lower control arm 1972 Duster

72bluNblu said
Moving the goal posts? Yeah, no. Still just talking about the slop in the LCA and why it's relevant.

My point in bringing up all of the different forces acting on the LCA was not to highlight the fact that there is flex in the LCA structure, because of course everything flexes. My point was rather that the SLOP in the LCA, which you say isn't important, is in fact an issue because the LCA is constantly subjected to loads that don't just act rotationally at the pivot point. Which means that the slop that's highlighted in Jim Lusk's video between the torsion bar socket, pivot arm and the halves of the LCA is in fact an issue. Because if those forces can flex the LCA, they can cause the LCA to move around in all of that slop. They can even spread the halves of the LCA which will allow the LCA to move about on the socket/arm. And if they can do that (they can), then the slop between the halves and the socket is an issue that should be corrected. You want the LCA to just pivot about the socket like it would in a simple force diagram, and any movement of the LCA around the socket will create alignment changes that you don't need. But doing that means the LCA has to be able to resist the forces that act in different directions, which it won't do if it's sloppy at the socket end.

I included the video only because it shows that even on a simple drive around the block (which is what was happening in the video), there is significant forward/backward movement of the LCA, even just under very boring driving situations. Maybe that wasn't news to you, but it isn't something that a lot of people understand. And because the video shows that the LCA does in fact move around forward/back it further highlights that in the real world you don't just have a simple rotational force at the socket. The video was an attempt by another member to locate a different issue, it's not mine (as I stated).

The diverging arcs of the strut rod and LCA are important, but only in that you need to make sure that the LCA and strut rod are not binding within the range of travel of the suspension (which is easily done even with delrin LCA bushings and adjustable strut rods). This is where you didn't follow at all. The arcs of the strut rod and LCA overlap well enough in the range of travel, and the ~5" to ~5.5" of suspension travel most of these cars have from the factory is a relatively small section of the total arcs created by the LCA and by the strut rod. You try to equate that relatively large suspension travel to the relatively small amount of slop/flex in the LCA, but those are completely different things. The suspension travel is a lot larger, but that distance is small compared to the diverging arcs. The slop and flex is much smaller than that, but it results in movement in the ball joint, which means caster and camber changes that are not built into the suspension geometry. That slop is small, but it results in variable changes in the geometry that are reactive to changes in load, which is what creates vague handling. The changes that occur on the defined arcs are what we plot out as camber gain, bump steer, etc, and because they're created by defined geometry that isn't variable we can plot it, evaluate it, and correct it if necessary. You can't do that with the changes you get from slop.

The QA1 LCA is also more rigid than the factory LCA's, so your claim there is false. Although this is obvious if you actually have seen both of them side by side or run them on your car, it was actually tested by Mopar Action in the April 2011 copy. This was actually when CAP was still making the arms, before QA1 bought them out and improved the design (they're stronger now). Mopar Action tested the factory LCA supported at the load points, the rivets popped at 2,800 psi. The CAP arm didn't fail until 7,700 psi (must not have had bad welds that CAP became known for), when it still failed in bending. From the article...
View attachment 1716188860

Regardless, this isn't just "postulation" on my behalf. I certainly didn't come up with the idea to box and strap the factory LCA's, that was done by the guys that were circuit racing these cars decades ago. Probably the most telling fact though is that Chrysler redesigned the control arms on the later cars, starting in '73 on the B-body cars and continuing from there on the later cars that were heavier and got factory radial tires.. The LCA's on those cars are not halves connected by rivets like on the A's, and their construction surrounds the torsion bar socket/pivot so the slop between the socket and the LCA is greatly reduced.

A Chrysler "kit car"- notice the large strap on the LCA tying the halves together
View attachment 1716188859

The later B-body LCA - no rivets, no halves, socket/pivot captured completely by the LCA to reduce slop
View attachment 1716188861

So, do what you like. Doesn't hurt my feelings if you leave the slop in the LCA's on your car alone. But, having driven Mopars with factory LCA's, boxed and strapped factory LCA's with the slop removed, and the improved QA1 LCA's, well, the handling is more responsive and less vague than the unmodified factory stuff was. So, I'm going to keep boxing and strapping the factory LCA's if I use factory LCA's at all.

So when was the last time you EVER heard of an OEM mopar LCA failing by popped rivets of the literally millions out there for over 6 decades of use?
Why is nobody marketing LCA high strength bolts as a weakest link rivet upgrade?

You haven't because the test loads are never achieved under ANY driving conditions that we here face. The reason being the real world loads are mostly limited by a torsion bar rated in the 200lbs/in and travel can't exceed approx 5 in with a preload of less than 1000lbs, hitting wheel destroying potholes or banked concrete retaining walls excluded. IF the LCA is flexing, and as agreed to earlier they ALL do, upping the TB rate ever so slightly completely offsets that minute flex.

I am skeptical of the CAP reported test results above anyway, as to what the complete test conditions were or if the test was skewed to achieve the results reported.

I have not seen the QA1/OEM LCA side by side. I have QA1 at PRI and in press releases. I find the claim unsupported and likely misleading and inconsequential anyway as to the "flexing" concern. My initial impression at PRI was how frail compared to OEM they might be but couldn't decide why they were not up to the task.
I completely disagree the LCA TB pivot points is to suppose the prevent LCA movement F to R, that is the sole task of the strut. If it was the LCA bolted single shear pivot point is a very poor engineering design with now even more tasks.

You note "small" in your ball joint movement concerns. You fail to quantify that. When you do you might want to mention how relative that movement is to those running say 40 aspect and up sidewalls. Not sure what OEM "slop" camber reduction method has been shared here anyway short of aftermarket replacement.
The lack of concern of conflicting arcs you voice, albeit minor, is mainly because the dreaded "slop" allows it.

The Kit cars were designed to reduce owner maintenance costs and improve reliability on Sat night wheel to wheel grudge rubbing and racing. That hardly applies to our uses.

The later B LCA pictured IMO seems to be actually a weaker design than the earlier design and its main success from a corporate standpoint, it was an easier/cheaper item to mass produce in quantity with a lot less parts and labor, and any "dreaded" slop reduction was simply a byproduct of the process. I may be wrong. You are welcome to prove me wrong.
However, since this newer design is fundamentally loaded much different than the older version, it really should not be included in this discussion.

Still nobody has yet quantified the downsides of the dreaded "slop" in real world use. IMO, you and others have drank the marketing koolaid on a problem, that in realty has not yet been proven to or may not exist.