Body stress cracks

[BroncoHauler]

Originally Posted by daveb

Body repairs are a done deal. Looks good, van still starts and no glitches so far.

They installed the re-enforcements and all I hope is another crack doesn't form. Pulling the weather stripping was not a wonderful experience. Seems the body welds are the source, but I'm sure both weight and where I take my van is an issue as well.

Hope that the worst is behind you now.

With the weatherstripping off, it would have been a good time to reinforce your door seals.

viewtopic.php?f=2&t=5101&hilit=latex+seals&start=1 5


Herb

[twogone]

[Chance]

Originally Posted by daveb

Looks like both cracks started on a spot weld. Body shop isn't that worried about it but attribute both due to body flex. They didn't come out and say it was the top being cut out as the root cause but said there is more flex due to the doorways. The window guy also told me the van front windows flex more than most vehicles. They plan to reinforce both areas but also warned more spots might pop up where no backing is.

To me it's clear these two cracks are related to same body flex, and also agree that cutting out a large part of the roof doesn't help matters because it allows the body to flex more and concentrate stresses in areas that were not designed for it.

The roof acts as a diaphragm which makes the body a lot stiffer, particularly in torsion (although stiffness isn't necessarily always a good thing). And the body man is absolutely correct that the side doorways don't help either. On the driver's side the continuous side body panel also serves as a diaphragm to limit twist, so when the body has to twist due to off-roading, movement or flex is concentrated where the body is least stiff -- and that's the passenger side.

Although it's hard to tell from pictures, it appears to me that both cracks are related from body flex in the same direction. That is, twisting the body caused the roof area to move back relative to floor (as in shear), which then caused these two areas to fail due to the areas being placed in bending.

And if both started at welds, it's somewhat likely that cracks started due to many load cycles leading to fatigue. I'd expect a one-time large event to fail different.

[Chance]

Originally Posted by E350

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Dude! I am so sorry to see those stress cracks. We both know that Ujoint and Bajasportsmobile are probably the best guys to answer your specific question. But (that won't stop me) to add to the knowledge pool, about 10 years ago I was sitting around a campfire and I disdainly commented about how a pickup truck bed flexes vs the cab, and about how my stock Bronco and my 4x4 van were better because they were more rigid, and I was schooled by the campfire guys that the reason vans were not made oem in 4x4 is because the body doesn't flex. And that even my Bronco did not flex as well as a 4x4 pickup truck.* And that for true offroading you can't beat a 4x4 pickup truck because the bed is disconnected from the cab.

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In my opinion the thing to remember is that everything flexes under a load, it's just a matter of how much. And also that in complex structures like a vehicle, flex is cumulative. Under a load everything flexes somewhat in proportion to the inverse of stiffness. It's common sense that the stiffest link in a chain flexes less, and the least stiff flexes more.

In the case of a vehicle like a van, the best solution I can imagine would be to stiffen the chassis so it doesn't flex appreciably while maintaining its strength. If the chassis is very stiff, then the van's body wouldn't get stressed as much. In this scenario the stiff chassis would force the vehicle's suspension to provide all relative movement. And of course the suspension would need more travel relative to chassis.

If you notice many newer vehicles have gone to "boxed" chassis cross sections because they are far stiffer in torsion than the open "C" channels used on Econolines.

In a perfect world if I were converting my van to 4X4 I'd remove the body and replace or reinforce the frame as part of the project's scope.