You already know your weight setting in Zwift affects your speed–especially up hills!
And you probably also know that the bike frame and wheels you select will affect your speed.
But did you know your height setting also affects your speed? Zwift uses your height in its equations to calculate CdA, which is a measurement of how much resistance your body gives to the air as you ride. Just like outdoors where aero is everything, Zwift calculates taller riders as offering more wind resistance.
We’ve done a lot of automated test laps with various height/weight/equipment setups, and confirmed this: all other things being equal, the taller you are the slower you’ll go.
The test laps we’ve done in the 225-watt power range work out to ~30 seconds longer on a full Richmond lap for each 15cm (approximately 6″) of height added. Or to put it another way: a 5′ tall rider will be ~2 minutes faster than a 6′ tall rider for every hour of riding on a fairly flat course.
Another way to look at this is every 15cm removed from a rider’s height is like adding 5-10 watts of power in terms of speed increase. We can figure this out by looking at average Strava speeds at different heights, and plugging those numbers into BikeCalculator.com to look at power differences.
These numbers fluctuate depending on the wattage range you’re looking at, and what type of route we are riding–long, slow climbs won’t be as affected by height as much as a flats or downhill.
But it is worth noting that Zwift, in attempting to emulate real-world physics, does take your height into account (even if your avatar doesn’t get any shorter or taller).
Is the difference the same between a road frame and a TT frame in Zwift? It seems like CdA differences should be much less for TT frames.
Been a while since we’ve tested it, but I don’t think it makes any difference.
I think you’re right (and Zwift is wrong for doing this). Huge differences observed due to height. https://www.iammike.org/2022/04/21/zwift-the-effect-of-height-on-tt-performance/
How about draft? It should be less behind little people.
According to Zwift, the draft IS less behind little people.
It’s incredible how Zwift penalizes tall riders. Using kreuzotter.de, in real life 15 cm would only make ~10s of a difference per Richmond Loop (at 225 W). From my estimates I suppose that Zwift simply scales the cda with the height squared, which assumes all the body and bike proportions scale up and down linearly (including head size and all the bike parts).
The height “bug” pisses me off actually. I have been considering just putting my height at absolute minimum because of this. There is no real world example where height would incur a penalty like this. They should have stuck with weight only for cda calculations.
I think this explains why shorter riders dominate the TT’s in Zwift, even if they are putting out much less power in a flat TT. It is completely counter-intuitive to how real world flat TT’s work.
Completely agree with you and share this bugbear with you. As a tall, heavy but strong rider it is annoying to be beaten in ITT’s by much smaller less powerful riders. Totally understandable that they crush me on the climbs, but the flat should be my terrain. And of course this bias just encourages Zwifters to not be truthful about their height and weight, so you end up with loads of Zwifters with pro-level power to weight stats (some will be real, but you don’t see so many out in the real world).
Hi Eric I have read lots of articles on weight in zwift and how riders who are lighter are biased. Low weight riders 65kg or less are lean say 15% fat maybe less against muscle composition. A heavier rider could have between say 15 to 35 % fat so a rider with more low fat to muscle ratio would fair better against a heavier rider with high fat. E.g the heavier you are doesn’t proportionaly increase power? How does zwift allow for this? Apologies if answer any where before. Also riding a tacx flux trainer against other brands seems to… Read more »
I’ve just roughly calculated my Zwift CdA at an absurd 0.33m^2 on the Shiv Disc with 808s. IRL, I’m at a measure 0.36-0.38 with baggy clothes and riding on the hoods on an aero road bike.
Does weight affect CdA, I’m 2m tall and 75kg. I’m guessing but I think my CdA would be lower than someone average height but much heavier.
I’ve just found this: https://zwiftinsider.com/rider-weight-speed/
So yes weight does affect CdA.
Eric, what about height and CdA effect from the pen, or standing start? I’m 189cm (6’2”+), 74kg (B rider) and forever getting spat out the back despite putting out arguably more watts than the leaders. It’s driving me a bit nuts..
Are you ramping up the power prior to the flag dropping?
Seems like a mistake: change on body size is already accounted in with the weight-CDA relationship. Basically they’re saying short and bulky is better than tall and skinny? Additionally, this was only @ 225 watts. Imagine doing 300. (air resistance increases quadratically). Zwift should open-source their formulas