Lecture 19Tire Print Evidence Evidence Often Overlooked
Tread Design http://www.jacks-tire.com/Tire101.aspx
Large Small Medium Small Medium
Direction of Travel • Closely examine tire tracks • Using known factors • Common sense
Direction of Travel Spinning tires Striations by sidewall in furrow Shows which way tire rolling Location where vehicle stopped, backed up to change direction Overlapping front & rear tire tracks Look to see if coming or going from scene Grass or small plants Direction they were flattened Directional tire tread patterns Damp soil or snow Tires lifted soil or snow slightly in direction of travel Deposition of transferred material in direction of travel Mud, dirty water or fluids splashed or thrown in direction of travel
Make the Appropriate Tire Track Measurements Wheelbase Front track width Rear track width Turning diameter Tread design width (arc width) Tread depth (skid depth) Tire circumference
Wheelbase Defined as the distance between the leading edge of the front and rear tires. Front Tire Track Width Approximate Wheelbase Rear Tire Track Width Front Tires Turned
WheelbaseFront Wheels Turned Inside leading edge of track will be rounded Make rt. Angles to center line of impression Will run laterally across the width of impression Project line along inside edge – parallel to center line Intersection is fixed point on inside leading edge Repeat of all 4 tires
Camber http://en.wikipedia.org/wiki/Camber_angle http://autorepair.about.com/od/glossary/ss/def_camber.htm
Camber • Tilt” of the tires as they • rest on the ground. • Mechanical issues with vehicle affect how impressions appear at scene • Differences between normal and abnormal camber (positive or negative) signifies • improper alignment or worn front-end parts of the vehicle. • Positive camber: tires tilted further apart, “out,” at the top. • Negative camber tires tilted closer, “in,” at the top, Normal Camber Narrower Positive Camber Tire tilted “out” @ top Wider Negative Camber Tire tilted “in” @ top
Front and Rear Track Widths • Defined as distance between middle of leading edge of the front and back tires. • Mechanical issues can affect how these impressions appear at the scene. • Among others, one is camber, which is the “tilt” of the tires as they rest on the ground.
Toe Difference between front & rear of front tires Normally set “in” only a few millimeters Compensate for normal front end tendency to toe “out” at highway speeds. Improper alignment Worn front end components Track Width Toe In or Out
Turning diameter • Diameter is defined as the diameter of the circle made when the vehicle is driven in a circle. • Determined from measurements taken @ scene.
Turning Diameter At the scene Don’t know if the turn is full-lock Curb-to-curb for the outer tires Not wall-to-wall for outside edge of vehicle Procedure Select segment reflecting sharpest portion of turn Measure imaginary line between 2 points of arc Measure outer margin to outer margin A-AA Bisect line & draw line to outer margin of track arc – (A-C) Draw another line between the bisect point and the inside of the track arc – (C-B) Calculate diameter
Tire circumference • Approximate the tire circumference by finding an accidental, repeated characteristic, such as a gouge or cut, along the imprint. • The distance between these repeated marks is the rolling tire circumference. • Investigators must understand that measured value – accidental characteristic-to-accidental characteristic – different from value obtained by wrapping the measuring tape around the tire in its center line, • Larger • Procedure • Measure distance between repeated accidental characteristics on impression • Considerations • Measured value is less than when the tape is wrapped around the tire • Tires have a curved arc width, impressions do not. • Measured circumference is larger in the center line of the tire than at the outer edges.
Tread design width (arc width) • Tread design width: • Measurement from one edge of the design to the other. • These measurements must be measured at the scene from the impression. • Information important so manufacturers can help investigators identify an unknown impression.
Non-dirt Impressions • 2D impressions visible because of contaminants adhering to the tread; dirt and dust the most common. • Other contaminants also create impressions • water, grease (oil) or blood. • Each leaves visible impression • Each can be enhanced, depending on the contaminant. • Consideration of Enhancement Choices • Variety of choices and investigators must make the proper decision. • Archiving the impression photographically is step-one • Enhancement choices and the variables offered by the scene: • Surface material, • Chemistry of the impression material (dirt, oil, blood, etc), and the • Chemistry of the enhancement method.