Forensic Tire Impression and Tire Track Evidence

Ch 19 p 377-389

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

• 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

I. Introduction1. 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

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

– 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

II. Original Equipment Tires, Replacement Tires, and Tire Construction

1. 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

2. 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

3. Most passenger tires today are radial-ply tires.– some bias tires are still mad

e and can be found on older vehicles

4. Tires are made from various compounds of unvulcanized rubber, steel and fabric– Tread and sidewall patterns

are molded into a green tire

III. Tread Nomenclature and Sidewall Information

1. 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

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.

Tire Measurements: P195/65R15

IV. Noise Treatment

1. Under the load of a vehicle, a rotating tire goes through a stress cycle.

2. 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

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

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

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

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

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

VIII. Tire Track Evidence

1. Tire track evidence consists of:– Tire track width–Wheelbase

dimensions– Turning diameter–Relative positions of

turning tracks

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

• 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

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

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’

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

IX. Recovery of Tire Track Evidence

1. The procedure for recovery and processing of tire impression and/or track evidence includes:– Photographs and DocumentationPhotographs and Documentation– CastingCasting– Comparison to known standard or source tireComparison to known standard or source tire

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

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

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

c. Comparison to a Known Standard

• Tire tread examinations Tire tread examinations compare the tire compare the tire impressions recovered impressions recovered from scene with tires from scene with tires taken from a known taken from a known vehiclevehicle

• Two categories of tires:Two categories of tires:– Suspect - Vehicle of Suspect - Vehicle of

suspectsuspect– Elimination - Vehicles of Elimination - Vehicles of

police, ambulance, etcpolice, ambulance, etc

• All tires should be seized All tires should be seized from suspect vehicle for from suspect vehicle for comparisoncomparison– Positioning is noted and Positioning is noted and

marked before tires are marked before tires are removedremoved

– Full circumference test Full circumference test impressions are madeimpressions are made

– Dual mounted tires are not Dual mounted tires are not dismounteddismounted

X. Tire Impressions, the Examination Process, and Conclusions

• Tire impressions are resulting transfer of tread detail of a tire Tire impressions are resulting transfer of tread detail of a tire against a substrateagainst a substrate

• Impressions can be three or two dimensionalImpressions can be three or two dimensional• Forensic examination begins with visual comparison and Forensic examination begins with visual comparison and

elimination of tires which do not match impressionelimination of tires which do not match impression• Forensic examination continues with full circumference test Forensic examination continues with full circumference test

impressionsimpressions• Impressions are superimposed on known impression over cast Impressions are superimposed on known impression over cast

or original tireor original tire• Other factors to be evaluated are tread design, tread dimension, Other factors to be evaluated are tread design, tread dimension,

noise treatment, wear features and random individual noise treatment, wear features and random individual characteristicscharacteristics