Tire Tread Transfer onto a Victim's Jacket
After a fatal hit-and-run, a tire tread pattern is transferred in blood and mud onto the victim's light-colored jacket. The pattern is a five-rib design with a distinctive angled sipe on the shoulder rib. The jacket fabric is stretched and folded, so the print is distorted; only a 12 cm by 20 cm area is usable. A suspect vehicle has four tires of the same model; one rear tire has a 2 cm sidewall cut and an embedded small stone in the shoulder rib. The jacket print shows the five-rib design but no sidewall area and no clear stone void. Test impressions on similar fabric are possible. The laboratory must decide whether to report that the suspect tire made the print, that the tire model is consistent, or that the comparison is inconclusive, and how to treat the missing sidewall and stone.
1. What conclusion is appropriate, and how do the missing sidewall and stone affect it?
Read the worked answer
The print shows class characteristics - a five-rib design with an angled shoulder sipe - that are consistent with the tire model on the suspect vehicle. The sidewall cut is not represented because the print does not include the sidewall; it cannot be compared. The embedded stone is an accidental characteristic, but the print shows no clear stone void. That absence is meaningful only if test impressions on similar fabric with the stone in place would be expected to show a void under the same conditions. Because the fabric is stretched and folded and the usable area is small, the absence may be due to distortion, incomplete transfer, or the stone not being in the printed area. Therefore the defensible conclusion is that the print is consistent with the tire model, not that the suspect tire made it. If test impressions show the stone produces a distinctive void on similar fabric and the questioned area covers that location without a void, the examiner could consider exclusion of that tire or conclude the stone was not present at the time. Without that, the comparison is inconclusive at the individual level.
2. How would you design test impressions to evaluate the missing stone void and sidewall?
Read the worked answer
Design test impressions on similar fabric with the same blood and mud matrix, using the suspect tire or a comparable tire section under controlled pressure. Make a series: stone in place, stone removed, and different orientations. The goal is to see whether the stone creates a void or a distinctive deposit on fabric and whether the sidewall cut could appear in a fabric print. Because tires are large, use a tire test rig or roll the tire over the fabric; document pressure and speed. Compare the five-rib spacing, sipe angle, and rib widths. If fabric stretch obscures the stone void, then the absence in the questioned print is not probative. Also compare all four tires, since the print could have been made by any. Document the limitations and use blind verification. Do not use the sidewall cut unless the questioned print includes sidewall detail.
3. If the suspect vehicle's tires were replaced before seizure, how does that affect the conclusion?
Read the worked answer
If the tires were replaced before seizure, the specific tires that might have made the print are unavailable. The examiner can still compare the questioned print to the tire model and to the replacement tires for class characteristics, but cannot individualize to the vehicle. Investigators should obtain the old tires through service records or other means; if they cannot, the evidence is limited to a class association. The fact of replacement does not exclude the vehicle; it limits the conclusion. The report should state that the print is consistent with the tire model and that no individual comparison was possible. The examiner should not speculate that the replacement tires are the same as the old ones unless records establish it.