Badly explained but.....
The problem is that the seat is NOT a 'rectangle' in a single PLANE.... it is a massively distorted 3 dimensional shape. Each of those distortions out of the single plane removes almost ALL of the stiffness obtainable within that plane by piece of material in a SINGLE plane.
Test your theory for YOURSELF! Hold the seat in your hand and then twist the seat in the way YOU suggest it RESISTS twisting. You'll see quite how little it resists twisting! Now hold the SEAT STAYS and try to twist the chassis in the way that you suggest the seat RESISTS. I estimate that the resistance of the chassis will be VASTLY more than 10 times that of the seat! At best, that would mean that a seat would add 10% more resistance than NO seat but that the differences between Seat A and Seat B would be almost immeasurable! I do NOT say that there IS NO difference, it's just that the differences would be MINUSCULE!
The second point is that another direction of twisting in the plane of the 'flat rectangle seat' MIGHT work...... except that it would require and INCREDIBLY strong 'grip' between the seat stay and the seat or else the it would just slip rather than applying torque.
Express which direction of 'torsion' you mean and it'll be easier to discuss. Do you mean the resistance to moving point A out of the (non existent) plane ABCD or do you mean the resistance of moving point A NEARER to point B within the confines of that plane? We should both know that it's the latter that has the highest POTENTIAL resistance.... but is that the direction you are claiming?
Ian
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