Collision Repair Supplement - 0820 - 12

COLLISION REPAIR DIAGNOSTICS
Photo credit: J. Anello.

repair procedure. Many of those state that
you must "perform four wheel thrust
alignment and/or measure unibody or
frame for structural alignment" after a
collision. So why is this consideration
so important?
In order for all the systems to work,
all systems have to be in alignment. The
body must be straight; tires must be
straight to the body, and the sensors must
be calibrated to the same straight line. If
the alignment is straight but the body is
sitting crooked, that will cause a problem.
Some sensors are calibrated to the direction of travel while others are calibrated
to the vehicle center line. Looking at and
understanding how a car interacts with
itself can often help you to further understand proper repair procedures.
Let's assume the wheel alignment
is perfect, steering wheel sits dead level
when driving straight, and the steering
angle sensor is calibrated to zero. Now
imagine that the front structure is askew
5mm to the left. We adjusted the gaps,
and the fender gap is good to the left door
well within factory tolerances; hood gap
is good, bumper looks good, our technician is proud of a job well done and
doesn't think twice that the adjustments
are maxed out. Afterall, if the OEM didn't
want you to use that much adjustment
room, why would they have made the part
with that much?
We follow the OEM repair procedures
for the ADAS calibration, hang a plumb
bob off the front emblem and another off
the rear, shoot a laser to intersect both,
and mark the ground. Targets are set at
the exact location that is required.
All measurements are checked and
double-checked. The location of that
target is square to the center line, but
remember, that center line is unknowingly off 5mm to the left. Now the sensors aren't 90 degrees to the back half
of the body, and they are certainly not
90 degrees to the direction of travel (or
if it's a front camera calibration, it's not

12 I Collision Repair I August 2020

perfectly parallel to the direction of travel). Instead, whatever target we place will
be off just slightly. At 100" (distance away
for many targets) it may just be 1" off; it's
only off by 1 percent, but these sensors
read 100 yards, which means that 1" turns
into one yard. One yard off could miss
seeing an object all together, and thus the
system won't operate correctly. The thrust
alignment can create the same issue; a
sensor can be aligned to a straight body,
but a car going down the road just 1 degree
off, due to a thrust alignment issue, at
100 yards is mathematically off nearly
one yard.

OTHER FACTORS
AFFECTING CALIBRATION
Many calibration procedures require that
all the tires are checked and set to the correct pressure, are the correct size, that the
car isn't overloaded with personal items,
and that the fuel load is at a specific level.
Why? The angle of the height of the car
front to back dictates where the sensor
will look up and down. When the target
is placed at the exact right distance from
the sensor, at the exact right height, the
computer is expecting the angle of the
car to be exactly right. If the front tires
are low, or the fuel load to low and those
items are corrected at a later date, the car

While performing collision work involving
sensor calibration, targets must be set at
the exact location that is required for
a proper repair.

will have an angle that the sensor cannot
compensate for.
The sensors do have a range to operate in, and it's just large enough for when
all things are perfect. Alter that range by
not checking all the boxes before performing the calibration and you may alter
the vehicle's angle by a degree or more.
That could mean the sensor is looking
at the sky or at the ground instead of at
the object directly in front of it. Again,
the same 1 degree at 100 yards translates
to 2.5' of misalignment. As the sensor
is looking for the center of a car at 100
yards, 2.5' of height misalignment could
have it looking at the ground, or right
above a normal size sedan.
Vehicles have a requirement that
calibrations are performed with a bumper R&I (remove and install) or a windshield R&I, not just after a replacement
or removal of the sensor itself, but after
simply taking the components off directly
in front of the sensors, why? When the
sensors are looking through a bumper,
an emblem, or the windshield it is like a
human looking through a set of glasses.
Glass lenses are cut in such a way that

Collision Repair Supplement - 0820

Table of Contents for the Digital Edition of Collision Repair Supplement - 0820