I am really surprised this didn't kick up
I hate to say it, but every post in this thread except one is off base. Please follow along.
Of all the Uncle Tony videos, this one is the only one that I disagree with.
I seriously doubt that any engineer would purposely design their cars to flex.
It's not so much that they do, but that they try to accommodate what they know will happen anyway. You have to acknowledge it and account for it. Kinda like....the plunge coupling on a steering box.
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.
Technically, some of the greatest engineering minds of the '60's
were working in the automotive industry; they just weren't working directly on automotive projects. We didn't get to the moon on a Chevrolet rocket....
This guy is only partly right- he needs to go to engineering school. I have been involved with motocross since the '60's. Those who really want to understand chassis stresses and flex and their effect on suspension and handling need to get educated. Beware this guys advise.
Having been through engineering school, I can tell you that 99% of engineers do not understand as much as Tony. He just doesn't do the math problems associated with it. Many engineers can only do the math, and understand very little.
This is like all the arguments about which kind of subframe connectors are best too. Until someone runs a finite element analysis, no one here can actually know.
Run it through FEA and it will correlate with what I explain below.
I guess one clue to this, is what is done to Nascar and the big time drag cars? Rigidity, safety, performance...
Apples and oranges, man.
For those that say ‘What is he smoking Tony’ provides good information,, I want to know more about those spot welds that get work hardened after sub frame connectors are installed.
Walk me through the physics of how a spot weld, that is already harder than the surrounding material gets ‘work hardned’ by removing flex.
See below.
I wonder if maybe he's talking about metal fatigue from the flex. Since the spot welds are already harder....and they are.......good point........maybe he means they get metal fatigue from having to carry more load from the added connectors? I know......I'm reaching here but that's all I can think of.
As usual, this non-engineer (at least to my knowledge) gets it more than most. Say it ain't so.
I had an actual electrician tell me that water doesn't conduct electricity but just the metal pipes it runs through.
Uhh huh, hold this wire.:lol:
Clean, pure water
doesn't conduct electricity. Salt water does. When you stick your hand in, the electrolytes from you skin
create conductive water, but I digress.
********************
What Tony is trying to say here, but not being concise about, is that subframe connectors DO reduce flex, but that the forces that create that flex don't magically disappear; they are distributed elsewhere.
When we study sheet metal structures, we have to look at what happens when forces are applied. A rectangle is free to become a parallelogram as soon as the material gives
at all. This is why a triangle is the strongest shape: It has no sides that are free to move, and only making a side non-linear will create a change in shape. This change in shape is what we are totalling up with the term 'rigidity'.
The key to maximum rigidity in any sheet metal structure is 'boxing'. If it's a cube, but open on two opposite sides, it's still going to fold where the remaining sides meet. Add a sheet to one side, and the other side will still try to fold. Add to the remaining side and you've max'd the strength for that particular shape. This gets into tetrahedrons, four-sided pyramids vs. three-sided pyramids, etc., that I won't get into, but if you picture it in your mind, you should be following along.
Still struggling? Go get a new box of cereal, and try to 'flex' it. Then open it, and try to flex it. Then open the other end, claim the prize, and try to again flex it.
If we look at the basic unibody of a Chrysler muscle-era car, it's a big rectangle and it's open on opposite sides: The passenger compartment. The doors contribute nothing to structure, even when closed, unless the flex is severe enough for the body to hit the door. Onto this rectangle, we have grafted two other rectangles: The front and rear subframes. These sub frames are heavily boxed, quite rigid, and assembled using almost exclusively spotwelds. They're rigid is as rigid does, and they're also where the suspension mounts. Think of the suspension as the 'input' for forces into the structure.
The passenger compartment, along the rockers, has some spotwelds but we're using the rockers to 'box' the floor section. It's still way weaker that the subframe sections, but it's strong enough to carry the load even without the roof, as in a convertible. The factory will still beef it in a convertible through torque boxes (there's that cereal box again), but it'll hold if we cut the roof off.
Proof:
http://www.imperialclub.com/Articles/EBerg/EBerg5.jpg
Let's go back to the cereal box. Open the top, and flex. The sides don't move straight. It
twists. Remember this.
Remember also that coat hanger that you flex repeatedly. It eventually (work hardens and) breaks.
In your front and rear subframe sections, the body is
not fully boxed. It also
twists. This twisting is what's stressing the hell out of those spot welds. This is what Tony is referring to in his video. Hit a bump on the front right, and every spot weld in that neighborhood gets twisting force (among others) applied to it. The suspension absorbs some load, but force in equals force out, and the chassis still sees the resultant force.
If you've ever spotwelded, you know it's not hard break them by twisting; that's usually the fastest way to break them.
So, what Chrysler has done is to effectively
direct this force to an area where flex is ok: In the middle of the passenger compartment. The center 'box' is flexing to relieve some of the force applied to the subframe body sections. The mid-body cracks cited earlier show this, in all it's glory. You can flex the middle and it's free to twist and flex and there aren't any critical structures there that could catastrophically fail. In fact, the long span means more flex with less force applied to spotwelds in those areas, because the force is distributed in the flexing of the metal as well as being applied to spotwelds.
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.
Edit: the imperial in the link above does not apply, as I just realised it is body on frame. I will update with a photo of a decapitated unibody as I find one.
