[ddmckee54]

Constraining things to a plane is what I do - sometimes.

I placed an axis in the centerline of the axle bores, and I've got an axis on the pinion shaft centerline for each axle. The origin for my suspension assembly is the tail end of the center of the frame rails at ground level. I've got offset planes where the center of each axle shaft should be located. I've also placed a plane at the ride height. I've constrained the axles so they can move vertically along the locating plane and the pinion axis is constrained to the YZ plane, this keeps the axle where it belongs front to back and side to side.

I constrain the axle shaft axis to the ride height plane when I initially place the parts/sub-assemblies. When I want to check suspension clearances I suppress that constraint. When working in the various sub-assemblies I will constrain the various parts to get them properly oriented, anchor the part, then delete the effected constraint/s.

I did something similar with the front axle assembly. When I was initially placing the CVA axle and the CVA half-shafts their axis were constrained to be coaxial. Once I had everything placed properly I suppressed the coaxial constraint, and constrained the axle so that it could pivot around what are effectively the king-pin bushings.

I stumbled across a "Flexible" check-box and hoped I'd found the mother-lode regarding the "moving" sub-assembly parts no longer moving issue. Alas, there was no joy in Mudville that night since it made no difference. I've been doing a little digging on-line and it seems that I was on the right track.
That option IS supposed to fix the no longer flexible issue - but it doesn't always work.

I was able to use the assemblies to check clearances, what broke the rear suspension assembly was trying to get the walking beam to play nicely. I'm pretty sure the full scale rubber bushings are there to absorb any minor alignment problems, and I've got silicon O-rings to substitute for those bushings in the model. I just don't think Alibre assemblies were designed to simulate something like that.