Speed Tests: How Rider Height Affects Speed In Zwift

Speed Tests: How Rider Height Affects Speed In Zwift

Astute Zwifters know the game calculates your virtual bike speed using a combination of “personal” factors (wattage, weight, and height) as well as external factors (bike frame and wheelset choice plus the virtual environment including road surfaces, gradient, etc).

Here at Zwift Insider we’ve done a lot of testing to attempt to tease out the speed effects of each of those factors. We’ve posted about how much each kilogram of weight loss or gained speeds or slows you down, including doing so across a range of w/kg. But we’ve never really done a thorough analysis of how height affects speed – until today!

Zwift racers know that height plays a significant factor in your in-game speed. Simply put, all else being equal, a shorter rider will be faster than a taller rider. But just how much of a difference does it make, across a wide range of wattages and rider heights? Let’s dig in and find out…

Speed Test Data

This chart shows the time it took 6 riders of varying heights to complete our standard flat test course at different power levels. Each rider was set at 75kg weight and used the same virtual bike setup – the only variance across the tests were rider height and wattage.

Height vs Speed at Various Power Levels

Conclusions

  • Zwift’s physics hold remarkably steady across a wide range of rider sizes (153cm to 203cm, or approximately 5′ to 6’8″)
  • Zwift’s physics hold remarkably steady across a wide range of wattages. We didn’t include it on the chart above, but we also tested at 750W and 1500W, and found the time gaps progressed as we would expect.
  • As power increases, speeds increase. As speeds increase, the time gaps between riders of varying heights are reduced.
    • For example, at 150W, the different between a 153cm and 203cm rider is 230 seconds. At 450W, those riders are only separated by 159 seconds.
    • While the gaps in seconds may change across power levels, in terms of percentage of overall time, these gaps hold pretty steady, with overall time increasing by 5.63% (at 150W) to 5.83% (at 450W) when rider height changes from 153cm to 203cm.
    • Why does the percentage difference increase at higher wattages? Because aerodynamics matter more at higher speeds.
  • At lower heights, adding a centimeter makes a bigger difference. Going from 193cm to 203cm slows you by approximately 1.06%, but going from 153cm to 163cm slows you by approximately 1.35%. This makes sense, because as you get taller each centimeter is a smaller percentage of your overall height.
  • Another way to look at the chart is that it shows how many watts each centimeter of added height costs.
    • For example, we can see that 163cm at 150W delivers the same time as 173cm at ~160W. So we know that a rider will need to hold ~10W higher to match a rider 10cm shorter. Or looking at it another way, every centimeter costs 1 watt.
    • At higher wattages this cost is even higher, with a 10cm jump costing closer to ~20W in the 450W range.

So how much difference does a centimeter make? Here are estimated numbers based on the chart above:

  • At 150W, every added centimeter
    • Adds ~4.6 seconds
    • Costs ~1 watt
  • At 225W, every added centimeter
    • Adds ~4.0 seconds
    • Costs ~1.1 watts
  • At 300W, every added centimeter
    • Adds ~3.7 seconds
    • Costs ~1.2 watts
  • At 375W, every added centimeter
    • Adds ~3.4 seconds
    • Costs ~1.8 watts
  • At 450W, every added centimeter
    • Adds ~3.2 seconds
    • Costs ~2 watts

I Wish I Were Smaller

Looking at the dramatic effects of height on Zwift speed, the inevitable questions of “height doping” will surface. Is this a real problem on Zwift? Are riders pretending to be midgets so they can win races?

Sure they are. But it’s a very small minority of the serious racing community. (And yes, I wrote that on purpose.)

With WTRL requiring random height verification videos, Zwift Cycling ESports requiring them for top-tier events, and ZwiftPower tracking every rider’s height history, serious racers know that lying about your height can get you into hot water. Quickly. And it’s not like weight, where you can perhaps make the case that you gained or lost 5-10kg in a short amount of time. With height, change is slow and minor. Significant changes stand out quickly in the data, and get riders flagged for cheating.

Is the system foolproof? No. But the system isn’t being overrun by wannabe Cavs, either. Perhaps someday we’ll have a Zwift Racing Passport, where our height can be marked as confirmed after submitting a video?

Should Height Matter So Much?

Whether or not Zwift’s height physics accurately reflect the real world is a matter of some debate. With some online bike speed calculators not even taking rider height into account, why does it figure so heavily into Zwift’s algorithm?

Zwift calculates your avatar’s frontal area based on your weight and height (your BMI, essentially). That frontal area, in turn, is the key factor in Zwift’s CdA calculation, which accounts for the majority of the virtual air resistance your avatar encounters.

At a glance this seems logical – a taller person will be in the wind more, right? But what about the example of two riders who weigh the same, but vary greatly in height?

