tire Choice 19 v 23mm=23w

gregkeller

Doesn't some of that 19W number come from the fact that the tire may lay more flush to the sides of the rim? When i look at a 25c tire on a wheel it bulges out from the rim more than a 23, i would imagine a 19 lays pretty flush and therefore there would be less disturbance of the air going over it, reducing the drag numbers. I don't know if i explained that well but it makes sense to me

wilmar13

Most if not all of that 19 watt # comes from poor test protocols, rather than the tire change itself.

rikoshea

I did consider this when I examined these numbers. Let's examine the underlying principles: drag force is portional to "drag coefficient (CD) * Frontal Area".
We know that the frontal area is only reduced by ~ 0.0018 m^2 (effectively the difference between the width of the two tires). This means that to obtain the 19W+ improvement would required the drag coefficient to be reduced by more than 9%. For example a CD value of 0.65 being reduced to 0.595. To put this in context this type of improvement is the difference between going from a CD value associated with the same cyclist going from an excellent aero tri-bar position to that of an absolute world class aero tri-bar position - all by changing the tire width by 3 mm.