Re: [T3] windshields

[Jim Adney <jadney@vwtype3.org>]

On 22 Aug 2008 at 5:10, Jason wrote:

> I had thought they were tempered.  I was under the impression that is why
> they break in so many small pieces.  If not tempered they would just break
> in large pieces and be more dangerous.   I know we talked about his in
> school but that was more than 15 years ago so I'm absoloutley sure I'm not
> absoloutley sure.

If you crack a side glass anywhere, the crack will propagate across the entire 
sheet, leaving you with a pile of little glass bits about 1/4" on a side. 
That's the hallmark of tempered glass. You can't drill, grind, or cut it at all 
without destroying the entire sheet.

Another indicator of tempered glass are two points where the glass was gripped 
at an edge while it was heated before it was air blasted to temper it. The 
grips are pincer-like and leave small indented points on each side of the 
glass. I see these in pairs on all our side glass, and I've seen movies of 
glass being tempered, and it shows those grips as they carry the glass out of 
the furnace. Those marks are hidden under the rubber gaskets on our cars. I've 
never seen such marks on a windshield.  

A windshield can be cracked and yet not shatter everywhere. We can get a stone 
chip that doesn't develop into anything more. If you ever get to see a badly 
smashed type 3 windshield, you'll see lots of long sharp slivers. I'm still 
picking some out of a part of our lawn where I once laid down a broken 
windshield.

It's possible that they would be even worse if they had not been partly 
tempered. I don't know. The article says that different amounts of tempering 
can be done to windshields to control the way they crack. Maybe that's true, or 
maybe it's something that has been developed since our cars were made.

That article, and several other identical ones, appear to have been plagiarized 
from a book that shows up in our engineering library catalog. If I can find 
time, I'll run over there and see if they copied it accurately. I'm quite sure 
that the glass and plastic thicknesses are reversed..

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI USA

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