Hood Design Flaw Causing Windshield Cracks?

[Not2Late]

Ok, here me out. Is my theory hairbrained? Any real engineers (rocket scientists) out there? I just played one on TV for a bit
I am on my second front windshield in two years! I have a Mojave with the hood bump. Both times my cracked windshield started with a tiny chip in the glass about an inch above the bottom edge of the windshield. Right where the airflow over the hood bump would bring a small stone as it flowed in the air stream. Before the math got really hard, I majored in aerospace engineering in college for 2-years. It didn't work out, but I remember a couple of things. One of which is air "flows" over smooth surfaces like a river until it is disturbed by something. It can follow the contour of the shape. So curving over a wing, for example, or over the shape of the hood on a car. Wondering if a stone is getting caught in the airflow and then it hits right at the worse spot as the flow comes back down from the hood bump?

The first time my windshield cracked was when I left my jeep out in the sun. I think a small chip (I remember seeing it and not thinking much of it) on the windshield weakened it and the heat from the sun then caused some stress/expansion - and it cracked starting at the edge.

The second time it cracked was when I was in a drive-thru. I knew I had a little chip in the windshield. It was a cold day, and I turned on the defrost. Pop! The long crack appeared starting at the bottom edge of the windshield. Exactly as before, but this time went to the right instead of left.

Here are some pictures:

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[Not2Late]

SO... Options?

1. Keep driving it and wait until the crack is really bad
2. Replace with Gorilla glass
3. Use one of the windshield film kits that are out there after putting on a new regular windshield
4. Build some sort of deflector or protective strip along the bottom 2" of the windshield
5. Put one of those bug shields along the front of the hood to disrupt the flow - although I don't like the look of them

Any other suggestions anyone can think of?

Thank you!

 

[rwu355]

Couple of observations, not opinions: 1). Just did the chip repair on 2 different nicks in the windshield, 1 was about 2” from the edge on drivers side about 4-5” up, the other one was littlepast the center on the passenger side about 8” up from bottom, so they are all over for me; 2) put a plastic hood deflector on mostly to protect the front edge from stones. Depending on web site it would or would not fit the Mojave. It fit it was right against the hood I pit a strip of rubber tubing to give it a little lip and stop rubbing. Over the last 2+ years it has a couple of nicks that could have done a number on the paint. in the last couple of years they may make a deflector that fits the Mojave hood???

so I’d suggest doing a search of post on the Gorilla glass, for some reason I’m thinking some haven’t been that thrilled with its performance (I could be wrong so check it out).

now for an opinion, I’d say get safelite (or any quality glass installer) to replace the glass, if the crack if it is causing distractions, and keep one of the chip repair kits handy to use befor a star becomes a crack. As they say your mileage may vary. (Had my Mojave since Oct 2020, with 46+K miles). Jeeps have a reputation for having windshields get damaged all of em not just the taller Mojave hood.

 

[LostMojave]

I'm not a relevant engineer for this but I do have a Mojave and I've watched mud and water go forward, against the wind on the scoop bump even at highway speeds. I've seen pieces of mud go forward and backward and then fly off. Anything close to the hood bump seems to be caught in an eddy current so I don't think its pulling rocks down you're just lucky enough to have two land in similar spots. I'm thinking similar speeds, similar rocks, similar trailing distance and similar weather or different enough variations to make the overall effect be the same. Be interesting to see a wind tunnel test to see how the scoop changes the airflow
I think the big issue is we just have a large flat plane of glass that loves to attract rocks.
Aside from running to the nearest glass center to have a chip filled I think its just something we have to deal with. From what I've read gorilla glass doesn't seem to be any better as they're cracking as well.
I'd run the windshield as long as you can, keep your distance as best you can, try to get chips filled, use cheap glass, and enjoy the drive without worry of the windshield as it will most likely crack. Maybe offer a prayer every now and then to whatever deity you believe in

 

[staying_tuned]

My only crack in 4 jeeps was up high just right of the mirror mount. If it happens that the stones in your area have evolved and prefer a hood hugging stealth attack vector targeting the hood bump to effectively tumble into your windshield hell yeah I’d use some PPF on the lower 6” of the windshield.

