Bob wrote "The keel effect of the bearing is real". Hmm.
Why don't they say it in textbooks? "The net force at the
C.M. , directed toward the center of path curvature is due
to the kill effect on the bearing". Are we outside the area in
which free body diagrams (FBD) are useful? I do appreciate
Bob's quick reflections very much; the sarcasm is directed
at authors who think that textbook drawings are self-evident.
Yes, the orientation of the front wheel (or two front wheels),
and the fact that the wheels are rotating, are certainly essential.
[The masses of wheels were declared to be negligible to avoid
issues associated with the angular momentum, etc.] The task
is how to explain the "kill effect" with in a FBD. There are
only three external forces (road acting on the rigid platform)
in our simple model. They are applied through turning wheels.
I woke up thinking that wheel were used for more that 5000
years and we are understand them. Sorry for the little Internet
"boxing match" with Bob last night. It did help me a little.
I hope somebody will add more clarity today or tomorrow.
Ludwik Kowalski