Fix physics simulation on the flat

If you read it out of context yes you are correct the conclusion should be changed so that it can be read out of context.

I will update that. Thanks

Edit: I updated that conclusion so It can stand alone.

I’ll just add this here:

Please go to the 13:00 minute mark onwards for the relevant comments and insight that Zwift modelling feels incorrect.

Theres also another video he does where he specifically discusses one of his real world friends that he races against and there is no way IRL that this person would beat him because of the watts difference (I’m trying to find the video and the timestamp)
UPDATE: found it here:

go to the 40:00 minute mark

This is Alex Dowsett, current world tour pro, former world hour record holder and time trial specialist:


Once Alex buys a fan I’ll pay more attention to his complaints.


:rofl::rofl::rofl: Fair enough!

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In real life, I always Tt faster than my lighter friends, which 10-20 kg lighter than me.
Even in my draft they always suffer.
But in zwift lighter guys can keep up no problem, as long as they can give same watt per kg.
50kg @ 4w/kg (200 watt) can stay with 90kg @4w/kg who put out 360 watt.
In real life it hard to do that on flat road even in draft


So this thread was really interesting to me as a) I like stats and b) I feel like I am way slower in Zwift than IRL with the exception of hills which feel about right.

So I recently did two laps through the ocean tunnels at somewhat consistent power (using garmin vector 2s as the power source) on the basic TT bike with enve ses 3.4 wheels.

I then pulled the average power, speed, elevation and cadence (why not) from Strava for an 1,100m section that cut off any extra speed from the descent.

So for me (180 cm, 89 kg) using the calculator site from above with measured cadence, 20C temp, -5m elevation , 0 slope, no wind, and assuming a 9.5kg triathlon bike with “narrow racing tires”

Power 157W @ 81 rpm
Zwift 2:10 @ 32.2 kmh 2:08 @ 31.0 kmh

Power 275W @ 94rpm
Zwift: 1:43 @ 40.5 kmh 1:43 @ 38.3 kmh

Despite being a very short interval (and the lack in precision on length from strava), that is surprisingly close, suggesting that any discrepancies in expected speed (for me at least) could be coming primarily from slopes (up or down) or the alternate road surfaces (why so much dirt zwift?). Or that after a drastic reduction in fitness from parenting and a long time off the bike I’m just way slower than I wish I was :wink:

How can the model predict a quicker time with a lower average speed? Is there a difference in assumed route length?

Has to be the rounding error from what Strava tells me I selected (to the nearest 100m) and the actual distance in zwift. As its a short segment I expect some error, but it’s the only true flat I could find.

Today we can see how each one of the light and short climbers is going to literally destroy Fabian Cancellara on a completely flat TT, even putting average 100W less:

Today we will see the evidence of the ridiculous and absurd advantage of being short and light in Zwift, which is huge on all terrains in addition to the obvious (ascents) that IRL does occur.
The reason so many cyclists do the “Zwift diet”.

The model should be corrected because it is absurd that the aerodynamics become lower and lower as a rider gets short and light, such that 155cm and 48kg could feel even tailwind… :joy: :joy:

Please Zwift programmers: ASK FABIAN.
Sure he will help.


really like your effort in this thread here @Susun_Corda !
this w/kg bogus should be unnerved.

next if this is fixed is the powermeter/trainer mess so many monitoring devices measure anything but real values …

Of course, next would be display power device in classification as Rouvy or Bkool actually DO.

Sorry to be all grumpy but as a lighter rider of average height (60kg/176cm) my distinct impression is that Zwift unfairly favours heavier riders, particularly on any kind of rolling terrain.

Ultimately the fact that we’ve come to opposing conclusions probably means that the physics model isn’t actually that bad.

Never seen IRL that if on the flat you (60kg) put 50W less than a 70kg rider, you will be faster, as is happening on Zwift.
Try yourself in real life, riding next to a cyclist heavier than you and then put 50W less.
Everybody knows what will happen!
You sure know this.

Never seen that in a 12km descent, average -8.5% slope, without braking,… a 7kg lighter rider (75kg VS 82kg) will be only 7 seconds slower thatn the heavier, as actually is happening on Zwift, and Zwiftinssider have demostrated.

So, lighter are CLEARLY favoured compared with how things are in the reality. :wink:


wake me up before you gogo

Hi, I just wanted to add my two cents of empirical data gathered on Zwift races held by my squad over two consecutive days.

The first is a TT held on Greater London Flat (no drafting) and the second one is a climb up Ven-Top on regular bikes.

I have plotted average speed vs average watts/kg for all racers, and included a linear regression line for each chart.

As can be seen, there is quite a bit is dispersion along the trend line for the flat course, while there is an almost perfect correlation for the climbing course.

Being that this is the same group of riders, my conclusion is that average power can be misleading as a speed predictor on flat terrain, due primarily to momentum effects, which penalizes variance in power.

Weight does not play a big role on a flat road. same as in real life.

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But as you can see in my first graph, speed is correlated to watts/kg, just not as closely correlated as in climbing. I am trying to find an explanation for the deviations from the trend line, and it is not weight.

If there was drafting involved, that would have been the obvious answer, but this was not the case.

I just think that averages in power can be misleading, as fluctuations play a big role

I can think of 2 additional factors to take in account. The first being rider height, which also contributes to the riders’ CdA. The second is pacing strategy.

I want to correct my earlier comment. I looked at the points above and below the line, and found that the average weight of those above is 81.9kg, while those below the trend line is 63.7.

This is a very interesting insights, since it indicates that in general, in flat terrain, heavier riders are more efficient in converting their watts/kg into speed than the lighter ones.

Put another way, in flat terrain, at the same average watts/kg, the heavier rider, who is putting out more watts will go faster than the light weight rider. Our 3 heaviest riders were those farthest above the trend line.

No way man.
Unless Zwift’s algorithm was corrected in the last 2 months, heavier and taller riders become big trucks at Zwift in terms of aerodynamics.
So, a light and small rider can be as fast as heaviers even on descents, just for the aerodynamics.

Aerodynamics improve more and more when weight and height are lower.The joke is that some users weighting 40kg and heighting 135cm can feel up to some kind of tailwind in Zwift… as the linearity of the algorithm is prety absurd