 

[Kevin_D]

In regards to the Gorilla Glass, it helped me avoid a couple of cracks.
Eventually, I took a large hit that left one hellua bulls-eye, and had to have it
replaced. I bought from HGP, and they honored their warranty.

Kevin

 

[Mr._Bill]

My suggestion would be to have the chips filled right away to try and reduce the possibility of it cracking. Most insurance companies will do that for free.

The bug shield and protective film are really all that's available to help minimize the damage.

 

[Kevin_D]

My suggestion would be to have the chips filled right away to try and reduce the possibility of it cracking.

That’s all fine and dandy, provided the chip cooperates and doesn’t crack during the remaining 300-mile-drive to the nearest glass shop.

Kevin

 

[Gvsukids]

Gorilla glass is no longer offered from the factory for a reason.
I have full glass coverage which has paid for chip repair and multiple new windshields.

 

[Boston Bill]

In regards to the Gorilla Glass, it helped me avoid a couple of cracks.
Eventually, I took a large hit that left one hellua bulls-eye, and had to have it
replaced. I bought from HGP, and they honored their warranty.

Kevin

HGP?

 

[Boston Bill]

A

 

[Kevin_D]

HGP?

Hyperformance® Glass Products

Kevin

 

[AyeCantSeeYou]

My opinion: There isn't a hood design flaw causing the windshield cracks. When I replace the cracked windshield this time (and no, it can't be repaired), I'll be on windshield #4 in 18 months' time.

I think part of the busted windshield problem with our Jeep's is because of the angle of the windshield itself. Next time you get on the road, take a look at the other vehicles around you, particularly the angle of their windshields; they do not have as steep of a slope as our Jeep's do (vertical). What that means for us is that when a rock flies into the windshield, it has a much harder time rolling off than it would on another vehicle. Kinda like a bird flying into a window on a building. Gravity wants to pull it down instead of letting it keep flying upwards.

 

[JRobes]

Mech engineer here... but more on the structural steel side... but did take a grad level aero course.

Anyways, for what it's worth, keep in mind that a rock is coming at your vehicle with all sorts of variables (attack angle, speed, jaggedness, mass, etc) which all greatly factor in to how any air flow over the vehicle may affect it and how much damage it'll do on impact. My opinion is if a rock catches a high arc from a vehicle in front of you (rear tire chucks it or skips badly off the road), any airflow around your vehicle is only going to slightly offset the rock's original course as it torpedos your front glass. The rock doesn't really travel in the air directly around the vehicle for very long... see pic below that shows a smoke trail over the vehicle's profile in FCA's wind tunnel.

I'm still a firm believer that it's just the shape (flatness) of the JL/JT window and it's angle (way more vertical than most) that cause them have such high rock chip impacts. If the windshield had a bit more rake to it I think it'd do a MUCH better job of deflecting rocks up and over the roof rather than just taking the impact square on to the angle of the windshield.

My best advice is just back off the big rigs, salt trucks, and off roaders with mud tires.

 

[Not2Late]

Mech engineer here... but more on the structural steel side... but did take a grad level aero course.

Anyways, for what it's worth, keep in mind that a rock is coming at your vehicle with all sorts of variables (attack angle, speed, jaggedness, mass, etc) which all greatly factor in to how any air flow over the vehicle may affect it and how much damage it'll do on impact. My opinion is if a rock catches a high arc from a vehicle in front of you (rear tire chucks it or skips badly off the road), any airflow around your vehicle is only going to slightly offset the rock's original course as it torpedos your front glass. The rock doesn't really travel in the air directly around the vehicle for very long... see pic below that shows a smoke trail over the vehicle's profile in FCA's wind tunnel.

I'm still a firm believer that it's just the shape (flatness) of the JL/JT window and it's angle (way more vertical than most) that cause them have such high rock chip impacts. If the windshield had a bit more rake to it I think it'd do a MUCH better job of deflecting rocks up and over the roof rather than just taking the impact square on to the angle of the windshield.

My best advice is just back off the big rigs, salt trucks, and off roaders with mud tires.

This is a great explanation! Thank you. The angle (lack of) makes sense. Sort of how an angled metal surface on a military vehicle better deflects projectiles? seems like a physics exercise and you explained it well. The wind picture is really cool!

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