The Forces Behind
by Steve Coyle
Many factors influence windshield retention, especially since the introduction of the passenger-side airbag. The Federal Motor Vehicle Safety Standard (FMVSS) 212 is the standard that determines specifications regarding windshield retention. It states that after a 30 mph frontal collision with two unrestrained dummies sitting in the front seats, the windshield must remain attached to the pinchweld along at least 75 percent of the windshield opening. Safe drive-away time is the amount of time required to cure urethane sufficiently to provide pull strength necessary to pass FMVSS 212.
When windshields were smaller and lighter, they only required about 15 to 20 psi to meet FMVSS 212. However, current designs make windshields dramatically larger and heavier, and their inertial load is much higher. Greater strength is required to prevent a heavier windshield from separating from the pinchweld during a collision. Adhesive manufacturers believe the urethane should provide at least two to three times more strength for windshields with airbags than for ones without airbags.
Since 1998, all passenger vehicles manufactured have been required to have passenger-side airbags. The implementation of the dual-airbag requirement has complicated windshield retention. The forces on the pinchweld and adhesive bond are much higher on cars that have dual airbags than on cars without airbags. There are three reasons for these forces caused by the airbags.
First, there is a far greater transfer of force to the pinchweld area when both airbags
deploy. During the FMVSS 212 test without airbags, a 165-pound dummy would place a force
of three tons on the windshield, according to the National Safety Council. However, that
force is localized to a very small portion of glass, which causes the glass and the PVB
laminate to absorb much of the force. Only a portion of the dummys inertia is
actually transferred to the
During the FMVSS 212 test with airbags the force of the deploying bags themselves and the inertial force of the dummy transfer to the pinchweld area. Instead of a small contact point on the glass, the passenger-side airbag can push the whole right-hand side of the windshield. There is almost a total transfer of airbag and inertial occupant forces to the adhesive bond.
Second, there are other combined forces applied to the windshield. Although the airbag is designed to be soft and cushion the passenger during a crash, it deploys in about 60 thousandths of a second. Because the bag deploys so quickly, it produces approximately one-half of one ton of force. In addition, the airbag door opening above the dash applies another force on the windshield.
Urethane is weaker when a longer duration of impact or pulling occurs. Therefore, our third consideration is that passenger-side airbags require windshields to have additional holding power. Vehicles entering a collision with passenger-side airbags experience a long span of force acting on the urethane. Although the airbag actually reduces the peak force on the dummy during the collision, it spreads the force out over a longer period of time. Because the duration of pressure on the windshield is lengthened, the urethane must be able to withstand the extended amount of force. n
Steve Coyle is part of the Performance Achievement Group (PAG), an auto glass training company located in Madison, WI. Special thanks to Sara Belda and Dean Mieske with PAG for their help with this article.
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