Woohoo! @Menithal and I were in the sandbox domain having wild rides with entities
He went off to sleep, so I’ll write up what we discovered, but the nature of he bugs are such that small targeted fixes can open up the ability to sit, ride, and have complex physical assemblies.
The first experiment was to see how an assembly of entities parented to another behave when physical. The first problem is that when you have such an assembly, the child objects do not inherit the parent’s physical properties, so very strange results occur. Second, when this is manually rectified (all entities of the assembly are made physical) they still interact, with respect to collisions and interpenetration, as if they were individual entities. But since they are linked, that too is observed in the simulation. The result is that two entities that are interpenetrated, when linked via parenting and then made physical, go into a never ending jitter spasm bouncing around constantly. When that assembly hits an avatar it packs quite the wallop.
Here again is the bug, linked entities must not have interpenetration or collision simulation between themselves.
A general case of this issue is that the linking relationship between a child and parent entities and/or child and child entiries requires parameters to define the interaction. It should range from no-interactions to some spring force & dampening factor for each local axis.
Do that and all of a sudden you get some very nice vehicle possibilities: the child tires of the parent car body will jiggle up and down slightly as the vehicle moves along the surface - all handled by the physics and physics framework code. But if that is too much then at the very least there needs to be a no-collision, no-interpenetration set of switches that apply to child entities.