  • Rider A: 160cm tall, 80kg
  • Rider B: 190cm tall, 80kg

If both riders hold the same power on flat ground, who would be faster? Our speed tests above show that, at 300W steady, Rider A would be faster on Zwift by approximately 2 minutes over an hour effort due to their reduced height.

But in an aero position with a flat back on a TT bike, Rider A would actually present more frontal area, and therefore be slower in real life. Zwift’s own graphics support this:

It all comes down to rider posture, really. Or at least, it should, if we’re trying to simulate outdoor riding. If Rider A and B are both “on the hoods”, height would matter quite a bit, and it’s reasonable to believe Rider A would be faster due to his reduced frontal area. But if they’re in the drops, height matters less.

Could Zwift change things so your rider’s CdA differs based on avatar posture? Sure they could. But they would need to change the logic which determines when your rider “sits up” or gets into the drops as well.

And would that be the best solution anyway? Possibly not. When you’ve got riders pedaling away in their pain caves in a variety of postures, how should we expect Zwift to fairly decide what everyone’s virtual front area should be?

(It’s worth noting here that calculating your front area in real-time based on a webcam feed showing your posture in your pain cave seems like a super-cool idea, even if it’s a bit before its time…)

It’s easy to say Zwift’s height physics aren’t quite accurate. But it’s also hard to propose a better solution than what they already have in place!

In the end, for better or for worse, Zwift takes a simplistic approach to calculating rider CdA. Some taller riders feel they are unfairly penalized by this, but it’s all part of the game. This is Zwift: a riding simulation. If you want to perfectly replicate outdoor physics, you’ll have to ride outdoors.

Questions or Comments?

Share below!

About The Author

Eric Schlange

Eric runs Zwift Insider in his spare time when he isn't on the bike or managing various business interests. He lives in Northern California with his beautiful wife, two kids and dog. Follow on Strava

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Craig Martin
Craig Martin (@viiich)
1 month ago

Nice, thanks Eric. I’d been trying to get Zwift’s factors for height out of Tempus Fugit TT race results myself. This fits with what data I’ve got, but actually doing bot experiments is much higher quality data. Very much appreciated!

Eli J
Eli J
1 month ago

Maybe I missed it but when you say gain 4s or loses 3s what is the distance duration? It’s early here so I could have glossed over it. Love the site ! Cheers

Paul Himes
Paul Himes
1 month ago
Reply to  Eli J

Somewhere up above he says their standard course, so I’d assume that’s 2 laps of Tempus Fugit (something like 36 km).

Neil Melville
Neil Melville (@neil_melville)
1 month ago

I’d understood that Zwift used one of the established CDA estimators (maybe you can find out which one). Speaking as a short guy (168 cm), I know there are other factors you haven’t mentioned here: I’m riding a 150cm frame and my saddle height is low, so it’s easy to get a good draft. Conversely, big riders complain that it’s pointless riding second wheel with me on the front. Try factoring that into a Zwift washing machine!

Paul Himes
Paul Himes
1 month ago
Reply to  Neil Melville

That is a good question. Do I at 188 cm and 108 kg provide a bigger draft in Zwift like I would in real life, or is amount of draft provided constant?

Paul Himes
Paul Himes
1 month ago

Thanks for doing this. I’ve gotten in a number of conversations with people who are of similar BMI but shorter weight than me who always claim that racing against me is unfair because of my advantage on the flats. I counter about my added wind resistance, but it appears that weight still trumps my added wind resistance based on your data.

Ramón
Ramón
1 month ago

They could add complexion, like thin, middle, wide, because I’m tall but thin as a rail, the opposite of wide shoulders. Cycling is the only place this is good for me, but not in Zwift.

Jeff H
Jeff H (@jrh26)
1 month ago

I totally feel this going down hills in Zwift. My 96kg should give me an advantage but my height takes some of it away. I see people with small avatars flying by me and the amount of watts I have to put out to stay with them is ridiculous. I say lets let Zwift fix the important things like racing and we can worry about dialing this in down the road.

Paul Himes
Paul Himes
1 month ago
Reply to  Jeff H

I’m right there with you. I think my 108 kg should help me descend with the slipperiest of folks, but I was part of a TTT that got dropped going down Fox Hill (the Box Hill descent) while all of us were supertucking and I weighed 10-15 kg more than the next heaviest rider. I tried pedaling to catch back on but didn’t do it. Fortunately, the team didn’t need me for the finish.

Francois
Francois
1 month ago

I’d love to see that realtime frontal area calculation on Zwift (in my dreams, I know…); this would turn out to be so much more useful as a training tool for outdoors riding as well as so much more fun for races.

Bruno
Bruno (@bruno-lorentin)
1 month ago

What I’m more curious about is how it affects the draft itself. I agree that I’m TT mode, it’s good enough the way it is (mostly), but in racing situation is where I feel height is really penalized

ShermanO
ShermanO
1 month ago

Hey Eric, “midget” is considered offensive to little people, as they generally prefer to be called. It’s jarring to read—maybe you can find a different term? Or just replace it with the minimum height number?

