CdA dependency on height issue

The calculated CdA of a rider in Zwift depends on the riders weight, height and equipment, which is all good. However, how the CdA is affected by the riders height is completely unrealistic and unfair. This issue has been discussed on a number of other threads here on the forums, aswell as zwiftinsider also posting a article about it. Here are some examples:

There is a youtube video called “Why riders Height Dope on Zwift | Tall vs Small”
This is a test where a 70kg rider holds 300W on a course 4 times with the only factor changing being the riders height. 155cm 170cm 185cm and 200cm. In short, the results are:
40:28 for 155cm
41:15 for 170cm
42:09 for 185cm
42:40 for 200cm
Comparing the 170cm guy with the 185cm guy there is a difference in speed of 2.3%, running the numbers in a bike speed calculator gives a difference in speed of 0.77% with hands in drops and 0.86% difference with hands in hoods.

The person writing this forum post comes to the same conclusion: how-should-zwift-calculate-rider-speed/547828/25

So, there is no doubt that taller riders are heavily punished in Zwift.

Zwiftpower wants to avoid displaying riders height and weight because this is personal information that should not have to be displayed publicly in order to participate in racing on zwift. This was postponed however because of the fact that it will be easier for riders to cheat and the racing community did not like that. To me it is strange that Zwift does not adress the core issue of why people are height doping in the first place, which is that people are frustrated that they are unfairly punished for being tall.

Zwift also wants to become a serious competative platform, with the goal of being a olympic sport. It will be alot harder to attract serious competative riders if there is no place for them as a top level racer on the platform, why would a strong big rider want to commit racing on zwift when smaller riders will be faster than him/her on the flats anyway (even if they are not IRL). One of the beautys of bike racing is that there is something that suits everyone, bigger riders with a strong engine can specialize as a TTer for example. High level racing on zwift is pretty much just small punchy riders.

As a 193cm tall rider I agree that this is more of an issue on Zwift than it is in the real world.

If you are riding bolt upright, then yes, you would see the kind of numbers that Zwift seem to use. But even riding on the hoods in the real world I don’t feel that I have as much of a disadvantage over my little friends let alone on the drops.

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Amen @ 218cm.

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I used to think this was a bit of an issue, now not so much. Despite being 194cm B cat I can usually draft the fastest A cat packs ok, it’s when things go up hill I drop but that’s gravity not air resistance causing the problem. Sure I’d like to have a slightly lower CdA especially for the TTTs but it’s low on the list of gripes I have with zwift! My W/kg is usually very low compared to riders around me, the additional height comes with extra weight and we know the advantages that brings under the W/kg cat system.

I agree that being a big rider gives advantages under the W/kg cat system. But that is a flaw of the cat system rather than CdA calculations.

Agree it doesn’t directly negate any argument about the CdA. But also, weight affects CdA in a way that may be unrealistic too, maybe worse even than height, I don’t have a strong opinion as to realism but have certainly noted the magnitude of the effects. None of this would matter much for the vast majority of riders if a results-based ranking was used for categorising riders, though at the open (A/A+) level it’s still better to get it right if the aim is to simulate real cycling accurately.

“Agree it doesn’t directly negate any argument about the CdA. But also, weight affects CdA in a way that may be unrealistic too, maybe worse even than height, I don’t have a strong opinion as to realism but have certainly noted the magnitude of the effects”

I have not seen any test on how weight affects CdA but it would be interesting to see and compare with IRL data.

“None of this would matter much for the vast majority of riders if a results-based ranking was used for categorising riders, though at the open (A/A+) level it’s still better to get it right if the aim is to simulate real cycling accurately.”

Completely agree.

I have previously shown that an extra kg takes about 2.5W to maintain the same racing speed (around the 40kph range). About 0.5W of that is rolling resistance, the larger part is due to a higher CdA. As for reality, I don’t know. I was just trying to understand zwift on its own terms.

I’m ok if short people have a slight advantage on zwift, I have a slight advantage in real life when I want something from a tall shelf

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I totally agree on this. 190 cm and punished for it harder on Zwift than in IRL. The algorithm needs to change.

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Agreed! Good topic Viktor

I think I saw this in a race last week. B category crit city race, people attacking from the front a lot. One guy in particular kept slow-rolling off the front, opening a gap, and holding it without seemingly doing any more wattage than the peloton behind. At about lap 6 of 12 he put in a small dig and opening up a 10 second gap that ballooned up to 40 seconds which he then held until the very end 6 laps later, something I’ve never seen on a crit city race. So I went to look at his power numbers, and the strange thing is he only averaged about 3.7 W/kg, while the front of the peloton was doing more than that (3.8, 3.9, one was at 4.0).

So then I looked at his other results. He wins every single race he enters. Every. Single. One. and his power rarely spikes more than a small attack somewhere mid race, and he almost never has to sprint. He’s nearly always below the power of the second place finisher, who clearly is finishing with the pack in a sprint finish. You should not be able to hold off a drafting peloton in the wind without putting out at least equal power. (He also is a sandbagging A, since I found one race where he did 4.4 W/kg for a half hour, clearly well above category limits).

But the only way I can see that his power numbers and long range breakaways on flat ground make sense is if his height is so low his CdA is as if he were in the draft. Otherwise, it doesn’t make mathematical sense. Also, calculating his weight from his W/kg, he’s just average. Like 70 kg or something, so its not a big guy on a flat course power thing. He’s not on Zwiftpower, so height abnormalities can’t be determined, but its got to be it, right?

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I did my own testing back when Zwift was proposing not showing height and weight on ZwiftPower due to people having body issues.

Height doping really is the best way to cheat. You’ll get picked up on ZwiftPower if you do some extreme height doping (the lowest you can go in Zwift is 90cm but ZwiftPower will flag that as abnormal) but dropping your height from say 185cm to 165cm, there is no way to pick that up unless you do a verification video.

If you aren’t on ZwiftPower then there really is nothing to stop you from entering in 90cm as your height and being able to drop people like a rock while still doing legitimate w/kg numbers.

mine was set at 168 since i made my account but i’m actually 165. i cant be bothered to get it changed though. if i ever lose a sprint by 0.01s i’ll probably regret it. does the cda really matter all that much if you’re drafting, anyway?