We do some more research about the relationships between torque, linkage lengths and torsional springs at different angle. It turns out that when we increase or decrease the linkage lengths, the angle does not matter and the whole torque that the patients feel will increase or decrease the same amount and any angle. However, the result is not the same when we use with different torsional springs. At neutral angle, it is the same when we change torsional springs, but at critical high or low angle, it increase or decrease as the shape or parabolic.
To sum it up, the choice of what torsional spring is important to patient comfort.