Why Do Some Cracks Start at the Edge of the Windshield?

Summary:

Mike gives two main reasons why a crack can start at the edge of a windshield. The first is simple: if a rock hits the glass near the edge, the damage is at the edge. The second is less obvious and comes from the manufacturing or installation process. Sometimes a urethane adhesive is used that is too stiff for that particular vehicle. As the vehicle flexes during normal driving, that stiff urethane can cause the glass to crack from the side, which Mike describes as an imperfection in the manufacturing process. Those two causes, an impact near the edge or a urethane that does not flex with the vehicle, are typically why a windshield develops a crack that begins at its edge rather than in the middle.

Answer:

Cracks can start at the edge of the windshield for a few reasons. Obviously, if you get hit by a rock and it is at the edge of the glass, then the damage is at the edge of the glass. That one is straightforward. But sometimes in the manufacturing process there is a urethane that gets used that is just too stiff for your vehicle. From the flex of driving, it can create your glass to crack. The vehicle is moving and flexing as you go down the road, and if the urethane does not give the way it should, that stress ends up in the glass. It is more or less like an imperfection in the manufacturing process, at least from what I am aware of. So those are typically the reasons why you would get something to crack from the side: either an impact right at the edge, or a urethane that is too stiff and the flex of the vehicle working against the glass over time.