Oh boy. Well, I'll start off by saying- it's on the internet, so it must be true right?
So, this concept that the factory engineers knew best. First off, they didn't. The greatest engineering minds weren't working in the auto industry in the '60's. Even if they were, remember that the goal of the automotive industry is not to create the best engineered car. It's to make a profit selling cars while not killing so many people that the lawsuits cut into your bottom line. So, the engineers aren't making all the decisions. In fact, they're frequently overruled. Now that doesn't mean the CEO decides how thick a piece of metal has to be or how many welds to do per inch. But it DOES mean that you can't assume that just because a car rolled off the assembly line a certain way means that way was the BEST way. It could very well mean it was the cheapest way that still squeaked by the safety standards. Ford Pinto fuel tank anyone? Literally over a few cents per unit.
Something the video did mention that is totally true- let's look at the big picture. What did the factory engineers have in mind when these cars were designed?
And then there's just the history of it. Unibody construction for mass production was still a relatively new concept when these cars were designed. They didn't get it all right. If you look at a modern car this is painfully obvious. Sure, unibody construction is still the primary way to build mass produced cars, but I think we all know that a new HellCat has a chassis that's orders of magnitude stiffer than our A-bodies.
- Bias ply tires: I can't overstate how big a deal this is. Just adding radial tires significantly increases traction compared to bias ply's, especially during cornering. That means significantly increasing the suspension and chassis loads above and beyond what the factory engineers were designing for
- Horsepower: We all run the stock amount of horsepower and torque right? Oh, no, right. First thing we all do is double the amount of horsepower you could get from the factory. Well, needless to say that has a big effect on chassis loads.
- Lifespan: These cars were designed to be on the road for less than 10 years and probably not more than 100k miles. Lots of decisions were based on just buying a new car when the old one wore out.
- Factory race cars: These cars were significantly modified from the factory. They may not have gotten frame connectors, but they got a lot more than just torque boxes. And, they were only expected to last a few seasons, with a dedicated crew of mechanics overseeing them in that time.
Can you make a chassis too rigid? Yes. And when you do, you get cracking from fatigue at high stress locations. That's true. But you can also get cracking from having too much flex. If the chassis twists beyond the capability of the metal it's made out of it also cracks. That's the kind of stuff that happened on some of those factory race cars, and what some high horsepower cars experience too.
The factory didn't design these cars to last 50 years. They didn't design them to use radial tires. They didn't design them to last very long with 400+ hp. They never designed them for modern tires, or increased wheel rates, or sway bars double the diameter of the factory stuff.
So if you add better tires, what happens? More chassis flex. More than was ever intended. Same for adding horsepower. So what do you do? Well, you stiffen the chassis so you don't tear metal at all the joints, work harden the spot welds and break stuff. You reduce the flex back down to more tolerable levels. Will that cause higher stresses at some of those new joints? Yup. It's a balance, like all engineering is.
For me, the bottom line is you can't start making changes and then say the factory knew best. They didn't know radials were going to be used when they designed these cars, so the moment you slap a set of radials on your car more of the factory engineering goes out the window than most people realize. You want to roll around with 9" drums, /6 bars and bias ply's and Tony has a point. Start changing any of that stuff, and it's not relevant anymore.
I've logged almost 20k miles since I did the frame connectors on my Duster. I run a 300 lb/in wheel rate, 3x what the factory ran. I use 275/35/18's, run 400+ hp, have 13" disks. And frame connectors, and torque boxes, and significantly more chassis stiffening than that too. I've popped exactly zero spot welds. No cracks.
