As I've already commented, I think that slip force and rolling resistance moment are the two main loads that could play a role efficiency-wise. Consequently, I have made an effort to complexify their models as much as possible. The following 4 sheets explain the contact model and the loads acting on the wheel (chain, reactions in the axles, normal force, tangential force and rolling resistance moment).
After reading these sheets, you can imagine why the solver has problems under certain conditions. Complexity is very high.
That's all for today