Jack, we’ve wondered the same thing and have looked into a relevant aspect of human perception: the Just Noticeable Difference (JND), especially as it applies to vibration transmission of different bicycles.
The concept of the Just Noticeable Difference comes from the field of psychophysics and is defined as the difference in a stimulus which is detectable as often as it is undetectable. In other words, a greater difference is reliably detectable, and a smaller difference is reliably undetectable. The just noticeable difference (JND) is at the edge of being reliably detected.
Some examples of just noticeable differences in familiar measures are the 4.8% change in loudness required for humans to detect a change in sound level and the 7.9% change in brightness of a light. These values will differ from one person to the next, and from one occasion to the next. However, they do represent generally accurate values.[ii]
Considering bicycles in this context becomes interesting because the difference in vibration transmission between different road bikes can be hard to notice sometimes. Some folks seem to notice everything, while others can’t seem to notice anything. And some riders notice differences that aren’t even there! So we decided to test peoples’ just noticeable difference.
Figure 1. The test procedure: (1) recording, (2) test rig, (3) human perception
To conduct this test, we:
(1) Recorded the vibration from the road
(2) Constructed a two-post vibration test rig in the lab
(3) Asked human test riders to tell us which altered road signal has more vibration
Each test subject compared two simulated roads back to back. The only difference between the two simulated roads was the amplitude: 100% of the real road recording, 95%, 90% 85% or 80%, so that the differences tested were 0, +/- 5%, 10%, 15% or 20%. Each subject made a total of 100 of these comparisons, with random pairs of various percentage differences.
No test riders could notice the smallest difference, 5%. Many riders could notice a difference of 10%. And most riders could notice a difference of 15% or more.
The bottom line is this: different bikes can transmit vibration differently, and sometimes some riders can just notice the difference. In your case Jack, either the bikes you’ve tried aren’t substantially different in how they transmit vibrations, or you’re one of the insensitive riders, or perhaps a combination. Either way, consider yourself lucky – among bikes you’ve compared, you don’t have to worry about comfort and stiffness differences. Instead, focus on things that make a difference to you: “wind tunnel, paint and price!”