What causes a windshield to spontaneously crack?

[Loomis]

My wife's 2010 Q7 just got a horseshoe shaped crack after sitting on the driveway in the sun for a few hours. She was using a silver reflective sunshade from our host. That thing absorbs a lot of heat, plus it was about 96 deg. yesterday. But we've had it a lot hotter than that and never gotten a crack. So what gives?

[masraum]

Weird. I would think that something would have had to hit the windshield, or it had some sort of internal flaw or stress.

[John Rogers]

Bad glass material coupled with a stress point. When the new Mini Cooper came out they had a problem with the windshields splitting right down the middle. The glass was too brittle and couple with the hole at the top of the windshield the glass would split when going over a bump or cornering or anything! The changed glass suppliers I was told and the problem went away.

[UconnTim97]

Possibly a really small chip that spread in the heat?

[john70t]

If not the above, mabye has to do with the expansion rate and shape of the chassis vs. the pilars and roof.
(haven't windshields popped out of convertibles on turns because of this?)

[TimT]

This happened to my F250PSD... I had not used it in a few weeks, and decided to drive it for a bit to warm the fluids, etc.

I got in waited for the glow plugs to warm.... and noticed a crack across the windshield...

The truck is 8 years old, so I'm pretty sure the chassis is normalized..

I examined the windshield, and there is just the pitting you would expect on an 8 year old windshield, and the crack does not look like it originated at a minor ding in the windshield..

[mudman]

I had a spontaneous crack in a 2500HD pick up a few years back. It had been sitting in the sun, and on very uneven cement with a very heavy load hitched. I got in the cab to pull the truck forward and noticed a very strange crack, kinda 's' shaped, then POW!, the crack went from side to side. Pretty neat to watch. I assumed it was the heat and stressed glass(from my chassis being so twisted).

[Hugh R]

My 08 Camry cracked from one side to the other in the direct sun, glass guy pointed to a tiny chip and said it started there.

[RWebb]

I hear that modern windshields are under stress and provide rigidity for the car (unit body) - that is nice but makes them more prone to crack.

Other causes include exfoliation and flatulence.

[Loomis]

Originally Posted by RWebb

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Other causes include exfoliation and flatulence.

Ding! We have a winner.

Not really. The Golden Retriever hasn't been in the car for a while and I sure as hell ain't going to blame the wife! That just leaves me...


I don't think I've filed an insurance claim in at least 15 years. Travelers handled it easily with a $100 deductible. It's just a little disconcerting to have a vehicle that is otherwise meticulously maintained decide to mess up on its own while just sitting there.

[techweenie]

I cracked the windshield in my 928 by putting a piece of cardboard at the base of the windshield slightly pinched against the dash. I think it was a note for the body shop that had the car. That slight pressure at one point, through a couple days in the sun, heating and cooling, cracked the windshield. Weird, but glass can take an impact better than constant slight pressure. And pressure toward the edge is more likely to cause problems than pressure in the middle.

I've also heard of windshields cracking when being doused with cold water on a very hot day. So be careful washing your car!

[wayner]

Rust behind the windshield growing and putting tension on the glass??

[GWN7]

Shock wave from a gun will cause a windshield to crack. Anyone been shooting a .30-06 across the bow?

[teenerted1]

the reflective sunshade behind the windshield increased the normal temp of the glass they dont absorb the heat, they reflect it back so what is behind it doesnt get hotter. so either a crack expanded due to the temp change or someother outside influence was effected by the hot glass.

[kaisen]

A few years back I was in the service department of a Toyota dealership when the service write-up door opened and a customer pulled in with his new-ish Camry. It was well below zero outside, like -15F real air temp. The shop was equipped with overheat infra-red heaters (long steel tubes with reflectors above).

BANG! Sounded just like a small caliber rifle.... and the rear glass imploded into the back seat in small tempered shrapnel.

Rapid temperature extremes can cause tremendous stress.

[Heel n Toe]

Originally Posted by Loomis

My wife's 2010 Q7 just got a horseshoe shaped crack after sitting on the driveway in the sun for a few hours. She was using a silver reflective sunshade from our host. That thing absorbs a lot of heat, plus it was about 96 deg. yesterday. But we've had it a lot hotter than that and never gotten a crack. So what gives?

1. Did it happen while it was sitting there and one of you noticed it with the car empty...
2. Or did it happen when she got in... in which case, the next question is, "how much does she weigh?"

Just kidding.

$100 deductible, huh? All policies here that I'm familiar with have windshield replacement at $0 deductible.

[Brando]

Originally Posted by Loomis

My wife's 2010 Q7 just got a horseshoe shaped crack [...] So what gives?

A horseshoe, obviously!

I jest... A lot of good explanations above. Could have been a miniscule chip and tension on the windshield...