# Windshield "explosion" Boooooooooggus

For a couple of guys that supposedly know something about physics, I was surprised at your response to the woman with the cracked windshield presumably from temperature. Of course the idea of creating enough air pressure inside the car is inane, but I don’t think that glass expansion in the frame is much more likely. Uneven expansion of the glass due to extreme temperature difference inside and outside the car sounds much more plausible. In this case, cracking the windows while the car is in the sun wouldn’t relieve air pressure inside (although one could see how someone might think that way), but it would significantly reduce the interior temperature and therefore the temperature gradient across the glass.
While it might take a bit of research to figure this out, I believe that the temperature gradient theory is much more likely to be the cause of most of these windshield failures.

I doubt the expansion explanation, as I believe the frame, being metal, expands more than the glass, and a hole in a material expands at the same rate as that material (consider what happens to a circle inscribed on a sheet of steel as it is heated, and the same experiment with a narrow cut along the circumference of the circle.)

This suggests an alternative explanation: could the glass be tightly-enough bonded to the frame (so it won’t fall out when hot) that the expansion generates tension within the glass? Because glass is not crystalline, cracks spread rapidly when it is subject to tension, and toughened glass counters this through processing that generates compression within the surface. I would have thought the toughening of the glass and the flexibility of the rubber seal would be enough to counter this effect, but I am not certain.

As for the pressure theory, if it were correct, then 1) there would be a noticeable push (and whoosh of escaping air) whenever you opened the door of a heated car; 2) the car would be so tightly sealed that there would be a real danger of suffocation, as in those tragedies when a child gets stuck in an old refrigerator. It’s not as if there’s a device to seal a car up tight if and only if it is empty.

I agree, the pressure theory is the most booooogus of all explanations. All cars have ventilation exhausts (usually in the trunk area) that allow airflow through the cabin. Even if you experience a “whump” when you slam a door, it is not possible to build up even a slight pressure inside the cabin for more than a second or two.

I still believe that the expansion of the glass on the inside due to heat in contrast with the cooler temperature on the outside is the most likely explanation. If you pour boiling water into a glass (drastically increasing the temperature on the inside with the outside constant) the glass is likely to shatter violently. Don’t actually try this without taking all safety precautions because this is likely to generate an explosive result.

The tragedy of children that perish from being closed in a car suffer from hyperthermia, not suffocation. Temperature in a closed car can significantly exceed 140 degrees Farenheit.

The guys got it right. The glass first had to have a defect. The defect could have been a manufacturing/installation error or it might have gotten a very tiny chip from who knows.

Cracking the windows does not significantly reduce the temperature inside the vehicle. People think that they can roll down the window by one or two inches for their pet, and come out to find a dead pet within a few minutes.

I don’t think the guys were referring to the temperature gradient between the inside and outside surfaces anyway. I though they said across the glass. The glass could have a different temperature from top to bottom or side to side. The frame or rubber molding would have no affect on this, but say its top to bottom, the top may be expanding more than the bottom, and glass isn’t know for its flexibility.

As I recall, the vehicle was an SUV of some sort. Almost all SUVs have tinted glass in the rear and the sides aft of the driver. The tint is going to aggravate the situation. Along with a slight curvature of the glass, solar heating will not be uniform. The glass could be hotter at top, at the side closest to the sun or in the center depending on the time of day and compass orientation of he vehicle.

I live in an area where Allstate won’t even insure windshields, just the cases where a collision happpens to break it.

Agree that if there was a flaw in the glass, it could suddnely shatter with a temperature buildup. Nortmal temperature variations won’t cause cracking. I spent time in the Middle East with 44C (111F) temperatires and there were few cracked windshiels, except those hit by flying stones.

If you take your car through the carwash at subzero temperatures, you will likley crack the windshield. The sudden appliaction of hot water and subsequent rapid expansion will do it in. I had a small crack on my previous car’s windshield and went through the car wash on a cold day. The crack instantly expanded right across the windshield.

I still don’t believe that solar heating, window tinting, top to bottom gradients, or anything else will create as much stress in the glass as the temperature differential from the inside of the glass to the outside of the glass. Just think about it, even if it’s 105F outside, it’s likely to be 160F inside - a much greater than any temperature gradient top to bottom or whatever. OK, now throwing water on glass that is significantly different temperature is likely to have catastrophic effect (e.g., the subzero carwash mentioned by Docnick or hot water in a room temperature glass in my original post). But aside from such a thermal or physical shock, I keep coming back to the inside/outside gradient as the most likely explanation.

All of the windows in a modern car except the front windshield are made of tempered glass. Tempered glass will not chip or crack. Anything breaking the surface of tempered glass will cause the whole window to shatter into small pieces. This is why firemen or police sometimes carry a device that looks like a retractable pen with a hardened tip to shatter a side window to rescue someone from a locked car.

I think the caller suspecting a design flaw in the vehicle’s rear window is valid. The Xterra’s rear window has a unique shape which contains a reentering corner around the “first aid” box. Because this rear window glazing surface is uniformly exposed to the elements, this reentering corner feature might create uneven expansion and contraction of the glazing over a thermal cycle. If it were possible to see the thermal stress accumulating in the rear window glazing, I imagine that the stress would be greatest at the reentering corner. Possibly to the point of failure.

Does the Xterra’s rear windshield have an electric motor to raise and lower it? If so, I suspect that the motor may have pushed the glass tightly against the top frame of the window, leaving no place for the glass to expand when it was heated by the sun.
I was told, when I moved to Texas, to always leave my windows opened a small amount if the car is parked in the sun. The windows that are fixed have enough space between the glass and the frame, although it is covered by the gasket and you don’t see it. Windows that can be opened and closed, however, can be closed so that the glass is held against the frame much more tightly. When the pane of glass gets hot, it might need only a small amount of space to expand, but if that space isn’t available because of an unyielding mechanism holding it tightly, then any imperfection in that glass may indeed be the location at which a crack will start. Once it starts, the whole window is going to shatter.

The windshield does not need to have a factory flaw…Playing softball one summer at Ft. Drum, a friends car parked on the 1st base side caught a fowl ball in the windshield, no signs of damage. The next day after training came out to a busted windshield, the heat of the day caused the flaw created by the softball to crack.