2. INTRODUCTION
A tyre tread is the part of an automobile tyre that is in contact with the
road. Tyre impressions are longer than footwear impressions but both the
tyre impressions and footwear impressions may be two dimensional or
three dimentional.
Tyre marks and tyre have acquired great importance in criminal
investigation and their evaluation help to:-
Identify the vehicles involved in cases of hit-and-run.
Indicate mode of arrival of the suspect and his departure from the crime.
3. Solve cases involving kidnapping.
Find out the hideouts of the criminals by trailing.
Many times criminals after committing a theft take material to another
place, Criminals, after killing a person, may take his body to a pond.
In such cases, help to investigation is rendered by tyres & tyre
impressions.
Skid marks created by tyres help in finding the speed of the vehicle at
the time of applying brakes in cases involving accidents or in other
crimes.
4. Examination of Tyre marks & Tyres
• Tyre tread impressions must be photographed at the crime scene
whether or not a casting will be made.
• In the case of three dimensional tyre impressions casting must be
made at the scene because the object containing the impression cannot
be taken to the laboratory for casting or further analysis.
5. • Some times there are difficulties with making a good cast and one has
to rely on photographs only.
In many cases the tyre impression may extend to several feet resulting
in very heavy bulky casts made with plaster of paris.
• Comparison be made by photographing the Tyre impression and
using its negative with the cast made. Thus negative is being
compared with a negative since likes only can be subjected to
comparison
6. IDENTIFICATION OF TYRE MARKS
• Width of tyre
The tyre width is the distance
between the two edges of the tyre.
The size varies for scooters, cars
and buses etc.
Circumference of tyre
It is an important parameter for
the determination of tyre size to
type the vehicle.
7. Tread pattern
Different vehicles may have tyres with different patterns
.They differ from one to another and thus help to identify the
make of the tyre.
8. Wear &Tear of Tyres
• Important evidence for the identification of
tyre mark consists of wear & tear marks on it.
• No two tyres even when fitted to the same
vehicle would have similar wear and tear
marks to the same extent.
9. • The wear & tear marks are different because the nature of surface and
contact angle are different, apart from the difference in pressure when
tyres come into contact with various surface.
• Wear patterns of tyres get affected differently by peculiar driving
habits, over under inflation, misalignment and distribution of load
inthe vehicle as well as suspension system. This results in difference
in wear & tear marks and adds to individuality of the tyres with
respect to these marks.
• If clear marks are available in the wear pattern, identification of
suspect vehicle becomes easy.
10. • Embedding of Foreign material. Many times small pieces of stones metal
pieces or glass pieces get embedded in a tyre giving it further individuality.
These individualising features help in the identification of vehicles through
their tyres having individualistic marks in their patterns.
• Arc turning by vehicles. A small turning arc indicates a small vehicle where
as a large vehicle will turn covering a bigger distance and covering a longer
arc having a greater diameter/ radius
11. • Embedding of Foreign material. Many times small pieces of stones
metal pieces or glass pieces get embedded in a tyre giving it further
individuality. These individualising features help in the identification
of vehicles through their tyres having individualistic marks in their
patterns.
12. SKID MARKS
• If brakes are applied to a vehicle they (brakes) lock the wheels
resulting in stoppage of revolving of wheels.
• When a certain vehicle traverse some distance with locked wheels, it
is said to skid.
13. • The marks created by locked wheel (without revolving) are known as
skid marks.
• The length of the skid marks depends on the velocity of the vehicle
at the time of applying breaks.
14. • The amount of friction between the surface of road
and the tyres would abrade the tyres and black tyre
material gets deposited at the surface over which
the vehicle moved creating skid marks.
• These marks are normally visible & conspicuous.
• Such marks created on tarry roads and dusty roads
are the result of pushing away the tar from the path
or displacement of earth from the track.
• These skid marks have great evidentiary value.
15. • The marks are clear to start with, in view of the fact
that wheels do not get fully locked at that moment
of starting.
• These skid marks are helpful in calculating velocity
of the vehicle at the time of applying brakes.
• At times, absence of skid marks is observed in cases
of serious accidents. This is an indication that
brakes have not been applied or could not be
applied due to brake failure . The other alternatives
are that vehicle has been used for making an
accident or killing someone. Yet another other
possibility that driver was dozing is not ruled out.
16. Length of Skid Marks
The length of skid marks depends on sveral factors
mentioned below:
• The weight of the vehicle.
• The speed of the vehicle. Higher speeds yields
longer skid marks
• Nature of road:- Wet and greasy surfaces reduce
friction resulting in longer skid marks where as
rougher surfaces will yield shorter skid marks.
Foreign material on the road like water, oil, and mud
will result in longer skid marks
• Conditions of the tyres, over & under inflation, state
of wear & tear affect the skid length
17. • Efficiency of the brakes and mechanical efficiency
also affect length of the skid.
• Temperature and weather
• Reaction time:- Time taken by the driver for
applying brakes fully after he perceive the necessity
for applying brakes is called reaction time which
varies from 0.5 to 0.8 second in a normal case
18. Measuring Skid Marks
• If 4 marks are found, they should be
measured individually.
• To get the average skid distance,
add the four measurements
together and divide by four.
19. Skid Marks Continued
• If three skid marks are found, add the
three together and divide by three.
• The same applies to two marks.
• If only one mark is found, measure the
entire length and use this as the skid
distance.
20. Drag Factor
• Tires slide on some road surfaces easier
than others.
• Can you think of road conditions under
which tires would slide easily?
What type of road surface would be the
hardest for tires to slide on?
21. Examples of Drag Factor
• Asphalt: 0.50 to 0.90
• Gravel: 0.40 to 0.80
• Ice: 0.10 to 0.25
• Snow: 0.10 to 0.55
22. MEASUREMENT
• You now have the three variables required to
complete the minimum skid speed formula.
Dfn
S 30
23. In this formula:
S = speed in miles per hour
30 = a constant value
D = skid distance in feet
f = drag factor for the road surface
n = braking efficiency in decimal form
Dfn
S 30
