lower control arm 1972 Duster
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...
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
The later B-body LCA - no rivets, no halves, socket/pivot captured completely by the LCA to reduce slop
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.
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...
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
The later B-body LCA - no rivets, no halves, socket/pivot captured completely by the LCA to reduce slop
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.
