It is important to emphasize that in the foot there is only one center of rotation . Once the shoe is nailed or otherwise fastened to the hoof, the shoe becomes mechanically a part of the hoof, and moments affecting the shoe operate around the center of rotation in the distal end of the middle phalanx. The resultant R defining the position of the force F . In order to determine the position of F we must know the position of the resultant R , Figs.1 and 4. Remember that R is normally parallel to the horn tubules of the hoof wall and remains parallel no matter the position of the pastern during movement.
Fa is clockwise - negative.
The situation with moments is somewhat more complex than with linear forces. One can accomplish that by trimming heels or toes or by the use of wedges or wedge-shaped shoes. While there has been disagreement in the past about this, I believe most now recognize that this is what happens. It is of interest that the change of pastern orientation with change in hoof angle was well know to Lungwitz among others in Europe at least as early as the 1880s. As already noted the bar shoe, per se , can only decrease the value of S , the linear force per unit area of the bearing surface, if it is in contact with the frog.