I don't expect that you're following all of my posts but please note that I'm trying to play devil's advocate on this debate and to entertain Tony's suggestion. My first post in this thread, I stated that I'd take as much stiffness as I could get in a chassis...and this is because I am weary of metal fatigue and because I know what changes I can make to the suspension and to affect handling where the inherent (and possibly evolving) variables introduced by a an overly flexible structure are not preferable, IMO. Honestly, that's
my intuition. I don't have the resources nor time to dynamically measure contact patch changes beyond chalk line over the sidewall of the tire...and that is telling more about tire pressure than suspension/chassis induced contact patch changes.
How have you used science back up your conclusions?
What hypothesis were you testing? What were you measuring? How did you control the variables? How did these measurements translate into real world performance data? What kind of real world performance were you measuring? Was your hypothesis valid?
I'm not doubting that you have done some actual science here.
Otherwise, your evidence is just anecdotal too, no?
No. The behavior of the chassis is absolutely part of the dynamic suspension system. If you're imagining that you can just stiffen up a chassis to eliminate the variable, good luck. It would take half a Plymouth Scamp worth of metal reinforcement to eliminate chassis flex to the point where it was negligible.
The flex was almost certainly engineered into the Mopar unibody platforms. Unless someone knows otherwise, the pennies worth of sheet metal that they saved by not connecting the front and rear subframes most likely does not explain why they didn't connect them, meaning that it was part of the design...a design which was meant to ride comfortably and perform safely and predictably, which it did very well.
Well, shame on anyone who just takes the word of any internet source without thinking critically. I certainly don't think Tony is right about everything but he has earned my time and attention. As far as anecdotal evidence goes, his is worth listening to, IMO.
Well, on the subject of A-body chassis stiffness and reinforcement pretty much
everyones evidence is anecdotal. I haven't seen any FEA models, and the closest I've seen to a legitimate experiment was run by XV probably a decade ago where they took some measurements with defined loads and compared before and after adding some of their chassis components. And even their experiment was pretty basic, and wouldn't necessarily tell you anything about how that would change real world performance or how the chassis would hold up over time. Quite frankly no one that has the capabilities to do that kind of all-encompassing testing cares about how an A-body would perform. The real racers build cages because their classes require it, those of us that run on the street are left to figure it out on our own.
There are a lot of people with experience, but of course they fall on different opinions. It's pretty easy to find people that have added big blocks and sticky tires and have done things like pop rear windows, form cracks at the quarter seams and before Nader pins even pop doors open, there's a picture of that right here in this thread. But those are anecdotal examples, just like the usual "I jacked my car up and it bent less" story. Which is common and still true, but still doesn't tell you about real world performance.
As far as the science that I apply, yeah, I have a degree in engineering and I built chassis' and did FEA analysis as part of FSAE teams in college. I know the science behind these chassis, even a lot of what was considered when these cars were built. And that's the biggest mistake most people make, they assume the factory did the best thing. Well, two things about that. First, they did the cheapest thing they could get away with, and only designed these cars to last about 10 years or 100k miles at BEST. Second, they were building a chassis for the time, which meant bias ply tires, super soft suspension for comfort that was the selling point of a lot of stuff in the 70's, and safety standards that would make most modern engineers shudder because that's just what they knew at the time.
And that's how I apply my training. These cars had very little tire grip when they were built, and bias plys have very stiff sidewalls. So the chassis and the suspension were very soft and flexible. This is my opinion, but they even missed the mark on how soft the suspension was, right from the factory, even for bias plys. When you add those considerations to results of what happens when you add weight and power - cracked chassis points, flexing so much window pop out, that sort of thing, then it's the logical conclusion that the chassis is way too flexible when you start adding power, weight, and tire grip. Tony misses here because he only considers stress fractures from an overly stiff chassis, which definitely can happen if a chassis is too stiff at certain points and the material can't flex enough to distribute the load. But that's not what these cars are, they're too flexible and you don't need an FEA to figure that out. These cars crack because of overwork, they work harden from flexing too much. To someone with no education or training those cracks look the same, even though there are two completely separate causes on opposite ends of the spectrum.
No, you can't ever take chassis flex completely off the table. But that's not what your SCCA champion vs a stock miata on the back roads example was. Yes, the SCCA car still has flex. Probably a lot less than a stock Miata. But you conjectured the stocker might do better because of road conditions. And hell, it might. But that performance isn't because of the chassis at all. It's because of how the suspension is set up. The SCCA car would win handily with the suspension set up for the right tires and road conditions with the same chassis. Compare a NASCAR to a Trophy Truck. Both chassis are VERY stiff- the trophy truck does not have a flexible chassis for the rough terrain. The NASCAR doesn't get stuck 100 feet off the line at the BAJA 1000 because its chassis is too stiff. And the Trophy Truck doesn't roll over in turn one on the banked oval because of its chassis. That's suspension set up, and so is the SCCA champ vs a stocker on a bumpy back road.
Nascar
trophy truck
Yeah, my evidence is pretty much all anecdotal. I've put 70k miles on my Challenger with no chassis reinforcement. When it had hockey pucks in 215/60/15 and 235/60/15's on it with 1.12" torsion bars and XHD's out back the chassis flex wasn't that terrible and it was a RUSTY beast. 100% tire limited and set up too stiff for the tires I was running. When I put 275/40/17's on there pushing hard on the winding mountain roads I like to drive would induce some down right
spooky chassis flex, you could literally feel the chassis twisting and flexing and that is NOT a good feeling. It was worse than most because it had even more rust than I thought. I don't get that with my Duster and all of it's chassis reinforcement, even though the 275/35/18's and 295/35/18's I run are even grippier than what I ran on my Challenger, my springs are stiffer and I've probably doubled the horsepower. I still have a decent amount of suspension travel because of how I've set up my suspension, and it does well on back roads. Even with 1.12" bars it would probably still be too soft for track tires on a nice smooth road course. And the same goes for the chassis, I'd need a cage if I really wanted it to be consistent at high speed on longer tracks with better tires. How will all that hold up in 10 years? I don't know. But I know without any chassis reinforcement I'll start popping spot welds and showing up cracks, so, hopefully better than that.
