How to create a closed chain KinematicTree

[ManuelZ]

I think I need to use three ObjectKinematicTrees and some MarkerKinematicTreeRigids, but I have some doubts:

a) Can I add three MarkerKinematicTreeRigids to Tree0 and Tree3 and connect them in three points?
-> Connect Tree0,Body1 with Tree3,Body1 with a revolute joint
-> Connect Tree0,Body2 with Tree3,Body0 through an ObjectConnectorSpringDamper
-> Connect Tree0,Body2 with Tree3,Body0 through a prismatic joint
b) To create the contact with the ground, can I just use AddSphereWithMarker?
c) Does this make sense?

[jgerstmayr]

You can combine several ObjectKinematicTree, but it requires constraints and thus requires an implicit solver. An alternative is to connect several ObjectKinematicTree with spring-dampers, but then you need to find appropriate values.

You can attach a rigid-body marker to ObjectKinematicTree and hereafter use it in GeneralContact.

[ManuelZ]

Thank you! That clarifies the matter a lot.

For future reference: I think one starting point on how to select the appropiate stiffness/damping values can be found in this part of the Drake's documentation: How to choose a force stiffness constant kx or damping constant dx. There is also information in the following document, in the section "Estimating bushing parameters".