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Forensic Tire Impression and Tire Track Evidence

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Forensic Tire Impression and Tire Track Evidence Ch 19 p 377-389 – PowerPoint PPT presentation

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Title: Forensic Tire Impression and Tire Track Evidence


1
Forensic Tire Impression and Tire Track Evidence
  • Ch 19 p 377-389

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

7
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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.

12
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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

16
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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

19
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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
  1. Tread Design Guide by Tire Guides, Inc. has
    provided photographs of most tire designs.
  2. Who Makes it and Where lists where tires are
    manufactured.
  3. Even of measurements made at a crime scene are
    accurate, in most cases the databases only
    provide a list of possible suspects

22
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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

25
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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

29
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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

31
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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
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