I am really surprised this didn't kick up
Stiffen that up, and those suspension loads aren't going away. We know it's stiffer; we can feel it. But the loads are still there, and our fatass torsion bars, mega shocks, and lo-profile badass tires aren't helping. They're transferring more loads to those not-very-well-boxed sub-frame body sections.
In other words, exactly the opposite as Chrysler intended. And exactly as Tony is describing.
TLDR: Chyslers' intent is that the passenger-compartment (Danger: Engineer lingo ahead) becomes a stressed member instead of something that's simply holding the front suspension to rear, as rigidly as possible.
Please note, I make no comparison to modern cars, and I make no comparison as to what's 'better', although I agree with Tony and I still consider subframe connectors on a car I don't drive much.
You fell into exactly the same trap as Tony. The factory engineering becomes irrelevant the moment you change the load inputs beyond what the factory intended. If you change the load parameters you can't assume the factory design will work as intended. If you want to run around on bias ply's with /6 torsion bars that's fine, but if you don't you'd better re-evaluate the situation.
The flex allowed by the factory was good for the amount of load these cars were capable of putting through the chassis at the time. Increase the loads, and I mean
SUBSTANTIALLY increase the loads, and the resulting flex becomes too much for the materials to handle. The distribution of the forces through the factory structures is no longer enough to keep the spot welds from failing. Which is exactly why you get the mid-body cracks that show up on these cars, as you correctly describe. Because the distribution of those forces is no longer enough to keep all the individual spot welds within their design parameters.
That means something has to change. The chassis must be stiffened to reduce the flex back down to something that can be handled by the spot welds in the body section. Does that increase the stress on other members? Yes. That's why you're adding material too. Subframe connectors don't just connect stuff, they also add more material to further distribute that load. The subframe connectors themselves still flex. They are absorbing load, and that load is in addition to the factory design.
There are certainly engineers out there that lack the hands on experience of someone like Tony. And that can lead them to make inaccurate assumptions on the design end, I've seen it. But Tony doesn't understand the concepts he's trying to explain, it's obvious from his description of work hardening. Too much flex can cause failure just as quickly as not enough, there is more than one mode for failure.
As for this-
, I'm not sure what you think that "proves". Just because it didn't fold in half doesn't mean it's as strong as it was before, it's not. Ask any 'vert owner if their car has more or less flex than a coupe or hardtop. That poor car would have folded right up if it hit anything, and it would have failed had it been driven any number or miles like that. All that's proof of is that less weight means a faster 1/4 mile time. And perhaps that the floor section of the car is stronger than you give it credit for.
I'm not saying I have all the answers, I don't. But I know for a fact that if I ran my car around with 1.12" torsion bars, sway bars, and 275/35/18 and 295/35/18 tires with 400+ hp and no chassis stiffening I would already have issues with cracks forming at the critical body joints- quarters to roof, rockers to subframes, etc. Instead, after almost 20k street miles I have no cracks. Not at the suspension points, not at the body. That's an indication that the subframe connectors, torque boxes, radiator support brace, seam welded and reinforced K frame, and J-bars are doing their job. Have I hit the "sweet spot" for the chassis? I don't know. It could be too stiff, maybe I will see fatigue cracks show up at the suspension mounts as I continue adding miles. It might not be stiff enough, in which case I'll still see cracks form at the body joints. But I know that what I've done so far is better than nothing, ie, depending solely on the factory engineering to carry the input loads that I've increased substantially by adding more grip and more power.
I'm reminded of a wrongful death trial that I was on some 25 years ago or so.
I opened my big mouth in deliberations and the ladies made me jury foreman.
By the time of the trial the death car had already been crushed and was not available for examination. It was an older car of some type. Late 70s I think.
Still, the expert witness for the plaintiff made conjecture about what speed the other car was traveling when it hit the death car.
The trial did not end in the plaintiff's favor.
We all make assumptions every day to get through life.
Which is why everyday people don't design cars, and why not all "experts" are worth their title.
Assumptions are necessary, even at the engineering and design level. But those assumptions have to be based on good information and must be valid for the conditions. If you don't understand the consequences of the assumptions you're making, they can't be good assumptions. Assuming that the span between the frame rails,
by itself, determines the overall stiffness of the chassis is a colossal mistake.
Just going to toss this into the discussion... my MP Chassis Manual has a section on structural chassis mods and covers subframe connectors. It's mentioned as something you should do to any Mopar you intend to modify for performance. If the factory engineers didn't think it was necessary or beneficial (or would potentially compromise other parts of the unibody structure) I don't think it would have been published in those books (written by Chrysler engineers) for the past 40-some years.
Exactly.
