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Imaging and Photography > tyre mark impression evidence > Flashcards

tyre mark impression evidence Flashcards

1
Q

the basic anatomy of the tyre

A
  • groove- a void to allow the efficient channelling of water
  • bead- holds the tyre onto the wheel rim
  • sidewall- provides structural rigidity, deforms to absorb impact
  • dimple- increases surface area of the tyre allowing improved cooling
  • shoulder
  • sipe- improves grip in the wet on ice and snow
  • tread blocks- make up the majority of the tyre tread
  • rib- circumscribe the tyre, increases lateral grip
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2
Q

groove void ratio

A

Low void – Racing (slick) tyre <5% void
Standard – General purpose suitable for dry and wet conditions
High void – Off road tyre significantly >35% Void

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3
Q

tread wear indicators

A

Wear indicators

  • Raised areas within the tread
  • Indicate when tread is becoming worn near safe limits
  • Legal limit – 1.6mm across central ¾ of tyre

Many wear indicators ‘read’ at 4mm
Well before tyre performance is degraded significantly

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4
Q

how are tyres made

A
  • inner lining
  • bead
  • filler
  • ply material
  • tread
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5
Q

tyre construction- compounding

A

Compounding

  • The bringing together and mixing of tyre components
  • Exact nature of constituents affects tyre performance
  • Balance of grip vs. Longevity – Usage dependant
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6
Q

tyre construction- constituents

A
  • Rubber
  • Other material
  • Reinforcing material
  • Plasticisers
  • Vulcanisation chemicals
  • Antioxidants
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7
Q

tyre construction- compounding

A

-The compound is brought together and mixed using large rotors
-After thorough mixing the compound
is fed into an extruder where thick sheets
are formed
-These are powdered and stored for further processing

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8
Q

tyre construction- calendaring

A

Some of tyre constituents are formed directly from extruded material

  • Tread areas may a different compound to side walls
  • Most tyres will include reinforcing ply layers and these are formed by a process known as calendering
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9
Q

tyre construction- carcass assembly

A
  • Takes place around a tyre construction drum
  • Components brought together and joined
    Steel Beads
    Inner lining
    Inner play
    Steel Belting
    Outer ply
    Sidewalls
    Tread

Forms what is known as a ‘Green’ tyre

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10
Q

tyre construction- curing and vulcanisation

A

The green tyre – Un-vulcanised and unshaped
Lacks strength and rigidity
Lacks intended shape

Must be cured
Placed in a mold and heated to ~170⁰C
Two piece or multi-piece design

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11
Q

vulcanisation

A

rubber- Extremely sticky when warm , Brittle when cold

The addition of Sulphur or other vulcanising reagents will form cross links between strands

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12
Q

tyre design

A

Asymmetric – Increased lateral stability
In-Side/Out-side

Unidirectional – Superior grip and braking
V shape normally faces the direction of tyre travel

symmetrical

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13
Q

noise treatment

A

Use variable block sizes and patterns
This helps to minimise noise produced
May induce destructive interference

  • pitch lengths and designs vary with tyre size
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14
Q

tyre information

A

155/70 R13 75 T

155 – Section width (mm)
70 – Aspect ratio (%)

R – Radial tyre type
13 – Wheel rim diameter (“)
Sometimes in mm

75 – Load index (386kg)
T – Speed Rating (190km/h)

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15
Q

tyre information

A

Section Width
Higher performance cars tend to make use of wider tyres to increase contact area

Aspect ratio
Height of the sidewall/Section width
Measured unloaded at maximum recommended pressure

