“But, My Windshield Is Distorted!”
by Russ Corsi
At the advent of bent windshields in the early 1950s, we became aware
of a distracting waviness in the glass that quickly became known as distortion.
As the automobile manufacturers began to design more aerodynamic vehicles,
glass manufacturers were challenged not only to bend glass to conform
to the ”radical” windshield openings, but also to allow the occupants
to see through the glass without getting nauseated. A good example of
one of the early demanding glass opening designs was on the 1953 Corvette.
Initially, glass quality control managers developed standards that to
evaluate manufactured product for acceptability prior to releasing glass
to the car companies. As one might expect, the automobile manufacturers
didn’t want any distortion, even in the severely bent corners of the windshield.
However, the glass companies knew that having no distortion was an impossible
requirement to meet.
“As one might expect,
the automobile manufacturers didn’t want any distortion, even in the severely
bent corners of the windshield. The glass companies knew
this was an impossible requirement to meet.”
Evaluating the Glass
OE glass manufacturers soon identified areas of the windshield that should
be evaluated. Detailed quality assurance standards were written and presented
to the car companies for acceptance. Once accepted by the auto companies,
these standards were so specific that most trained quality control technicians
would arrive at the same pass/fail conclusion after inspecting the same
piece of glass.
Typically, distortion is evaluated against four basic criteria:
1. Cross car distortion;
2. Reflective distortion;
3. Transmitted distortion; and
4. Lower corner distortion.
All four of these standards require an evaluation of the subject windshield
from the installation angle that is observed in the vehicle.
• Cross Car Distortion—The windshield is rotated both left and
right to a position that allows the observer to look through the windshield
(across the car) against a blackboard with both horizontal and vertical
white lines that form a grid. The windshield is rotated in both directions
to a position where the angle between the grid and horizontal line is
50 degrees. The level of distortion (the grid squares observed develop
wavy lines and/or become diamond shaped) observed is compared to photographs
that exhibit various levels of distortion that have been approved by the
car company manufacturer. It’s important to note that some level of distortion
• Reflective Distortion—Reflective distortion is defined typically
as distortion observed on the outside glass surface that is present when
viewing the windshield surface, looking at the reflective image on that
surface. The windshield manufacturer’s process (bending iron/frame edge
support) typically causes this distortion. Windshields are placed at the
appropriate installation angle. Wide dark and light strips are suspended
above the windshield, parallel to the normal distortion evaluation position.
The observer faces the windshield and observes the level of distortion.
Acceptance criteria is very subjective. Like the cross car standard, the
automobile manufacturers approve limit samples that are used by the glass
companies to evaluate their products.
• Transmitted Distortion—A windshield being evaluated for transmitted
distortion is also mounted at the installation angle of the windshield
in the car. The windshield is observed parallel to the grid board. Each
individual windshield pattern is laid out in zones. The most critical
zone is located 75 mm from the inner edge of the paint band (zone A).
(If a windshield does not have a paint band, this zone starts at 100 mm
from the edge of the glass). The balance of the glass that is not covered
by the peripheral paint band is in the next zone (zone B). The paint band
is zone C while the area of the glass that is covered by mouldings or
body panels is zone D. (In many cases, zone C and D can overlap.)
The quality control technician employs a vertical bobbing motion while
observing the degree of intensity of the oscillation of the horizontal
grid lines. Acceptable levels of distortion are defined for both zones
A and B.
• Lower Corner Distortion—After the introduction of larger and
larger windshields that are press bent (glass is sagged both top to bottom
and side to side), while being installed at more and more severe installation
angles, a new type of distortion appeared. It is called lower corner distortion.
The technician places his chin approximately 400 mm above the bottom of
the windshield. Distortion is then observed against a luminous grid board.
As with the other types of distortion, the degree of acceptable distortion
has been determined by the car manufacturers with concurrence by the glass
All bent windshields do have some level of distortion. However, acceptable
levels of distortion are clearly defined by agreement obtained between
the car guys and the glass guys. The A zone of a properly manufactured
windshield will appear distortion free to the average driver. However,
all non-vision areas will exhibit some level of distortion. (General reference:
PPG Industries Automotive Quality Assurance website).
Russ Corsi retired as manager of technical services from PPG Industries’
Automotive Replacement Glass business unit after 31 years in the glass
industry. He now serves as a consultant to the industry. Mr. Corsi’s
opinions are solely his own and not necessarily those of this magazine.
I received an e-mail from Steve Harmon who read my January/February article,
“How Do I Get My Glass Clean?” Mr. Harmon advised that anyone looking
for alternate ways to clean glass should visit the website www.yourviewplus.com.
Steve’s company, Clarity Glass and Surface Restoration, is listed as a
wholesaler. I am not endorsing any of the products represented; merely
offering the reader some more information to evaluate.
© Copyright 2009 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.