Air Pressure and Cargo Weight Affect the Width of Tire Impressions

Technical Note
Air Pressure and Cargo Weight Affect the
Width of Tire Impressions
Jan LeMay 1
Thomas W. Adair 2
Angela Fisher 2
Jennifer James 1
Brittany Boltman 3
Abstract: It is commonly understood that varying air pressure in a
pneumatic tire can affect the size of the contact patch (the area of tire
in contact with the road) [1]. A tire with low air pressure will have a
longer contact patch than one with higher pressure. In this study, the
authors made test impressions of tires at various air pressures and with
various weights of cargo in the vehicle to determine whether the width
of a tire impression will change based on those variables. The results
of these experiments supported the hypothesis that the contact patch
width will vary with changes in tire pressure and cargo weight.
Introduction
Tire impressions documented at crime scenes are commonly
compared to known tires. Characteristics that are compared are
physical size, tread design, noise treatment, wear, and individualizing characteristics. The authors hypothesized that varying
air pressure in pneumatic tires can affect the overall width of
impressions made by those tires. The authors also hypothesized
that varying weights of cargo in a vehicle may affect the overall
width of impressions made by tires. In this study, tires were
Weld County Colorado Sheriff ’s Off ice, Greeley, CO
Westminster Police Department, Westminster, CO
3
Fort Collins Police Department, Fort Collins, CO
1
2
Received Febr uary 14, 2008; accepted May 20, 2008
Journal of Forensic Identification
660 / 58 (6), 2008
tested at var ying air pressures, and under various loads, to
measure any degree of change in the width of impressions made
by the tires. By creating these conditions, the authors hoped to
answer the question, Can one tire under these various conditions
create impressions that vary in width?
It has been documented that changes in tire inf lation will
change the size of the tire footprint and thus will also change the
rolling circumference of the tire [2]. The authors could find very
little in the literature to support this with documented studies.
Chavigney [3] brief ly notes that “the surface of the tires varies
in width according to the inf lation of the tire, the weight of the
car and its load”. However, Chavigney does not go on to discuss
any experiments supporting this position. The authors are not
aware of any other studies describing the affects of changes in
air pressure and vehicle cargo on the width of tire impressions.
Common changes in tire pressure occur as a result of such
conditions as changes in ambient air temperature and air leaks.
Tire air pressure decreases by about one pound per square inch
for every 10-degree drop in outside air temperature [4]. If the
tire pressure is tested when the temperature is 80 degrees outside
and retested when the temperature is minus 20, the same tire
will be underinf lated by 10 psi. A vehicle that is in a poor state
of maintenance may have very different air pressure in all four
tires. Other factors that may inf luence air pressure include the
specialized use of vehicles (e.g., towing, racing, off-road travel,
and hauling heavy loads). Tire construction may also inf luence
the effect of the air pressure. Analysts should also consider any
modifications that may have been made to the vehicle by the
owner for such activities as off-road driving and amateur racing
that may also affect the load on the tire.
Method
To examine the width of impressions made by tires with
varying air pressure, tires were tested under load at pressures
of 8 psi, 16 psi, 24 psi, 32 psi, and 40 psi. At each air pressure
setting, a two-dimensional test impression of the tire was made
by rolling the tire onto a tire test impression medium. Width
measurements in metric units were taken of each test impression
and compared to the measurements of the same tire at various
air pressures.
Journal of Forensic Identification
58 (6), 2008 \ 661
To examine the width of impressions made by tires with
varying weights of cargo, tires were set to the tire manufacturer’s
recommended air pressure setting of 32 psi. A test impression
of a rear tire with no cargo in the vehicle was taken. Then 250
pounds of weight was added to the rear cargo area and another
test impression was made. An additional 250 pounds of cargo,
making the total cargo 500 pounds, was evenly distributed in
the rear cargo area, and another test impression of the tire was
made. Weights were obtained from department exercise rooms
to ensure proper loads were maintained. Width measurements in
metric units were taken of each test impression and compared to
the measurements of the same tire at various cargo loads.
