tyre mark impression evidence Flashcards
the basic anatomy of the tyre
- 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
groove void ratio
Low void – Racing (slick) tyre <5% void
Standard – General purpose suitable for dry and wet conditions
High void – Off road tyre significantly >35% Void
tread wear indicators
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
how are tyres made
- inner lining
- bead
- filler
- ply material
- tread
tyre construction- compounding
Compounding
- The bringing together and mixing of tyre components
- Exact nature of constituents affects tyre performance
- Balance of grip vs. Longevity – Usage dependant
tyre construction- constituents
- Rubber
- Other material
- Reinforcing material
- Plasticisers
- Vulcanisation chemicals
- Antioxidants
tyre construction- compounding
-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
tyre construction- calendaring
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
tyre construction- carcass assembly
- 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
tyre construction- curing and vulcanisation
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
vulcanisation
rubber- Extremely sticky when warm , Brittle when cold
The addition of Sulphur or other vulcanising reagents will form cross links between strands
tyre design
Asymmetric – Increased lateral stability
In-Side/Out-side
Unidirectional – Superior grip and braking
V shape normally faces the direction of tyre travel
symmetrical
noise treatment
Use variable block sizes and patterns
This helps to minimise noise produced
May induce destructive interference
- pitch lengths and designs vary with tyre size
tyre information
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)
tyre information
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
tyres of impression
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
treatments of impressions
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)
intelligence- multiple impressions
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
examination
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
trademark databases
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!
tyre circumference
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
tyre size- what if we don’t have a full tyre rotation
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
reference tyre mark impressions
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
tyremark comparison
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
reference comparison
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
wear characteristics
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
sub class characteristics
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
identifying characteristics- accidentals
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
