Tomas Slancik

Lightgodlike
Right now, if I apply the Car Rig to the truck, and following your instructions everything works fine — but the trailer ends up with no wheels.

not sure what you are referring to as "your instructions" but I assume it's the image with yellow descriptoions
which in general looks like a proper setup for this kind of sim

the immediate thing that comes to mind when you say that the wheels disappear may be that your trailer wheels have the same name as the wheels on the truck

in general it's a good idea to prefix everything to avoid potential name collisions to begin with
additionally make sure that your RBD Unpack has Enforce Unique Name Attribute... checked to resolve any outstanding name collisions

while I personally havent used it, COPs since 21 support UDIMs to certain extend

if you already tried it and hitting issues, you may want to be more specific

your path in
is invalid to begin with, since the node holding the iteration metadata is called foreach_begin1
but even if it was valid, detail() VEX function expects geometry, so either input number, file path or op: string pointing to sop node
since you want to directly point to sop node you need to specify it as

but it may be just easier to connect foreach_begin1 to 2nd input of wrangle and do: