Opinions on rider HEIGHT vs SPEED

Hi everyone,

Just wondering what the general consensus is on the accuracy of Zwifts algorithm in terms of rider height vs speed?

I train with a friend both on Zwift and on roads, when we are out and about he destroys me, but when we are on Zwift I am able to drop him, so something doesn’t seem right.

We are both using calibrated smart trainers with our real height/weight numbers, so I don’t think there is a problem with our equipment or setups.

I have noticed that a lot of A/B category riders seem to be a specific height/weight, there do seem to be an abundance of riders around the 170cm / 70kg mark, but that may just be to do with the distribution curve of the population in general more than anything else.

Still, there are definitely a few youtube videos around, which seem to suggest than changing a rider’s height in their Zwift profile has a HUGE impact on ride time of say a 1 hour ride, with taller riders taking several mintues longer than shorter riders (with all other stats equalised).

Any tall riders out there who train with shorter guys having similar issues…?

I don’t have any direct comparisons like you do, but I don’t think generally that being tall (I’m 193cm) in the real world affects you as much as Zwift seems to think it does. Perhaps if you’re sitting completely up right, but when do we do that?

bumping for more thoughts

Zwift makes simple assumptions about aerodynamics from height and weight.

But look at a photo of yourself on a bike. Then look up a pro with the same height and look at a photo of them on a bike. I’m guessing you’ll observe the following:

  1. their back is a lot flatter
  2. their clothing fits a lot tighter

These two factors alone are huge in aerodynamics. Yet Zwift will treat you the same as the pro (perhaps wider if your BMI isn’t at the pro level).

Additionally, not everyone is as good at riding trainers. Outdoors, the bike moves back and forth when we apply lateral forces. On most trainers, the bike simply pushes back. So the pedal dynamics are different. Different riders experience different amounts of power loss going from outdoors to indoors. Maybe some have more power indoors if they only ride indoors.

Then there’s the environment. Consider you’re riding at 200 watts, which is low power. But this means your body is producing 200 watts, but internally dissipating around 640 watts, assuming 24% metabolic efficiency. That’s a lot of heat being generated in the room (840 watts total). So unless you have adequate airflow, it’s easy to overheat, and that leads to cardiac drift, and that causes power to drop. Outdoors this is far less an issue.

So the model is good, but it’s very simple. Maybe a future generation Zwift will have a camera filming your position as you ride, checking the fit of your clothing, and using image processing to send CdA numbers to the game. But it’s not there yet.

There’s a video on-line of Alex Dawsett riding a time trial on Zwift – he’s one of the top time trialists in the world among pros. Yet he got beaten on Zwift, because part of his success is his aero positions. Zwift doesn’t know anything about that – it just assumes an average for his height and mass.

I’m trialling another cycling app and in it you can change your cdA which might sound like a good idea but then everyone would just put that at the minimum.

For apps which mimic real-life, like one which uses videos of actual climbs that I might also climb IRL and I want to compare times, it makes sense to allow a user to adjust CdA to better reproduce reality, for example “time trial position”, “road race position”, “recreational/climbing position” (even pros sometimes have their jersey hanging open on climbs, for example), as deviations from a baseline formula based on height and BMI. But Zwift isn’t so devoted to modeling real life. So it makes sense Zwift would use an optimistic number.