Tire impressions reflect the tread design and dimensional features of the individual tires on a vehicle. If properly collected the tire tread impression is extremely valuable in proving a suspect vehicle was present at a crime scene. Tire track evidence can:
Tires are made of semi-hard rubber and are characterized by class and individual characteristics. Class characteristics include size and general patterns. Individual characteristics include regular wear and tear as well as accidental cuts or holes. These characteristics may be reproduced in the tracks left by the tire, depending on the surface and the circumstances under which the track occurred.
When tire tracks are present at a crime scene or in the immediate vicinity, one must properly observe and record them. First, they need to be photographed at a 90-degree angle. This allows for a permanent record of their class and individual characteristics. Then, the tracks are carefully measured. Not only it is important to note the width, but also the circumference of the tire, if the lengths permit it. In addition, if multiple tire tracks from the different wheels of the vehicle are present, it is important to make as many measurements as possible in order to determine the toe (the distance between the front of the tires and the rear of the tires on the same axle) and turning radius of the vehicle. Finally, if the tracks are present in relief, it is also possible to make a cast from it. Usually, plaster of Paris is used and, if the whole track cannot be cast, the most pertinent spot is cast.
Once the general size and pattern of the track is determined, it is possible to consult a database of tires to determine the brand and model of the tire that left the impression. In addition, there are also databases that list which tires are installed from the factory on which vehicles. Finally, with the dimensions of the vehicle, it is also possible to determine which vehicle could have left the tracks.
Recent advances in technology and research allow for the characterization of tire tracks by chemical analysis of the rubber. If a tire leaves rubber residues on the surface, as is the case with skid marks, crime scene investigators can collect these traces and compare them with the tires from a suspected vehicle. The result of this analysis is not as powerful as having individual characteristics that will identify the exact tire, but it adds one more piece of information to the investigation.