Originally Posted by spasm3
15 degree. That angle absorbs less energy and allows the hailstone to continue moving imparting less energy to the glass. Seems to me the closer you get to 90 degrees, the more energy is absorbed by the glass.
Windshield Damage
- Thread starter MolaKule
- Start date
Isn't 90 degrees is straight up? Perpendicular to the ground. Car is not moving. The hailstone wouldn't even hit it. We must not have have the same understanding as to the angle of the dangle.
I'm interpreting 15 degrees as almost flat. Say at 9:30 o'clock. 30 degrees at about 10:00 o'clock.
Originally Posted by spasm3
Too add to my previous post, think of a stone skipping on the water vs dropping straight down in to the water.
Originally Posted by Gebo
Originally Posted by spasm3
15 degree. That angle absorbs less energy and allows the hailstone to continue moving imparting less energy to the glass. Seems to me the closer you get to 90 degrees, the more energy is absorbed by the glass.
It depends heavily on which vehicle is parked in the garage. ;-P
All else being equal, the windscreen that is more vertical will be less likely to be damaged.
As Molakule said the hail is assumed to be coming straight down and the windscreen is weakest to impacts that are perpendicular to its face. A horizontal windscreen ( which would make it a sunroof I guess) would take 100% of the energy of the hailstone as the vector of the hailstone's velocity would be perpendicular to its face. A vertical windscreen would take 0% of the energy as the vector would be parallel to its face.
MolaKule
Staff member
Originally Posted by zorobabel
Are the windshields identical in size?
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Wait, I'm lying in the bed thinking about hail.
The glass is harder than the frozen water. Therefore, the ball of frozen water hitting the "flatter" windshield would shatter thereby dissipating its energy before it could transfer it to the windshield.
So, I now believe the frozen water hitting the more upright windshield (30 degrees)would stand a better chance of breaking it. Because the hail would not break open. It would transfer all its energy into the harder laminated glass.
Do I win a years supply of any of your new concoctions?
Say as an experiment?
MolaKule
Staff member
Glad to see many responses and thanks to all who submitted answers.
Those who started their analysis with a horizontal windshield had the right idea. A horizontal windshield would receive the full energy of impact, whereas the more we tilt it toward the vertical the less the impact energy the windshield would receive, If the windshield were purely vertical the vertical component of impact energy would be zero.
For impact studies of this type we assume a purely elastic collision where there is no mass exchange of the two bodies at collision nor are there any frictional forces involved. I.e., Energy is conserved. Both bodies are thus solid bodies.
A free falling hailstone is limited to a certain velocity because of the aerodynamic drag on the surface of the mass.
Hail with a free fall velocity of 91 feet/second (27.34 m/s) has an impact energy of 7.25 ft.lbs or 9.83 N.m.
If we use a bit of trig, (the sine of the angle of tilt or slope) we can determine the total impact energy for any angle of tilt or slope.
For a 15 degrees slope the impact energy would be Kimpact 15 degrees = 0.26X7.25 ft.Lbs = 1.88 ft.lbs (2.6 N.m) of energy.
For a 30 degrees slope the impact energy would be Kimpact 30 degrees = 0.5X7.25 ft.Lbs = 3.63 ft.lbs (4.9 N.m) of energy.
So the windshield with a tilt of 30 degrees would have the higher probability of being damaged because it would receive the greater impact energy..
For a fully horizontal impact, at 90 degrees the impact energy would be Kimpact 90 degrees = 1X7.25 ft.Lbs = 7.25 ft.lbs or 9.83 N.m.
For the same reason, roof shingles on highly sloped roofs receive less damage than shingles on less sloped roofs. The difference here is that in hail collisions with roofs, some of the energy is absorbed by the shingle (an "inelastic" collision), so it takes a larger hailstone to damage asphalt shingled roofs.
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MolaKule
Staff member
Originally Posted by Gebo
So the "hardness" of the hail is irrelevant? Interesting 🤔
Quote
For impact studies of this type we assume a purely elastic collision where there is no mass exchange of the two bodies at collision nor are there any frictional forces involved. I.e., Energy is conserved. Both bodies are thus solid bodies.
Originally Posted by Gebo
Wait, I'm lying in the bed thinking about hail.
The glass is harder than the frozen water. Therefore, the ball of frozen water hitting the "flatter" windshield would shatter thereby dissipating its energy before it could transfer it to the windshield.
So, I now believe the frozen water hitting the more upright windshield (30 degrees)would stand a better chance of breaking it. Because the hail would not break open. It would transfer all its energy into the harder laminated glass.
Do I win a years supply of any of your new concoctions?
Say as an experiment?
MolaKule
Staff member
Impact mechanics is an interesting field of study:
https://digitalcommons.unl.edu/cgi/...2&context=engineeringmechanicsfacpub
We all forgot one of the most important variables-is it OEM glass or aftermarket? Because the aftermarket windshield, regardless of impact angle, will break every time! 
I was in a hailstorm about ten years ago. My car looked like a golf ball but none of my glass was broken. I would estimate that at least 90% of the vehicles in the area had some kind of glass damage. I was driving a "low tier" car at the time so the glass wasn't stronger in any way.
After asking around, all I could figure out was that most other drivers slowed down or stopped while I chose to speed up significantly. Does the speed of the vehicle matter?
Forward motion at speed would create a shroud of denser, displaced air.
Much like a hood deflector pushing some of the bugs and stones over your windshield.
Now then, our (Earth's) ionosphere deflects solar radiation such that concentration of the Sun's charged particles create the "Northern Lights".
Driving fast enough to deflect and compress the snow will not, however, change the hail's color.
