Ñ Spotting corrosion. Rust and corrosion can
compromise the structural integrity of a vehicle.
Visual inspection helps in identifying areas of
corrosion that need to be addressed to prevent
further damage.
Ñ Conducting safety inspections. Critical safety
components such as seat belts, airbags, and lights
are visually inspected to verify that they are func-
tioning properly.
Ñ Examining the engine and exhaust system. The
engine and exhaust components are inspected for
signs of damage, corrosion, or unusual deposits
that might indicate underlying issues.
Ñ Evaluating the suspension and steering. Visual
inspection of the suspension and steering systems
can reveal issues such as worn shocks, struts, or
other suspension components that could affect
the vehicle’s handling capabilities.
Visual Inspection in Automotive Repair
Auto mechanics use visual inspection to identify
a broad range of vehicle issues. Following is a
short list of vehicle issues where visual inspec-
tion plays a crucial role in both identifying the
problem and executing repairs: tire wear and
tread depth assessment, detection of vehicle
dents, and alignment evaluation. This article
will discuss the application of machine vision
methods by focusing on these issues and compar-
ing typical inspection processes with and without
machine vision systems.
Tire Tread Depth
In the US, the minimum legal tire tread depth is
2/32 in. (1.6 mm). This standard applies to all pas-
senger cars, light trucks, and SUVs. The measure-
ment should be taken in the major tread grooves of
the tire and across different points along the tire’s
circumference to ensure accuracy, as tires can wear
unevenly.
VISUAL INSPECTION CAPABILITIES AND
LIMITATIONS
The 2/32-in. standard is based on the fact that tires
significantly lose their ability to grip the road surface
and effectively disperse water as their tread wears
down. This increases the likelihood of hydroplan-
ing and accidents, particularly in wet conditions.
To easily check whether tires meet the minimum
tread depth, the US has popularized the “penny
test.” Insert a penny into the tread groove as far
as possible, with Abraham Lincoln’s head facing
down (Figure 1) if the top of Lincoln’s head remains
visible, the tire’s tread depth is below the legal
threshold, indicating the need for replacement.
Although the legal minimum tread depth is
2/32 in., numerous safety experts recommend
replacing tires when they reach 4/32 in. (3.2 mm) of
the remaining tread depth, especially for wet driving
conditions—and even more so for winter driving
conditions—to ensure optimal traction and safety
on the road.
In much of the rest of the world, the legal
minimum is 1.6 mm (approximately 2/32 in.).
FEATURE
|
AUTOMOTIVEVT
Figure 2. Measuring tire tread depth with a depth gauge. Figure 1. Measuring tire tread depth with a Lincoln penny.
26
M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 4

The penny test is a common method for deter-
mining whether the remaining tread depth meets
the minimum legal requirement. However, this
method is highly dependent on the user. A more
robust approach is to use a micrometer or depth
gauge (shown in Figure 2).
Because tires wear unevenly, to properly assess
if the remaining tread meets local legal require-
ments, multiple measurements should be taken
at various circumferential positions around the
tire and along each tread. Care must be taken to
ensure that the depth gauge is properly positioned
for accurate readings. Additionally, the depth gauge
must be calibrated to eliminate any systemic bias
that could affect the measurement results.
To determine if a measurement approach can
be used to meet a specific requirement, a gauge
study, specifically a Gage R&R (Repeatability and
Reproducibility) study, should be conducted. A
gauge study assesses the measurement system’s
overall performance via the following factors:
Ñ Repeatability (equipment variation). This
assesses whether the same operator can get consis-
tent measurements using the same depth gauge
on the same tire tread multiple times. Low repeat-
ability suggests that the gauge itself or the measure-
ment process introduces significant variability.
Ñ Reproducibility (operator variation). This
evaluates whether different operators can achieve
consistent measurements using the same depth
gauge on the same tire tread. High variability in
this area indicates differences in how operators
use the gauge or interpret its readings.
Ñ Overall measurement system variation. This
combines repeatability and reproducibility to
assess the total variation introduced by the
measurement system, encompassing both the
depth gauge and the operators.
We conducted a gauge study on the use of
a depth gauge for tire tread measurement and
found that the operator contributed approximately
0.039 in. (1.0 mm) to the overall range of measure-
ments for a given tread. Therefore, to ensure that
the potential error introduced by the operator (the
auto mechanic) does not result in tires worn beyond
the legal limit being incorrectly assessed as passing,
the pass-fail measurement result must be adjusted
to account for this potential error (Figure 3).
To ensure a pass condition with a 95% con-
fidence interval, where the minimum legal tread
depth is 2/32 in., and considering the inherent
variability in measurements, we must account for
the uncertainty in the measurement process. This
involves adjusting the nominal pass threshold to
accommodate this measurement variability. By
doing so, we can be 95% confident that the true
tread depth does not fall below the minimum legal
limit.
The calculation entails determining the
standard deviation (​ ​ of a set of measurements and
using the Z-score associated with a 95% confidence
level. For a 95% confidence level, the Z-score is 1.96.
Because the range of measurements is 0.0394 in., if
we assume this range represents the total variability
(six standard deviations in a normal distribution, as
per the Six Sigma methodology), we can approxi-
mate the standard deviation as: ​ ​
(1)​ σ =​ Range _
6 ​ ​
To ensure the measurement result meets the
minimum legal tread depth with a 95% confi-
dence interval, we adjust the threshold by adding
the margin of error (MoE) to the fail criteria. The
margin of error is calculated as: ​ ​
(2)​ MoE =Z × σ​
where
Z is the Z-score (1.96 for a 95% confidence level),
and ​
σ​ is the standard deviation.
To ensure a pass condition with a 95% confi-
dence interval, given a fail criteria of 2/32 in. and
a range of measurements of 0.0394 in., the mea-
surement result for tread depth must be at least
0.0755 in. This adjusted threshold accounts for
measurement variability and ensures, with 95%
confidence, that the true tread depth meets or
exceeds the legal minimum tread depth.
While the above approach ensures that the
tread of all tires measured with a depth gauge
meets the legal minimum, the significant variance
in the measurement process means that many tires
with tread measurements shallower than 0.0755 in.
may actually have true tread depths deeper than
the legal limit of 2/32 in. This means that tires
with remaining usable life might be discarded
unnecessarily.
Pass with 95% confidence
Fail
Tread fail limit 0.0755 in.
based on GR&R results
Minimum legal tread
depth 0.0625 in.
Figure 3. The
impact of a visual
inspection on the
tread depth fail
limit.
J U L Y 2 0 2 4 • M A T E R I A L S E V A L U A T I O N 27