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Q & A: Windshields, rocks, and aerodynamics

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Most recent answer: 10/22/2007
Q:
I’m looking for a brief explanation of whether aerodynamics, specifically windshield design, of a car, could possibly have any effect on rocks or debris hitting and cracking the window. If there is good design, is it possible for a rock to be deflected and not hit the windshield of the car? I don’t know what department to contact, but I would appreciate any information you have, or to be redirected to the source of such information. Thank you.
- Paula
Grenada, MS
A:
Hi Paula,

I just drove along I-55 through Mississippi a couple of months ago and noticed that a large fraction of the cars on the road had cracked windshields. I was told that this had to do with loose rocks on the road, esp. near construction sites, and the rocks get kicked up in the air by cars and trucks.

I don't think aerodynamics are going to have any noticeable effect on any but the smallest of rocks. A flying rock probably has rather little horizontal speed relative to the ground, and a car comes zooming into it at 65 MPH (if it's doing the speed limit on I-55). To deflect the rock with air means blowing on it so hard it'll be out of the way by the time the car gets to it, and I'm afraid that most rocks, being dense, don't get blown around very easily.

The best thing to do is to try to make windshields that don't crack when you hit them with a rock going at 65 MPH relative to the windshield -- this is quite difficult. A lot of car glass has a layer of plastic to hold the shards together in case of a shattering collision, but won't protect against the cracks. You could imagine making a windshield out of some other material like acrylic plastic or kevlar, but there could be issues with transparency and scratches blurring the driver's vision. Some kind of netting might help, but that again would be hazardous if it interfered with the driver's vision.

Tom

(published on 10/22/2007)

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