Forensic Tire Tread and Tire Track Evidence

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.

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.

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

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.

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

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

Contact Patch – portion of tire touching the ground

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

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

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

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.

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.

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

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

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 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

Front wheels turning produce unreliable measurements Rear wheels accurately record turns

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 15-25 pounds of dental stone 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(s) of suspects) Elimination (Vehicles of police, ambulance, first responders, etc.) All tires should be seized from the suspect vehicle(s) 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