Originally Posted by
Carbonfiberboy
I think we all know that, no matter how perfectly we try to pedal circles, there's a net downward force pushing on the pedals. The further back our butt, the more torque to hold our torso up we generate about our butt. Were our feet directly under our butt, there would be zero torque. So it's not the angle of hips and back, it's simply the distance of the butt behind the BB.
This is roughly how I see things. When we pedal, we push down on the pedal, which by the laws of physics results in a force being pushed back up on our feet. This force, transmitted up the body through bone, muscle and joint, can help support the torso, and thereby reduce the forces through the arms and hands. The further back the saddle from the pedal, the greater the lever arm and the more support (torque, in CFB's words) our torso receives from the feet. Also note that the higher the pedaling forces, the higher the support to the torso from pedaling.
At the same time, physiology should play a role. If we have weak muscles for transferring the force exerted on our feet by the pedals, the torso may receive less support from our pedaling forces than it otherwise might. Similarly if the muscles are at angles that don't allow them to work as effectively. So, a weak core might mean we don't get as much benefit from a rearward move of the saddle. So, I think physiology is also a part of it.
I suspect that there is more detail than I've described above (eg there are dynamic issues as our legs are moving masses that are constantly undergoing acceleration and deceleration, and our centre of gravity plays a role), but I believe the above is the gist of part of the story. If one draws a diagram showing the forces acting on the body from the bicycle, one starts to see how things play out.
This also means that the analogy to the "balance" we create by moving our butts back when we are standing on the ground and lean forward is not really a correct analogy. I think it is more that we have three points of contact with the bike and the vertical and horizontal forces must balance each other (if the legs are not pedalling, the forces would simply have to add to zero, but it is a bit more complicated than that). When I think in terms of balance and saddle setback, I think in terms of the body/torso seated on the saddle trying to fall forward and the arms and legs suppling balancing forces/torques to support the body/torso.
Getting back to OP's issue, I think the saddle height *could* affect setback requirements because of the physiological aspect (muscles at different angles, etc). I wouldn't guess as much of a role as OP describes, though.