1. Forensic Tire Impression and Tire Track Evidence
Ch 19 p

2. Terminology
Original Equipment (OE) tires – the original tire installed on a vehicle
Green tire – an unfinished tire that has not been molded
Design elements – raised rubber design; components that form the overall design
Tread wear indicator – raised rubber bar 1/16 inch above the base of the tire

3. DOT number – a number that appears on every tire
Retread tires – tire carcass to which new tread rubber is added to produce a new tire
Noise treatment – arrangement of design elements used to reduce tire noise

4. I. Introduction
It is more likely than not that a vehicle is used in the commission of a crime or in transporting a criminal to and from the crime scene.
Most surfaces retain tire impression and track information
Evidence should be collected to prove a suspect vehicle was at the crime scene

5. 2. Tire Impressions reflect the tread design and dimensional features of individual tires on a vehicle.
Can be compared directly with the tread design and dimension of the tires from a suspect vehicle

6. Provide information regarding the relative positioning of the four tires
Can be used to profile the size and type of vehicle
3. Tire Tracks are relative dimensions between two or more tires of a vehicle.
tracks reflect general characteristics about the vehicle
can be used to determine the track width, wheelbase, and turning diameter of the vehicle

8. II. Original Equipment Tires, Replacement Tires, and Tire Construction
Tires sold as equipment on new vehicles are known as original equipment (OE) tires.
Same size and brand are used high volumes of the same make and models cars
Replacement tires are those that are purchased to replace worn or damaged tires.
Not the same design as the OE tires
Choice of design is made by the owner

9. 3. Most passenger tires today are radial-ply tires.
some bias tires are still made and can be found on older vehicles
Tires are made from various compounds of unvulcanized rubber, steel and fabric
Tread and sidewall patterns are molded into a green tire

10. III. Tread Nomenclature and Sidewall Information
Tire treads are composed of design elements.
Can be arranged in ribs or patterns
2. Designs are separated by grooves .
Small grooves are called sipes
3. Tread wear indicators or wear bars can be seen in the grooves patterns

11. 4. In addition to the tread design, much information is molded into the sidewalls of the tire.
Outer side is called the label side, faces outward
Inner side is called the serial side, not visible
5. Tire brand and size is usually on both sides. The serial side contains DOT and mold numbers.

13. Tire Measurements: P195/65R15

14. IV. Noise Treatment
Under the load of a vehicle, a rotating tire goes through a stress cycle.
The portion touching the ground is the contact patch.
The tread contracts where it contacts the ground
As the tire rotates, the tread that was contacted will expand
Tire designs vibrate and produce noise

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

17. 4. Forensic Tire examiners must understand the concept of noise treatment and include it in the evaluation of the dimensional aspects of a tire impression as compared to a tire.
Exact location on a suspected tire
Different on opposite sides of the tire, can help when direction is in question

18. V. Tread Wear Indicators
1. Tread Wear Indicator, or wear bar, is a raised rubber bar 1/16 of an inch above base of grooves of tire
DOT requires all tires contain a minimum of six tread wear indicators
As tread wears these become noticeable and indicate tire needs replacement
can be retained in 3-D impressions

20. VI. Retread Tires
1. Retread tires are primarily used for commercial or fleet vehicles in United States
Some retread tires have valuable individual characteristics which provide information to the examiner

21. VII. Tire Reference Databases
Tread Design Guide by Tire Guides, Inc. has provided photographs of most tire designs.
Who Makes it and Where lists where tires are manufactured.
Even of measurements made at a crime scene are accurate, in most cases the databases only provide a list of possible suspects

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

24. 1. Track Width (Stance)
The measurement made from the center of one wheel or impression to the opposite wheel or impression
Front stance is normally different from the rear width
When a vehicle is traveling forward, the rear tires will track over the tracks left by the front tires

26. If a vehicle is turning, the track width between the front wheel will become narrower and cease to be a reliable measurement
The rear tire stance will stay the same
Databases can produce lists of vehicles that have the same stance dimensions

27. 2. Wheelbase
measurement of center of hubs of front wheels to center of hubs of rear wheel
normally not present at crime scenes, unless the tracks show evidence of being parked

28. 3. Turning Diameter
Diameter of circle a vehicle makes when its steering wheel is fully turned
pertains to front wheels only
Smaller cars have a smaller turning diameter
Formula for calculating turning diameter:
Turing Diameter = (B²/A) + A,
B = distance between two points ( x and x’) on turn circle
A = distance between outer margin and a mid point between x and x’

30. 4. Tire Positions in a Turn
When a vehicle moves in straight path, the rear tires run directly over the front tires
So there are only two sets of tracks to recover only rear tire tracks
When a vehicle
turns, rear and front
tires track separately
Rear are inside the
front

32. IX. Recovery of Tire Track Evidence
The procedure for recovery and processing of tire impression and/or track evidence includes:
Photographs and Documentation
Casting
Comparison to known standard or source tire

33. a. Photographs and documentation
First general crime scene photos are taken from various angles
Diagrams and written notes describe the number of tracks, track width, relative positions, surrounding area, and direction of travel

34. Finally, examination photographs are made for identification of tread design.
Longer impressions should include overlap

35. b. Casting Cats offer the best physical evidence for later comparison
Shows 3-D features, contours, uneven qualities
To allow examination of the noise treatment of a tire, a long cast must be recovered
Any impression 4ft of less is always cast
Dual tire assembly is always cast as a single unit

36. c. Comparison to a Known Standard
Tire tread examinations compare the tire impressions recovered from scene with tires taken from a known vehicle
Two categories of tires:
Suspect - Vehicle of suspect
Elimination - Vehicles of police, ambulance, etc

37. All tires should be seized from suspect vehicle for comparison
Positioning is noted and marked before tires are removed
Full circumference test impressions are made
Dual mounted tires are not dismounted

38. X. Tire Impressions, 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
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