1. Forensic Tire Tread and Tire Track Evidence

2. Tire track impressions are classified as pattern evidence because the tire track impressions leave behind a unique pattern. Just like shoe impressions can help narrow down, the brand, style and size, tire tracks have the ability to do the same thing.

3. Tire tracks are important in forensic investigations and are usually found in road accident scenes or in the access and escape routes of other crime scenes.
Tracks help investigators identify the type of vehicle that left them. Investigators may make ink prints of a tire or plaster casts of a track. They will also take photographs that can later be used to prove a match.

4. Features to analyze:
Tread pattern
Width & depth of the tread pattern
Unique characteristics due to the wear pattern or defects

6. Tire databases are available help investigators determine the brand and model of the tire that left the impression , which can be used to determine the type of vehicle that made the tracks.

7. Tire impressions reflect the tread design and dimensional features of individual tires on a vehicle

8. Tread Nomenclature
Tire treads are composed of design elements separated by grooves
Grooves which run across a tire are called slots or transverse grooves
Tread wear indicators – raised areas which indicate tread wear

10. Contact Patch – portion of tire touching the ground

11. Stress Cycle – Under the load of a vehicle, rotating tire goes through stress

12. Tread Wear Indicator, or known as a wear bar, is a raised rubber bar 1/16 of an inch above base of grooves of tire

13. The Department of Transportation requires all tires contain a minimum of six tread wear indicators
As tread wears down on tire, these become noticeable and indicate tire needs replacement – these indicators can be retained in 3-D impressions

14. Noise treatment – tire industry created tire designs that vary the size (pitch) design elements of tire. This reduces or controls noise emitted by the tire.

15. Could two vehicles leave the same tire marks?
Write out your answer in the space provided
Explain why or why not. Be specific and provide examples!
It is difficult for two vehicles to leave the same tire mark. This is because as tires are used, the wear on the tires ends up changing the impression pattern of the tire. An example would be a unique wearing of the outer edge of a tire because the alignment of the vehicle is off. This unique pattern is of great importance to the investigation because this pattern will only be present in the tire of the vehicle that was used.

16. Tire Track Evidence Tire track evidence consists of: Tire track width
Wheelbase dimensions
Turning diameter
Relative positions of turning tracks

17. Track Width (Stance) – measurement made from one center of one wheel to opposite center of wheel

18. Wheelbase – measurement of center of hubs of front wheels to center of hubs of rear wheel
Normally these measurements are not present at crime scenes

19. Tire Positions in a Turn
When vehicle moves in straight path, only the rear tire tracks are available
When a vehicle turns, rear and front tires track separately
Important to understand and document at crime scenes

20. Front wheels turning produce unreliable measurements
Rear wheels accurately record turns

22. Recovery of Evidence Tire impressions must be:
Photographed from all angles with and without measurement tool (ruler)
Measurements – width, length and depth
Direction of travel noted
Impressions made if possible
Cast – a 3 foot impression requires pounds of dental stone
Pictures of the source (tire) must be included for comparison

23. Known Tires and Exemplars
Tire tread examinations compare the tire impressions recovered from scene with tires taken from a known vehicle
Two categories of tires:
Suspect (Vehicle(s) of suspects)
Elimination (Vehicles of police, ambulance, first responders, etc.)
All tires should be seized from the suspect vehicle(s) for comparison

24. Tire Impression, the Examination Process, and Conclusions
Tire impressions are resulting transfer of tread detail of a tire against a substrate
Impressions can be three or two dimensional
Forensic examination begins with visual comparison and elimination of tires which do not match impression

25. Tire Impression, the Examination Process, and Conclusions
Forensic examination continues with full circumference test impressions
Impressions are superimposed on known impression over cast or original tire
Other factors to be evaluated are tread design, tread dimension, noise treatment, wear features and random individual characteristics