Unexpected behavior when moving unrelated joints in an assembly

Hello,

I am modeling my 350Z's front suspension in Fusion. I extracted the suspension mounting locations from a 3d scan and have modeled them as solid objects to mount the suspension components to. I have also modeled a few control arms as separate components. The bushings and ball joints are modeled as spherical joints in separate sub-components and sub-assemblies. The component with the mounting points is grounded to the top level of the assembly, and the control arms are attached to the mounting locations using rigid joints that connect the inner spherical bearing spacer to the mounting point.

When test fitting a few of the components to the front suspension mounts, I'm am observing the following behavior:

* When the upper control arm is not attached to the upper mounts, you can drag the lower control arm via one of the modeled components and it will move freely

* When the upper control arm is attached to a single mount, the lower control arm moves freely and the upper arm also moves freely

* Once the upper control arm is attached to both mounting points, the lower arm either cannot move via dragging, or if it does move it also causes the upper control arm to move. Moving the lower arm left, right, up, or down causes the upper arm to move up and down.

* When moving the lower arm via "animate joint relationships / motion study" on the revolute relationship of the spherical bearing in the lower arm, the upper arm does not move

* If I attach the control arm to the rear mount via an "as-built > rigid" joint, then suppress the lower control arm in the timeline, the "as-built > rigid" joint changes position and is no longer in alignment with the mount. The components that make up the joint were aligned to the construction axis present on the mounting point, so I did not expect this behavior.

* If I attach the control arm to the rear mount via an "as-built - rigid" joint, the upper arm moves very erratically when dragged, despite the bearing design facilitating smooth operation when attached to the other mounts via rigid joints

* If I attach the upper control arm to the rear mount via a cylindrical joint or revolute joint, the upper arm operates more predictably, but still not as smoothly as the lower arm. Also, the upper arm will still move when dragging the lower arm, despite them not being connected with each other.

I have attached the project file. The issue can be observed by removing / re-adding a joint to the rear mount of the upper control arm.

I've also created a post in the Autodesk fusion forums just in case it's something basic they can help me figure out.

Best Regards,

Levon

Hi Levon,

Sorry I don't have time at the moment to go through your file and have a good look at this. But I have had similar experiences when building suspension assemblies.

I think you are using all the correct approaches with separate components and your idea of using spherical joints inside the component with subcomponents and then "as built" rigid joints is great in theory. But I find that fusion tends to have some issues with this.

I usually try to represent all my joints within the single "master" component. To be honest I don't even set up a joint between the bodies of my spherical bearing components anymore because it's not usually necessary and often causes issues.

Rather I'll just create a spherical joint between the control arms and chassis and use references to control the position. Hopefully that makes sense.

Cheers,

Connor

Hi Connor,

Thank you for taking the time to reply. I had a hunch that was the case and am reworking some of the models to simplify some of the joints where it makes sense and hopefully reduce how much math Fusion needs to do during the motion simulations.

As for using references to control the position, is there an example of this workflow in any of the video courses?

EDIT: I think what you're describing just clicked for me. Are you saying that you use references to keep the bodies from moving around in the sub-component / sub assembly design file, and then in the "master" assembly you define the joints there?

Many Thanks,

Levon

Yes that's essentially what I'm saying.

By references I just mean solid bodies, datums or sketch points in each sub component that we can pick up on when building the joints in the master component assembly.

This might help: https://www.hpacademy.com/previous-webinars/384-suspension-assemblies-in-cad/

Perfect, will give this a try soon!