marcag wrote:
however, chest movement is not that exciting. When on your pads, your chest is 99% in the same spot all the time. If you sit up, sure it changes. But a set of pressure sensors on the pads, would be as almost as good. I am over simplifying, but not much. This is not where the gold is.
The head : now that's another story.
Most riders can move their chest position by about 3-5cm while resting on the pads, wide shouldered riders can move by as much as 7cm. You can try this without a sensor just using a tape measure from the top tube to the center of the riders chest. For most riders their most aerodynamic position will be with their head stretched forward towards their hands, this will also result in the lowest measure chest position. As they tire, they stop stretching their head forward and their chest position raises.
Our experience is that chest position has a CdA sensitivity of about 2 to 3 times greater than the effect of a riders head height. In general, the head and chest move together, with the head movements being larger in magnitude, but not 2 to 3 times greater. So we find the head movements less sensitive than chest movements.
For a rider whose upper shoulders are above the mid back, we find that CdA moves by 0.003 to 0.005 per centimeter of combined head and chest movement. I will also put forward that CdA reductions from stack height follow the same slope as CdA reductions from chest and head height changes, sometimes stack is less sensitive as riders on a very low stack tend to lift themselves up, reducing the effect of the lowered stack. You can see similar results in a wind tunnel when using optical tracing from the side profile.
Chest and head height are not the only thing, but they are the most sensitive thing, and most riders when fatigued do not keep their stretched aero position, but sit up higher, all while remaining on the pads.
John Buckley
https://streamlines.aero