This is pretty much the same kind of discourse that occurred prior to PD4 being implemented. Lots of speculation, lots of contradictory opinions and data etc.
Zwift is a unique riding environment. No one knows what the overall impact of changing CdA will be (on group dynamics, speed in races and the fun-factor) until it’s tried, tested and refined.
I think we should all keep an open mind and let’s see how it pans-out. It’s not a done deal, it’s just an idea at this point.
I played around with chatgpt to test out some CdA calculations.
Using the Faria et al. formula (aerodynamic position with aero-handlebars) for estimating the frontal area (A):
A = (0.0293 * H^0.725) * (M^0.425) + 0.0604
Cd=0.55
H=height, M=mass
| Weight (kg) | Height (cm) | Frontal Area (m^2) | CdA (m^2) |
|---|---|---|---|
| 40 | 135 | 0.348 | 0.191 |
| 45 | 142 | 0.377 | 0.207 |
| 50 | 149 | 0.403 | 0.222 |
| 55 | 155 | 0.425 | 0.234 |
| 60 | 161 | 0.446 | 0.245 |
| 65 | 166 | 0.464 | 0.255 |
| 70 | 172 | 0.481 | 0.265 |
| 75 | 177 | 0.496 | 0.273 |
| 80 | 182 | 0.511 | 0.281 |
| 85 | 187 | 0.525 | 0.289 |
| 90 | 192 | 0.538 | 0.296 |
| 95 | 197 | 0.550 | 0.303 |
| 100 | 201 | 0.561 | 0.309 |
| 105 | 206 | 0.572 | 0.315 |
| 110 | 210 | 0.582 | 0.320 |
Weight is the total weight of the rider.
I’m not sure if I got the Cd part right. Cd can also be estimated based, but that complicates things.
A Cd value of 0.55 is very low. I have seen values in the 0.7-0.9 range. It could also be that they are using another formula for the frontal area.
I’m not sure if this is way off?
EDIT: Changed total weight to the only rider. CdA for bike and wheels is probably added as fixed values in addition.
I don’t know much about this stuff, but intuitively I would think that the CdA will be very sensitive to the combination of real-life weight and height. In these examples, I’m assuming that these values scale in some kind of ideal fashion i.e. a very athetic physique is assumed in all cases as the rider gets bigger/smaller?
What happens, for example, if you take the 95 kg case, but reduce the rider’s height from 197 cm to, say, 183 cm? How much does the CdA change.
Similarly, what happens if you take the 65 kg case and increase the height to, say, 175 cm?
I will create a diagram for it later. I’m om a phone now.
Your CdA is not a result of your size and weight. It’s a result of how aerodynamic you get on a bike. Hence aero bikes, TT bikes, skinsuits etc.
2 riders of different height and weight can have the same CdA.
Zwift does not work like real life. It uses game physics.
Wow, thanks for taking the time to run the numbers.
I told you I didn’t know much about this; I was really expecting a much bigger change in CdA 
I wonder how Zwift estimate their CdA values; I’m presuming there must be a height & weight component to the calculation; maybe not?
kreuzotter.de uses a formula for rider frontal area that include the position on the bike.
There are many formulas to calculate frontal area. The problem is to know which one Zwift uses.
Cd is used to factor in the rider position:
Upright position: Cd of 1.0 - 1.2
Drops position: Cd of 0.7 - 0.9
Aero position with arms parallel to the ground: Cd of 0.5 - 0.6
Aero position with arms tucked in: Cd of 0.3 - 0.4
Shouldn’t this simply be the rider’s weight (see Gerrie’s post)? Adding in the bike’s weight will introduce a bigger contribution (error) for the smaller/lighter riders. Not sure how much of a difference this will make to your CdA values?
Good point. I have to check it out. I was just assuming it had to be both since both the rider’s area and the area of the bike will have to be in the calculation for it to be correct. It might be that they just use a fixed CdA addition for the different bike+wheels combinations in addition to the CdA for the rider.
I’m 175cm talk and lighter than that so I appreciate the efficient aero. IRL I can tuck down well on the bike still. The kit I wear is form fitting - and it still fits.
On the weights, the bike should also be lighter for smaller riders as they ride a smaller bike. The smaller bikes can be a lot lighter.
The large bikes quite a bit heavier.
You seem to have mistaken Cd for CdA.
CdA is Coefficient of Drag (Cd) x Frontal Area (A)
A perfect sphere has a Cd 0.47. Cd is constant regardless of the size of the object.
CdA changes depending on the size of the object because A is equal to the frontal area of the object. 2 riders of different height and weight may have the same Cd (although this is very unlikely, taller leaner riders will have a lower Cd than shorter rounder riders because aerodynamics but let’s pretend for this example that they have the same Cd) but they certainly will not have the same CdA.
A sphere that is 1 metre in diameter has a CdA of 0.47 x 3.1415926 x 0.5^2 = 0.369136825
A sphere that is 2 metres in diameter has a CdA of 0.47 x 3.1415926 x 1^2 = 1.4765473
Notice how the CdA is 4 times for the sphere? That’s because the frontal area is 4 times larger so you’re going to need 4 times the amount of power to move that sphere down Fuego Flats.
Now, I don’t know if Zwift adjusts Cd based on height and weight (for a truely accurate model they should but I don’t know if they have that data) but I’d be willing to bet they adjust the frontal area based on height and weight and therefore the CdA.
I do an agree BUT a sphere has no ability to change it frontal area. A cyclist does. Hence wind tunnel testing etc.
David already said further up in this thread that CdA is different for different hieight and weight riders in game.
Cda can be different because of different A but same cd OR because of different cd and A…
50kg 168cm rider should have very low cd and A when compared to a 80kg 168cm rider (who can’t be very aero with their big belly…)… he is wondering if this flexibility thing which changes the cd is taken into account, or if only the A is taken into account.
Zwift can’t tell if the 80kg/165cm rider is a flexible body builder or a sofa potato. Assuming one or the other can get wrong.
Cd is not static and can be affected by:
I doubt Zwift takes all this into the calculation.
Well Aleluja…i wonder who has a better CdA in a windtunnel test 168cm/80kg or 186cm/80kg…it’s like comparing Basketball with an Arrow…but in Zwift Basketball wins 
Maybe Zwift needs to introduce a third possible restriction to races: “camera required”. Then you need to set your camera up in front of your bike, configure it, and Zwift uses the camera to see your real-time frontal area while you ride - you also need to put a fan in front of you so Zwift’s CdA can notice and react accordingly to flapping clothes etc. 
I am sure that would run really well on mobile devices and old PCs. It has been a dream of Apple TV users to Zwift at 10 fps after all 
