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I was driving to Montrose on County Rd 1, which is unpaved, at about 35 to 40 miles an hour when the rear window in the FJ shattered into a million pieces. I don’t think it was a rock as I can’t figure out how a rock could hit the rear window that hard when I am going the other way.

Any ideas on what might have caused this?


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Lots of stories on 4th Gen 4Runners where the rear window shatters when they turn on rear window defrost (and there is corrosion on the elec connectors). Did you happen to push the rear window defrost button?
 

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Lots of stories on 4th Gen 4Runners where the rear window shatters when they turn on rear window defrost (and there is corrosion on the elec connectors). Did you happen to push the rear window defrost button?


Nope. Just driving down the road.


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unless you find a stray bullet in the vehicle, I would guess it most likely was a rock -- i suppose once in a blue moon a rock could fly off the tire that has some english on it and curve around and nick the back window.

You didnt pass a vehicle heading the other direction, right?

All it takes is a tiny little nick in the glass for tempered glass to explode into a millions pieces
 

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unless you find a stray bullet in the vehicle, I would guess it most likely was a rock -- i suppose once in a blue moon a rock could fly off the tire that has some english on it and curve around and nick the back window.

You didnt pass a vehicle heading the other direction, right?

All it takes is a tiny little nick in the glass for tempered glass to explode into a millions pieces


Did not pass a car but I was pulling my trailer Considering direction of wheel rotation and fenders it seems unlikely a rock could have come from the trailer’s wheel but I suppose it could have. Oh well.

Going to take a week to get a replacement. Luckily I have another car to drive in the mean time.


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Rock off of FJ tire hit the trailer and ricocheted back into the window.
 

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Or, a small chip, or scratch, could have gotten into the edge of the glass somehow (either during manufacture, or from something during use, probably around one of the attachments through the glass), not large enough to cause it to immediately fracture, but enough for it to be sitting there, waiting. Then, just the right combination of stress, temperature change, or just time, and the thing could let go.

Remember that tempered glass is under tremendous stress, by design, all of the time. It is ready to let go with a bang, like that, once a point is provided for it to begin to release somewhere. Once such a point is provided (the most typical is a stone chip somewhere in the middle, but much more subtle scratches or chips on the edge somewhere can also lead to it), then the suddenly unbalance between the built in stress, and the one locally released stress, creates a pressure wave which propagates through the sheet of glass at the speed of sound. As the wave progresses, it bounces back and forth, through the "softer" inner layer, between the "harder" outer layers. The resulting ripples are what makes the glass break into those tiny cubes (if you can hold all of the little pieces together, the patterns in the fracture lines will reflect the relative thicknesses of the three layers, and show the path the pressure wave took through the sheet, and the lines lead back to wherever the release started).

Tempered glass is made by taking sheet glass, heating it up to nearly molten, and then suddenly cooling its surfaces to create the two outer "hard" layers. The inner material, which naturally cooled slower, remains "soft". The difference in cooling rates creates the tremendous stress that gives tempered glass is incredible strength. A 50mm sphere of steel, dropped from several meters height won't break it, but a small chip will cause it to let go with a bang.


The cool thing is that, even though the edges of glass are incredibly sharp (sharper than the edge of a scalpel), the nature of fractured, tempered glass, is that those edges are all at nearly 90 degrees, making it far less able to cut skin than plate glass would have been. The edges of a fractures windshield will cut your skin to ribbons, but a fractured tempered side/rear glass, you can almost sweep up with your bare hands.

I am sorry that this happened to you, and hope that the new piece of glass arrives soon. Take a close look at your hinge and latch fasteners, you might want to replace them if any look rusty, or if the plastic bushings seem at all worn.


Norm
 

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Discussion Starter #18
Or, a small chip, or scratch, could have gotten into the edge of the glass somehow (either during manufacture, or from something during use, probably around one of the attachments through the glass), not large enough to cause it to immediately fracture, but enough for it to be sitting there, waiting. Then, just the right combination of stress, temperature change, or just time, and the thing could let go.

Remember that tempered glass is under tremendous stress, by design, all of the time. It is ready to let go with a bang, like that, once a point is provided for it to begin to release somewhere. Once such a point is provided (the most typical is a stone chip somewhere in the middle, but much more subtle scratches or chips on the edge somewhere can also lead to it), then the suddenly unbalance between the built in stress, and the one locally released stress, creates a pressure wave which propagates through the sheet of glass at the speed of sound. As the wave progresses, it bounces back and forth, through the "softer" inner layer, between the "harder" outer layers. The resulting ripples are what makes the glass break into those tiny cubes (if you can hold all of the little pieces together, the patterns in the fracture lines will reflect the relative thicknesses of the three layers, and show the path the pressure wave took through the sheet, and the lines lead back to wherever the release started).

Tempered glass is made by taking sheet glass, heating it up to nearly molten, and then suddenly cooling its surfaces to create the two outer "hard" layers. The inner material, which naturally cooled slower, remains "soft". The difference in cooling rates creates the tremendous stress that gives tempered glass is incredible strength. A 50mm sphere of steel, dropped from several meters height won't break it, but a small chip will cause it to let go with a bang.


The cool thing is that, even though the edges of glass are incredibly sharp (sharper than the edge of a scalpel), the nature of fractured, tempered glass, is that those edges are all at nearly 90 degrees, making it far less able to cut skin than plate glass would have been. The edges of a fractures windshield will cut your skin to ribbons, but a fractured tempered side/rear glass, you can almost sweep up with your bare hands.

I am sorry that this happened to you, and hope that the new piece of glass arrives soon. Take a close look at your hinge and latch fasteners, you might want to replace them if any look rusty, or if the plastic bushings seem at all worn.


Norm
Thank you for the detailed description. The FJ is approaching 12 years old and that was original glass. There is some washboard on the road so some vibration was in play. And of course it is a road I drive frequently.

I spent an hour or so yesterday evening taking everything out of the back of the FJ and cleaning up the glass and as you said nothing sharp that could cut you.

New glass had to be ordered and will not be here until next Tuesday so I will take some time to examine the hinges, etc. for rust or other imperfections.
 
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