20 Forensic Science Powerpoint Chapter 20 Forensic Tire Impress

Forensic Science An Introduction to Scientific and Investigative Techniques Stuart H. James and Jon J. Nordby Chapter 20 CRC Press: Forensic Science, James and Nordby, 3rd Edition

Chapter 20

Forensic Tire Tread and Tire Track Evidence

Chapter Author: William J. Bodziak

Presentation created by Greg Galardi, Peru, Nebraska

Edited by Stuart H. James, Fort Lauderdale, Florida and

Dan Mabel, Richmond, Virginia

Chapter 20 CRC Press: Forensic Science, James and Nordby, 3rd Edition

Forensic Tire Tread and Tire Track Evidence

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

Tire Tracks are relational dimensions between two or more tires: tracks can be used to determine the wheelbase of the vehicle

Original Equipment Tires, Replacement Tires and Tire Construction

Original Equipment Tires - tires sold on new equipment

Replacement tires – those used to replace worn or damaged tires

Most tires today are of radial ply construction

Tires are composed of liner, sidewall rubber, beads, belts and tread rubber

Tread Nomenclature and Sidewall Information

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

Department of Transportation number, known as DOT number – consists of:

DOT AC L9 ABCD 4901

DOT impression

Plant code and manufacturer

Date of manufacture

Tires made after January 1, 2000 have four numbers in last group, while prior to that there are three numbers

Retread tires – have slightly different numbers

DOTR YPY 1201

DOTR – Department of Transportation retread

YPY – Manufacturer retread

1201 – date and year of manufacture

Common tire size designation includes:

P 195 / 65 R 15

P – passenger tire

195 – approximate width in millimeters

65- aspect ratio

R – radial tire

15 – rim diameter in inches

Noise Treatment

Stress Cycle - Under load of a vehicle, rotating tire goes through stress

Contact Patch – portion of tire touching ground

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

Noise Treatment

Mixture of three design elements, small (S), medium (M) and large (L) allow manufacturers to change size of tires and minimize noise

Noise treatment is different on opposite sides of tire

Tread Wear Indicators

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

DOT 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

Retread Tires

Retread tires are primarily used for commercial or fleet vehicles in United States

Two types of retread processes:

Mold Cured Process

Pre-Cured Process

Some retread tires have valuable individual characteristics which provide information to the examiner

Tire Track Evidence

Tire track evidence consists of :

Tire track width

Wheelbase dimensions

Turning diameter

Relative positions of turning tracks

Tire Track Evidence

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

Front wheels turning produce unreliable measurements

Rear wheels accurately record turns

Tire Track Evidence

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

Tire Track Evidence

Turning Diameter – Diameter of circle a vehicle makes when its steering wheel is fully turned – pertains to front wheels only

Turning diameter is measured by (B squared/A) + A, where:

B = distance between two points ( x and x’) on turn circle

A = distance between outer margin and a mid point between x and x’

Tire Positions in a Turn

When vehicle moves in straight path, only rear tire tracks are available

When a vehicle turns, rear and front tires rack separately

Important to understand and document at crime scenes

Recovery of Evidence

Tire impressions must be:

Photographed from all angles with and without measurement tool (ruler)

Cast – a 3 foot impression requires 15-25 pounds of dental stone

Measured – width, length and depth

Direction of travel noted

Impressions made if possible, and

Pictures of the source (tire) must be included for comparison

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 of suspect

Elimination - Vehicles of police, ambulance, etc

All tires should be seized from suspect vehicle for comparison

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

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

20 Forensic Science Powerpoint Chapter 20 Forensic Tire Impress

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