But you never know the power and weight for sure, right?
This comment referred to the specific example of Indurain's hour record, were all the parameters are known (approximately). For climbing efforts, I use 60kg standard weight for all calculations. I also use era-adjusted standard CdA and CRR for every rider based roughly on an average 60 kg rider.
Someone who is more aredynamic than normal has to push less power than my formula spits out, but being aero is also a skill.
At what speed does CDA really start to matter +/-?
Could anyone provide a practical example of say 15, 20, 25 kph for climbs?
And how much is the difference between the most aero and least aero rider on a 25 kph climb in terms of watts/kg?
CdA is essential for calculating the watts needed to overcome air resistance at a certain speed.
Let's use an
unrealistically extreme example: Rider A with a CdA of 0.25 (mythical ultra aero climbing position), Rider B with a CdA of 0.4 (very unaero, more standing efforts).
15kph: Rider A needs 9.8W <--> 15.8 W for B
20kph: Rider A needs 23.3W <--> 37.3 W for B
25kph: Rider A needs 45.6W <--> 72.9 W for B
30kph: Rider A needs 78.8W <--> 126 W for B
This is for a climb in standard conditions. The exact watts can change depending on weather, draft and altitude.
So at 20kph the difference would already be 14 Watts = 0.23 w/kg in my example. Of course, in reality the differences in CdA are not that big (unless someone is riding out of the saddle all the time). More draft and higher altitude also lower the consequences.
More realistic example:
The CdA is just one factor in the formula and effects the watts proportionally. Double the CdA means double the watts needed to overcome air resistance. If rider A has a CdA of 0.3 and rider B of 0.3333, then Rider A only has to use 90% of the Watts to overcome air resistance at any given speed.
15kph: Rider A needs 11.8W <--> 13.1 W for B
20kph: Rider A needs 28W <--> 31.1 W for B
25kph: Rider A needs 54.7W <--> 60.8 W for B
30kph: Rider A needs 94.5W <--> 105 W for B
At 20 kph, the difference would still be relatively negligible, at 25kph it is ~ 6 Watts = 0.1 W/kg. You can also see that around 1 w/kg is needed to overcome air resistance at 25kph. The rest is divided between gravity (lion's share) and rolling resistance (and potential changes in velocity).
(All examples are for solo efforts with 0% draft. As fo the least and most aero rider, I have no idea. If you take riders with huge size/weight differences, then big diferences in CdA are possible. CdA also depends on the climbing style)