Dan
Dan
1 month ago

At 6′ I’ve always felt that I am being unfairly penalised in ITTs, which has been borne out both in the numbers for my races but also it appears to be the case here too. I’m pleased that you have addressed the TT position here in your post as I have long questioned how a tall rider in a TT position can be so dramatically slower than a shorter one in the same position.

Daniel Connelly
1 month ago
Reply to  Dan

Average height of top 5 in the Olympic time trial was, by my calculation, 6′ 1.4″. (sorry for imperial units but Dan used them first :)).

Wally
Wally
1 month ago

“(It’s worth noting here that calculating your front area in real-time based on a webcam feed showing your posture in your pain cave seems like a super-cool idea, even if it’s a bit before its time…)”

No it doesn’t, that would be awful. Why would I want to ride in my living room all hunched over? I want to sit up and be comfortable and expose as much front area as possible to my cooling fans.

What next, a check to make sure we’re only drinking from bike bottles and not reaching for drinks or snacks on a tray?

JamesT
JamesT
1 month ago
Reply to  Wally

Because it would make it more realistic, and would encourage better training. If you’re out the saddle smashing away on a flat course, you should be penalised (by CdA) versus someone who can put out the same power in an aero position.

Not that it matters to me, at 6’7″ I just have to be happy getting dropped everywhere on Zwift by the little people.

SMAD
SMAD
1 month ago

IRL – 5″8

Bumble – 5’10

zwift – 5’8

Smashquatch
Smashquatch
1 month ago

I disagree Eric. If Zwift is all in on making their game about racing (which is clear by the lack of fixing basic QOL issues that have existed since beta), then they need to make their game fairly reflect everyone’s performance not just people who are skinny and short.

Even a previous hour record holder commented how the games physics arent there. https://forums.zwift.com/t/fix-physics-simulation-on-the-flat/317921/82

Jacek
Jacek
1 month ago

Eric, will you perform a similar test on a climb as well? Would really like to see the difference there. Although the speeds are lower on a climb, the heavy and high rider is disadvantaged by both height and weight…

Joakim Lisson
Joakim Lisson (@joakiml)
1 month ago

Hi Eric You sort of missed a couple of big points when it comes to aerodynamics and height. Aerodynamics is never just about rider posture because you can never adjust position to remove the drag from your legs. So riders with long legs will always have a penalty compared to riders with shorter legs in CDA. Then in real life, calf size, is again quite a big factor for the legs and big calves agains makes for a higher CDA. Zwift obviously can’t account for that but at least leg length it will be the reason for the difference your… Read more »

Francis Norman
Francis Norman
1 month ago

Maybe it’s time for some special events for riders sorted by height to give pocket rockets and string beans the chance to race against similar 🤔🚴‍♀️

AmbientShades
1 month ago

It’s kind of hard to work out what the first graph means. I can see the difference between riders of different heights, but without the distance of the course it is hard to see how significant they are.

Niels
Niels
1 month ago

Hi Eric thanks for the great article. I enjoy reading all your articles and it makes me enjoy Zwift much more. I have one question: can you check at Zwift if and when they plan to introduce a new challenge? Most of us have completed the Everest, Italy and California, but I do not understand why Zwift is not adding new challenge like: Cycle around the world (40K KMS) for new shoes. or to the moon: (400k KM) for new helmet. To me this sounds fairly simple to implement and will give lot of us a new long-term challenge. Thanks… Read more »

Daniel Connelly
1 month ago

Your analysis of the effect of the *torso* is good, but don’t forget leg length (and bike size) also presumably scale with height, and legs are normal to the wind. So while the cross-sectional area of the torso may be higher in the higher BMI, shorter rider, their legs are shorter. Think Cavendish or Ewan: each get extremely low. I think rider CdA proportional to sqrt(height times mass), frame CdA proportional to height, and wheel CdA constant is a decent heuristic approach. The results, however, are counter-intuitive. At higher speed, wind resistance becomes a larger fraction of the total. A… Read more »

Doug Smith
Doug Smith
1 month ago

At on point last year I was adjusting my weight, having lost quite a bit. I think I accidentally rolled the scroll wheel while the cursor was on the ‘height’ box, and reduced my height from 6′ to 3′. I rode one of the Tour de Zwift rides that day, I think the one that did the Epic KOM and the Radio Tower. In addition to being able to keep up with riders doing a lot more w/kg on the flat, I FLEW down the descents! As soon as the ride was done, I checked my settings and saw the… Read more »

Ade Paul
Ade Paul
5 days ago

Filippo Ganna has such a disadvantage right? Height should only be a factor while sitting up. On the drops/TT bars taller riders can achieve an equally good if not better aero tuck.

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