Load index
Maximum load per tyre

Speed rating
Maximum rated sustained speed

DOT number- department of transportation

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16
Q

tyres of impression

A

3D impression
Impression in soft ground – Sand, Soil, snow etc.
Most prolific

Positive impressions – Less common
Fine detail may be obscured by road surfaces

Negative Impression – Uncommon

Other impressions
Tyre marks on the body of victim

17
Q

treatments of impressions

A

3D impressions
- casted using dental stone

wet origin impressions- blood oil water
- always consider the medium, powders (aluminium, bristol black)
- alternative light source
-chemical treatment
(blood= KM, LMG, luminal/blue star, protein stain) (mud= thiocyanates- iron reacts to form red/brown colour) (physical developer- paper or cardboard media)

dry impressions

  • powders
  • alternative light source
  • chemical treatment
  • lifting could be viable (ESLA)
18
Q

intelligence- multiple impressions

A

Can indicate the size of the vehicle…
Track width is the distance between the centre point of one tyre contact patch to that of the opposing tyre

Track width can eliminate certain vehicle types

Front and rear marks
Can indicate length of wheelbase
Turning circle

19
Q

examination

A

The mark will go to a mark expert
As strange as it may seem most footwear marks experts also work with tyres

Begin with a possible make
Many brands use distinctive or generic pattern types

There are a number of tyre databases

20
Q

trademark databases

A

An extremely valuable tool for the examiner
Even the best examiner requires a reference
May give:
-A definitive identification
-A list of possibles
-Examples of similar patterns
Frequently updated

Extremely valuable tool
33 million registered cars in the UK!

Over 1 billion tyres are manufactured every year Worldwide

There are more than 8000 different tyres models currently available!

21
Q

tyre circumference

A

Feature to feature length
A specific feature is repeated along the tyre mark at intervals of ~1760mm

This tells us that tyre circumference is ~1760mm
From this we can work out tyre diameter

Diameter = Circumference/ π (3.14)

Diameter =1766mm/ π = ~560mm

22
Q

tyre size- what if we don’t have a full tyre rotation

A

We can use the size of tyre block to ascertain tyre size

  • Noise treatment
  • Contact manufacturer

Same tyre design…

  • Often larger tyres have stretched block designs
  • Tyre tread blocks will be longer on tyres with larger circumferences
  • Tread blocks can be measured and compared to a database or manufacturer information or an original tyre

Wear indicator spacing

23
Q

reference tyre mark impressions

A

Tyre should be on the vehicle
The weight of the vehicle should cause the same deformations seen in the scene mark

Car driven/rolled across a section of inked paper and onto a section of thick paper

Thin layer of Vaseline applied to tyre and driven or pushed over thick paper
Bristol Black powder used to develop tyre mark image

24
Q

tyremark comparison

A

Class characteristics
Repeatable characteristics shared by all tyres of the same type

Is there a gross concordance between the reference and crime scene marks?
Shape
Size
Gross pattern

A single point of difference is sufficient for an elimination

25
Q

reference comparison

A

Correlation
There are a number of methods
-Imaging techniques
-Physical overlay –

Align features and pitch block elements
-Tread wear indicators or damaged areas where present may help

Does the variable sequence correlate?

  • It should do if the tyres are the same model and size!
  • Minor variations may occur due to differences in vehicle loading/surface
26
Q

wear characteristics

A 
Influenced by 
Driving style
Load
Vehicle model
Tyre pressures
Tracking
Toe in/Toe out alignment
Mechanical integrity

Wear patterns do not constitute positive ID
Two tyres can be shown to produce identical wear

27
Q

sub class characteristics

A

Sub-class mold characteristics

  • Features resulting from the variability of individual molds
  • Molds are either handmade or laser cut
  • Handmade molds are unique

Molds may become damaged

  • Damage may exhibit itself on a number of tyres from that mold
  • Eventually picked up by QC process
  • Number of released tyres with this feature may vary
  • Not unique but discriminating
28
Q

identifying characteristics- accidentals

A

Identifying characteristics

Damage events caused by random processes

  • Cuts/abrasions
  • Tears or perforations
  • Nail or screw head damage/puncture – Common
  • Trapped material – Stones which may persist

Imaging and Photography (20 decks)

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