TIRE IMPRESSIONS AS EVIDENCE
It is surprising to learn that more than two-thirds of major crimes in the United States involve an automobile, if only as the “getaway car.” It is also true that a tire impression is the most effective way of positively linking a motor vehicle that has been at a crime scene with some degree of certainty. Many crime scene investigators, however, do not look for or record tire impressions nearly as often as should be the case with such potentially important evidence. As with footwear impressions discussed previously, there are three methods for recording tire impressions at a crime scene. Tire impressions may be two or three dimensional and may be negative or positive depending upon how they are produced. As with any other crime scene, photography and drawings are the best methods of faithfully recording the overall scene and this should be done before examination-quality castings are made of tire impressions. As with footwear impressions, both recording the impression photographically and casting for three-dimensional impressions should be done. The main advantage of a cast is that all three dimensions can be easily seen. With tire impressions in particular, there is often a need to make a three-dimensional cast at the scene because, unlike footwear impressions, tire impressions often cannot be taken up and moved to the laboratory for further analysis. Figure 1 shows a portion of a tire tread cast in plaster. There are disadvantages to casting tire impressions relative to photography. Some of these do not apply to footwear impressions. First, it can be difficult to make a cast on a steep incline because the casting material may tend to flow downhill and part of the cast made at the top of the hill may be too thin and fall apart. Second, unlike footwear impressions that are usually about a foot long, tire impressions may be many feet long and require very large casts that can be bulky and unwieldy. It is much easier to take a series of photographs of a long impression. Finally, there is the problem with three-dimensional impressions being negative; the raised areas of the tread become depressions in the cast. Negative impressions should never be compared to a positive image. To correct this problem, a photograph negative should be taken of the tire tread, which adds time and expense to the project. Not surprisingly, tire impressions are made from the same materials and are done in the same way as footwear impressions. Dental stone is the preferred medium for casts in soil, and snow-casting wax is best for impressions in snow. A suitable measuring instrument should be placed in all photographs.
FIGURE 1 A plaster cast of a portion of a tire tread.
FIGURE 2 A portion of the tread of a known tire that has been inked and rolled out on paper to provide points of comparison with an unknown tire tread.
Lifts of two-dimensional tire impressions are made in the same way as with foot wear impressions. Because of the great length of many tire impressions, several lifts of the same impression may have to be made in order to get the whole impression. In some cases, a roll of Mylar film, the full length of the impression, can be used to lift a long tire impression all at once. Figure 2 is a portion of a tire tread that was made in ink for comparison with an unknown tread.
In addition to the tread patterns, other data can be derived from tire impressions. For example, the Michigan State Police Forensic Science Laboratory collects data on wheelbase and tire tread stance measurements. The wheelbase of a motor vehicle is the distance from the center of the front hub to the center of the rear hub. The stance is the distance from the centerline of the right tire to the centerline of the left tire. The Michigan State Police Laboratory keeps a database of automobiles organized by these three measurements, and from these data and the tire tread design, one may be able to determine the make and model of a car or truck.
