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View Full Version : Has anyone actually cracked a windshield from Summer Heat?


aceplace57
07-29-2011, 10:42 PM
Growing up, I often heard people warn that windows should be cracked in the summer. Otherwise the windshield might crack. :eek:

I stopped doing this when thieves with coat hangers were a nuisance. Figured a cracked windshield was better than a stolen car.

Is cracking a windshield or any car glass in the summer heat a old wives tale? Anyone know of this really happening?

Closest I came was finding my rear view mirror on the floor. The heat had ruined the glue holding it to the windshield.

aceplace57
07-29-2011, 10:48 PM
Whoops. Wrong hole. Thought I was in IMHO. I predict a quick trip by the Mods. Sorry Guys.

Not A Tame Lion
07-30-2011, 02:01 AM
Not exactly, but...

I used to live over on the other side of the hills, where it gets a lot hotter in the summer than where I currently live. A few years back, I had a couple of small cracks in my windshield, cracks that came from rocks or other road debris hitting the windshield while driving. Just a short line here, a short line there--no "spiderwebs" or any such thing.

Anyway, one very hot day I had my car parked in the driveway with the sun beating down on it for a period, long enough for the glass to get quite hot. When I came out to leave, I saw the car was filthy (as usual) and decided to hose it off to get some of the dirt off it. Well, when the cool water splashed over the hot glass, that rapid temperature change caused some physical flux--I'm assuming shrinkage--and both cracks immediately spread all the way up and down my windshield, where they remain to this day, dividing my windshield into three parts (somewhat like Gaul, I guess).

The moral of the story: if you've got a little crack in your windshield, don't splash cool water on the glass on a very hot day, or you will have a big crack in your windshield.

Here endeth the lesson.

Girl From Mars
07-30-2011, 05:51 AM
There was a recent Car Talk podcast on this - their opinion was that cracking glass could occur due to heat, but only based on the glass expanding and putting an internal flaw in the glass under too much stress. The caller had been keeping her windows rolled down slightly, but they believed the increase in pressure from the air expanding wouldn't be enough to cause the damage. No idea how accurate this is though.

Joey P
07-30-2011, 07:03 AM
Growing up, I often heard people warn that windows should be cracked in the summer. Otherwise the windshield might crack. :eek:

I'm not sure about cars when you were growing up, but all (most?) cars in the last at least 15 years or so that I've seen have vents in the rear, under the bumper to allow your heating/cooling system to bring in fresh air and expel the old air. These vents (just one way valve type thing) would let out any pressure long before it would be enough to crack a windshield.

It's usually easiest to see the vent (http://t3.gstatic.com/images?q=tbn:ANd9GcSvmf4vpyVZaVRFowuZcWI1xpzQ3BrwAOWYp90Y589Brmti_5fO) on a car who's rear bumper has been lost in an accident.

mattwan
07-30-2011, 05:33 PM
When I was a teenager, our family car's windshield blew out in the summer heat. I can't remember exactly when, sometime in the Alabama summer in the late 80s or early 90s. I can't even remember which car it was; it was probably either a late model Cutlas Ciera or a mid-70s Bonneville. I just remember being in the living room, the car parked on the driveway just outside the door, and a a loud pop when it gave way.

jz78817
07-30-2011, 05:42 PM
cars and trucks have cabin vents (little plastic grids with rubber flaps that let cabin pressure out but block shit from coming in.) And even without them, the interior of your car isn't airtight; the door seals aren't perfect, the HVAC system isn't airtight, the trunk seals aren't airtight, hell, even the speakers aren't usually airtight.

what will crack glass is a thermal shock. i.e. driving for a half hour with the defroster on full heat and suddenly a lump of slush decides to flop onto the windshield.

I just remember being in the living room, the car parked on the driveway just outside the door, and a a loud pop when it gave way.

then it wasn't the windshield. Windshields have been multi-layer glass-PVB-glass for a long time, they might crack but they'll hold together. Side windows and backlites are tempered glass, which are under tremendous amounts of internal stress. If you have a piece of glass that just goes "pop" and blows tiny little bits everywhere, it's tempered glass.

SnakesCatLady
07-30-2011, 08:50 PM
I'm not sure if heat will crack an otherwise undamaged windshield, but it will expand small cracks to the point they become large cracks.

A dumptruck-thrown rock hit the windshield of my Wrangler on the driver's side edge, right at the gasket. Over the space of about two Georgia summers the crack had exlended halfway across the windshield right at eye level.

I had it replaced before I went to Tennessee last month.

Dallas Jones
07-30-2011, 11:01 PM
If there is any flaw or chip in the windshield the heat may cause that chip to run into a long crack. Or as already mentioned, sudden temperature change can do the same thing.

I ruined one windshield by not opening the windows to let the interior cool down before washing my car one day. I hit the car with the water hose and heard a screech as a small chip at the bottom of the glass, one I hadn't even seen before, ran all the way across the window.

As jz78817 says, tempered glass is held under a great deal of stress. You can think of tempering as if the exterior skin of the glass has been slightly shrunk and is holding a tight grip on the interior of the glass. Tempered glass is up to 7 times stronger than regular glass, you can hit it pretty hard on the face of the pane before it will break. But if you even nick it on the edge and break that tempered skin it will fall apart almost by itself.

I worked in a glass shop for a year and helped unload delivery trucks. We had to be especially careful to cushion where we set down the tempered. One piece of gravel or a nail in the wrong spot could cause a self destruct.

Bryan Ekers
07-31-2011, 03:38 AM
I once cracked a windshield with the corner of a cardboard box.

I must have hit the sweet spot.

GilaB
07-31-2011, 02:07 PM
We once returned from a summer vacation to find the rear window of my father's car totally shattered, almost certainly due to the heat wave we'd had that week.. This would have been in the mid-to-late nineties, on a relatively new Honda Accord. As far as any of us knew, it hadn't been previously chipped.

blondebear
07-31-2011, 02:22 PM
No cracked windshield, but I did have a bottle of Everclear explode in my car on a hot day. When I opened my car door an invisible cloud of alcohol vapor wafted out--rather a bizarre surprise after spending a 100+ degree day at the State Fair. There wasn't a drop of liquid left, just shattered glass in a brown paper bag.

Phlosphr
07-31-2011, 02:30 PM
I've heard of it, but have not actually seen it with my own eyes.

Northern Piper
07-31-2011, 03:42 PM
At the opposite end, I've cracked a windshield by turning on the defroster in the winter without fully clearing off the snow form the bottom of the windshield and the wipers - the temperature contrast between the heat and the snow caused a crack all along the bottom of the windshield, tracking the snow level almost exactly.