Typically, when representing frame rigidity, BB rigidity is mentioned, and when representing ride comfort, seatpost rigidity is mentioned.
While this distinction is intuitive and has some validity, there are limitations in generalizing it as a standard for complete bikes, and in particular, responsiveness and ride comfort must be understood as having different structural characteristics.
First, regarding responsiveness, the responsiveness of a complete bike structurally has characteristics similar to a series connection system. The force generated from the pedals is transmitted through the crank, BB, frame, and wheel to the ground, and if any single part lacks rigidity in this process, deformation occurs at that point, which immediately leads to a decrease in overall responsiveness.
Due to these characteristics, BB rigidity is not a factor that solely determines responsiveness, but when it falls below a certain level, it acts as a 'limiting factor' that is difficult to compensate for with any other configuration. In other words, it is not the only one, but can be considered essential. Without sufficient BB rigidity, no matter how high-rigidity wheel sets or other components are used, there is a clear limit to improving the responsiveness of the complete bike.
On the other hand, ride comfort has a completely different structure. Impacts coming from the road surface are transmitted and dissipated simultaneously through multiple paths: the tires, wheels, various parts of the frame, seatpost, saddle, and so on. In other words, ride comfort is not serial but can be seen as similar to a parallel and complex system where multiple elements act simultaneously.
Therefore, seatpost rigidity is only one of several factors that affect ride comfort, and if anything, the expression one of them would be appropriate. In fact, since ride comfort is formed by various variables such as tire setting, wheel characteristics, frame structure, and rider posture, it is difficult to represent the whole with just one specific factor.
Moreover, even if we look at the frame alone excluding wheels and tires, ride comfort cannot be explained by seatpost rigidity alone. Since various factors such as the shape and thickness of the seat stays, the structure of the chain stays, the flexibility of the top tube and seat tube, the cross-sectional shape of the tubing, and the material and thickness distribution act together in the frame's ride comfort, it is reasonable to consider that ride comfort is ultimately the result of the comprehensive compliance created by the entire structure of the frame.
Nevertheless, the reason seatpost rigidity is frequently mentioned as a representative factor is that, whereas making the chain stays or wheels flexible often involves some sacrifice to responsiveness or efficiency in improving ride comfort, the flexibility of the seatpost can effectively mitigate the shock transmitted to the rider with almost no impact on pedaling responsiveness. In other words, while seatpost rigidity is only one of several factors that constitute ride comfort, it is characteristically mentioned as a efficient adjustment factor that can improve ride comfort without performance loss.
In conclusion, the responsiveness of a complete bike has a structure that is limited by the weakest part, similar to series connection, and in this process, BB rigidity acts as an essential limiting factor. On the other hand, ride comfort has parallel characteristics where multiple elements act simultaneously, and seatpost rigidity is only one of them.