Results and Discussion
Thirteen tires of different makes, models, and sizes were
tested using the above described methods. All tires were used,
but were in good condition, with little wear. Results were as
expected: tires with lower air pressure generally made impressions that were wider than the impressions made with tires
having higher air pressure (Table 1). An average was calculated
from the 13 tires tested and was found to be 1.83 cm (about 3/4 of
an inch) difference in impression width from 8 psi to 40 psi. The
tire that showed the greatest difference from 8 psi to 40 psi was
a “General Grabber” tire, with a difference of 4.4 cm (Figure
1). The tire that showed the least difference from 8 psi to 40 psi
was a “Goodyear Wrangler”, which showed no difference in tire
impression width at any of the air pressure settings. This may
be due to stronger side wall construction.
Tires under greater load generally made impressions that were
wider than impressions made by tires under no additional load
(Table 2). An average was calculated from the 13 tires tested and
was found to be 0.83 cm (about 3/8 of an inch) difference from
0 pounds of cargo to 500 pounds of cargo. The tire that showed
the greatest difference from 0 pounds of cargo to 500 pounds
of cargo was the “Goodyear Regatta” tire, with a difference of
1.6 cm. The tire that showed the least difference from 0 pounds
of cargo to 500 pounds of cargo was the “Firestone Destination
LE” tire, with a difference of 0.1 cm.
In the study, two tires of the same brand and model but different sizes were also tested. They were a Goodyear Regatta, sizes
P206/70R15 and P215/60R16. These two tires had very similar
results and further reinforced the hypothesis.
Journal of Forensic Identification
662 / 58 (6), 2008
Conclusion
Tires under various cargo loads and with varying air pressure
can make impressions of varying width. When comparing test
impressions to crime scene impressions, tire impression examiners should not eliminate a tire based simply on the impression
width when class characteristics such as t read design and
noise treatment correspond. In fact, examiners should expect a
difference in the width of the contact patch of the impression,
especially when there is significant time between the crime and
the recovery of the tire. In most cases, the examiner will have
no indication of the air pressure setting or cargo load in a tire
when it made a crime scene impression. Obviously, there may
be times when an examiner might suspect that a difference in
contact patch width may be the result of a larger or smaller size
tire than the known exemplar. In such cases, examiners should
utilize our model, or a similar model, of varied tire pressures and
cargo loads to test the range of variation for the given tire. In
addition, careful consideration should be given to the variables
that can cause impression width to vary when performing a tire
track comparison. Examiners are encouraged to report similar
experiments or observations so that we may better understand
the dynamics of these phenomena.
For further information, please contact:
Jan LeMay
Criminalist
Weld County Sheriff’s Office
1950 O St.
Greeley, CO 80631
970-356-4015 x4654
jlemay@co.weld.co.us
References
1. McDonald, P. Tire Impression Evidence; Elsevier: NY, 1989;
p 10.
2. Bodziak, W. J. Tire Tread and Tire Track Evidence; CRC
Press: Boca Raton, FL, 2008; p 4-5.
3. Chavigney, M. Tracks of Vehicles. Am. J. Pol. Sci. 1930, 1
(2), 166.
4. Be Tire Smart Pla y Your Part. Rubber Manufact u rers
Association: Washington, DC, 2008.
Journal of Forensic Identification
58 (6), 2008 \ 663
Figure 1
A test impression of the “General Grabber” tire at 8 psi (below
the scale) is 4.4 cm wider than the test impression of the “General
Grabber” tire at 40 psi (above the scale).
