Tyre mark impression evidence-Will Flashcards
what is the bead?
holds the tyre onto the wheel rim
reinforced with a steel band encased within the sidewall
what is the sidewall?
provides structural rigidity, deforms to absorb impact
standard tyres=will collapse when punctured
run-flat tyres=designed to retain rigidity when air pressure lost and allow continued driving
what are tread blocks?
make up majority of the tyre tread
used to improve traction and allow efficient efflux of water
designed to minimise noise and dissipate heat
what is the sipe?
improves grip in the wet on ice and snow
increases the number of edges presented
allows more flexibility in the tyre blocks which although not energy efficient does improve heat dissipation
what are dimples?
increase surface area of the tyre allowing improved cooling
what are ribs?
circumscribe the tyre
increases lateral grip allowing more stable cornering
what are grooves?
a void to allow the efficient channelling of water to the rear of the tyre
groove void ratio
the ratio of open space to rubber in the tread
low void=racing tyre
standard=general purpose suitable for dry and wet conditions
high void=off road tyre
tread wear indicators
raised area within the tread
what is the legal tread wear?
1.6mm across central 3/4 of tyre
compounding - tyre construction
bringing together and mixing of tyre components - rubber and other additive materials
brought together and mixed using large rotors
shearing action generates considerable heat which can cause premature chemical reaction of components
whole machine must be cooled
after mixing the compound is fed into an extruder where thick sheets are formed
these are powered and stored for further processing
tyre rubber
tyre rubber is a mixture of synthetic and natural elastomers, carbon and sulphur
the synthetic rubber can be varied allowing the designer to produce softer or harder compounds
constituents of tyre
rubber carbon reinforcing material plasticisers vulcanisation chemicals antioxidants
calendering - tyre construction
some of the tyre constituents are formed directly from extruded material
tread areas may be different compound to side walls
most tyres will include reinforcing ply layers and these are formed by a process known as calendering
rubber compound is forced around fabric or steel to form ply material
carcass assembly - tyre construction
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
curing - tyre construction
green tyre places in a mold and heated to 170 degrees
a bladder within the mold is inflated and subjects the tyre to 25 ATM
this forces the tyre to adopt the shape of the mold, become fully airtight and most importantly causes a chemical change in the rubber
Vulcanisation - tyre construction
the addition of sulphur or other vulcanising reagents will form cross links between strands
by anchoring the strands to each other the properties are radically altered and the mass becomes more elastic and harder
altering the amount of vulcanising agent and length of vulcanisation will affect hardness and elasticity
what are the 3 tyre designs?
astmmetric=increased lateral stability
unidirectional=superior grip and braking
symmetrical
noise treatment
use variable block sizes and patterns
may induce destructive interference
can be designed by computer to test relative noise of multiple pattern types
most common = variable pitch lengths
what does 155/70 R13 75 T stand for?
155=section width 70=aspect ratio R= radial tyre type 13=wheel rim diameter 75=load index T=speed rating
aspect ratio
height of the sidewall/section width
measured unloaded at maximum recommended pressure
standard = 70
what does DOT stand for?
Department of Transportation
3D impression
impression in soft ground
most prolific
cast using dental stone
wet origin impressions
powders = aluminium, bristol black alternative light source chemical treatment -blood = KM, LMG, Luminol -mud=thiocyanates -physical developer=paper or cardboard media
dry impressions
powders=aluminium, bristol black alternative light source chemical treatment -bromophenol blue may work well with alkaline marks lifting could be viable
3 examples of environmental deformation
weather
other vehicles
human/animal activity
track width
distance between the centre point of one tyre contact patch to that of the opposing tyre
4 examples of treadmark databases
treadmate
tire global information
tireprint
tire guides
how to work out diameter?
circumference/Pi
reference marks
tyre should be on vehicle
weight of vehicle should cause same deformations seen un the scene mark
car drive/rolled across 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
class characteristics
repeatable characteristics shared by all tyes of the same type
reference comparison
correlation
imaging techniques - overlay software
physical overlay - transparency materials
sub class characteristics
features resulting from the variability of individual molds
not unique but discriminating
identifying characteristics
damage events caused by random process
