I forgot to specify one important detail. At time zero, when
the platform has a velocity v, the pushing force is removed
and the steering cylinder is turned left by a passenger. If the
front wheel were not turned the net force would be the sum
of the three rolling friction forces along the direction opposite
to v. The net acceleration would also be back. How come
that by turning the front wheel to the left the vector sum of
Fr+Fl+Ff becomes the centripetal force directed to the left
(and equal to m*v^2/R )? We know that R, the radius of
the path curvature, depends on the amount of steering. The
textbook shows the net force acting at the center of mass.
Ludwik Kowalski