Could expanding air in a closed, overheated car really cause the windows to burst?

[Zsofia]

So what could have caused it? Both my grandfather and my mother swear it happened - got in the car, didn’t roll the windows down, started it up, started the air, drive a minute, kapow.

It would have been an old car. Possibly in the 60’s?

[Kevbo]

@Hyperelastic

I could swear I read somewhere that car doors have a flapper valve that prevents an uncomfortable (to the ears) spike in pressure when you slam a door. That valve would open way before the pressure would get high enough to burst the windows.

Exactly so. They are often built into the lower rear corner of the doors. This is also where the air exits when you have the HVAC set to allow fresh air to be taken in from the cowl.

[Critical1]

tempered glass can shatter with just about any sudden temp change its just very rare.

this case is much more extreme than hot cars but I saw a glass door on a rottiserre oven shatter when a worker touched it to close the door.

my guess is its all about temp change unevenly hitting the window.

[flight]

@Gary T

Occasionally car windows do spontaneously shatter. All the cases I know of were on hot days. The one I saw personally did send most of the glass fragments outside of the car, I’m not sure why. Perhaps it was assisted by a small pressure buildup. But pressure doesn’t burst the glass.

It is not due to pressure at all, but rather the curvature of the glass. Car glass is almost never flat, but curves. In cars the center is bulged out rather than in, so a break due to heating would cause the glass to shatter outward. It may be that is was designed to do this or this is just a natural result of the aestehtics.

[Larry_Mudd]

@Zsofia

So what could have caused it? Both my grandfather and my mother swear it happened - got in the car, didn’t roll the windows down, started it up, started the air, drive a minute, kapow.

Same thing everybody’s been saying for the whole thread: Uneven thermal expansion.

@Larry Mudd

If the car gets very very hot inside, the possibility exists that the inside of the glass will be very hot and the outside of it will be cool – especially if you get in and drive off quickly.

As the car moves through the air, the outside surface of the glass (which was alreadly cooler than the inside surface) rapidly cools and contracts further. Kapow.

Nothing to do with the air conditioning. Opening the windows would slightly minimize this risk with or without air conditioning.

[Fridgemagnet]

The flapper valve Hyperelastic mentions serves another purpose. Improved production techniques in the 1980s led to some car models that had such airtight door seals that it was difficult to shut the last remaining door, the volume of air inside the car forming a sort of spring as it’s compressed when the door is swung shut. Deliberately allowing some air leaks prevents this.

I had a cousin with such a car - it was almost impossible to shut the last door, so he had to wind down a window a tad as a workaround.

It doesn’t take much to shatter safety glass if you catch it just right. It’s already under continuous tension, and will explode into thousands of little cubes if you’re unlucky.