Journal of Forensic Identification
664 / 58 (6), 2008
Tire make
Tire model
Size
DOT #
Position
Width
@ 8 psi
Width
@ 16 psi
Width
@ 24 psi
Width
@ 32 psi
Width
@ 40 psi
Bridgestone
Insignia SE
P215/60R16
08X8E20-0006
LR
17.2 cm
16.8 cm
16.5 cm
15.7 cm
15.6 cm
1.6 cm
KS
Ultra GT
205/60R15
7DTMW60R0407
LR
14.5 cm
14.4 cm
14.2 cm
14 cm
13.5 cm
0.9 cm
4.4 cm
Difference
General
Grabber
P265/70R16
RR
20.4 cm
19.5 cm
18.0 cm
16.7 cm
16.0 cm
Goodyear
Wrangler ST
P235/75R16
M63DDADR
RR
18.3 cm
18.1 cm
18.1 cm
18.0 cm
16.5 cm
1.8 cm
Goodyear
Regatta
P206/70R15
MDMONTR4805
RR
15.4 cm
15.3 cm
14.5 cm
13.8 cm
13.2 cm
2.2 cm
Goodyear
Regatta
P215/60R16
MPX8NTJR1505
RR
17.9 cm
17.1 cm
16.5 cm
15.9 cm
15.6 cm
2.3 cm
Goodyear
Assurance
P195/65R15
M60618HR
RR
14.5 cm
14.3 cm
14.0 cm
13.3 cm
13.1 cm
1.4 cm
Goodyear
Integrity
P225/60R16
M6YOE6DP
RR
17.7 cm
17.0 cm
16.2 cm
15.8 cm
14.0 cm
3.7 cm
Goodyear
Wrangler
LT225/75R16
DJILKVHV3705
RR
18.0 cm
17.9 cm
18.0 cm
18.0 cm
18.0 cm
0.0 cm
Goodyear
American Eagle
P225/55R16
MKT424HR
RR
18.3 cm
18.1 cm
17.6 cm
17.0 cm
16.6 cm
1.7 cm
Hankook
Radial H714
175/70R13
1GPHAP
RR
12.0 cm
11.9 cm
11.2 cm
10.7 cm
10.5 cm
1.5 cm
Delta
Fortune
P235/75R15
1GHL8B090
RR
15.5 cm
15.4 cm
15.3 cm
15.0 cm
14.0 cm
0.6 cm
Firestone
Destination LE
P235/75R15
WIHLD10
RR
17.5 cm
17.4 cm
17.0 cm
16.9 cm
15.8 cm
1.7 cm
Table 1
The results of tires tested with varied air pressure.
Tire make
Tire model
Size
DOT #
Position
Width
Load = 0 lbs
Bridgestone
Insignia SE
P215/60R16
08X8E20-0006
LR
15.7 cm
Width
Width
Load = 250 lbs Load = 500 lbs
16.7 cm
16.6 cm
0.9 cm
KS
Ultra GT
205/60R15
7DTMW60R0407
LR
14.0 cm
14.3 cm
14.6 cm
0.6 cm
Difference
General
Grabber
P265/70R16
RR
16.7 cm
17.0 cm
18.0 cm
1.3 cm
Goodyear
Wrangler ST
P235/75R16
M63DDADR
RR
18.0 cm
18.1 cm
18.2 cm
0.2 cm
Goodyear
Regatta
P206/70R15
MDMONTR4805
RR
13.8 cm
14.8 cm
15.2 cm
1.4 cm
Goodyear
Regatta
P215/60R16
MPX8NTJR1505
RR
15.9 cm
16.8 cm
17.5 cm
1.6 cm
Goodyear
Assurance
P195/65R15
M60618HR
RR
13.3 cm
14.0 cm
14.4 cm
1.1 cm
Goodyear
Integrity
P225/60R16
M6YOE6DP
RR
15.8 cm
16.0 cm
16.7 cm
0.9 cm
Goodyear
Wrangler
LT225/75R16
DJILKVHV3705
RR
18.0 cm
18.1 cm
18.2 cm
0.2 cm
Goodyear
American Eagle
P225/55R16
MKT424HR
RR
17.0 cm
17.5 cm
18.0 cm
1.0 cm
Hankook
Radial H714
175/70R13
1GPHAP
RR
10.7 cm
11.5 cm
12.0 cm
1.3 cm
Delta
Fortune
P235/75R15
1GHL8B090
RR
15.0 cm
15.1 cm
15.3 cm
0.3 cm
Firestone
Destination LE
P235/75R15
WIHLD10
RR
16.0 cm
16.9 cm
17.0 cm
0.1 cm
Table 2
The results of tires tested with varied cargo weight.
Journal of Forensic Identification
58 (6), 2008 \ 665