LookAt is a simple constraint (not and IK constraint – the video http://bongo.rhino3d.com/video/simple-constraint-versus-ik-in-bongo-2-0-a-clarification illustrates the difference). In the Excavator model it can used for the hydrolic cylinders because they are symmetrical in regard to the XZ-plane - a rotation of 180° isn't noticed. Do observe the pivot of object ‘cylinder_C’ making a twist (when it becomes vertical) between ticks 34/35 and 56/57 and 88/89. LookAt is used in the Excavator-model not to overload the IK with constraints hence slowing down.
The twisting (reverse 180°) of the LookAt is due to the way 3 dimensional directions are computed. Maybe @Lars can tell us something more about it.
Rolf’s step 1 is surely right: making the ‘Trunion’ object a Hinge will avoid any twisting – a Hinge will only rotate about the specified axis .
But then the connection to ‘Jackshaft’ and his ‘followers’ needs to be taken care of:
• ‘Jackshaft’ must be connected (child of) to the ‘Trunion’ in a sliding manner (Telescopic). In order to get the sliding motion right its pivot needs to be rotated so that the Y-axis (it could as well be Z) aligns parallel to the groove in ‘Trunion’.
• ‘Jackshaft’ also needs to follow JackshaftFocusPoint. You could make ‘JackshaftPin’ child of ‘Jackshaft’ and (IK-)constrain it to JackshaftFocusPoint, but in the model I used the ability of objects to have a double function “Joint and constraint’ on ‘Jackshaft’ itself. Simplicity first.
• The “followers” can then simply be made child of ‘JackshaftPin’
RetractsTest 002.3dm (288.0 KB)
I also made an alternative for ‘ActuatorRam’ thinking in reality it wouldn’t scale but rather slide (like a telescope). I needed to elongate the object to make is feasible though. Good idea?
For “the strut needs to move rearward and rotate so that the wheel axle points up.“ I think a simple parent/child relation Trunion/Strut will do?
• I don’t see any “goal not reached” messages on my system??
• The reason the IK-decoration (rubberband) turns out soOOO big is because the objects in your model are very small. I see you turned down the size of the pivot to 1 (instead of default 5) to have proper proportions. Unfortunately the size of the decoration isn’t (yet) size-able. I disable their display in Bongo's Options to get them out of the way.