Fix physics simulation on the flat

I realize W/kg aren’t directly used, although of course equations can be recast in terms of W/kg, or whatever else. That said, W/kg is a convenient parameter which is particularly closely correlated to climbing speed. I claim W²/kg is better for flat speed.

That Keuzotter website is excellent. Here’s the estimate for CdA:

This calculates the cross-section of an inclined rider. My crude model is simpler, the case where λ = π/2. In this case, sin λ = 1, and the m / (h · ρ) terms cancel. And sure enough, just as I claimed, CdA ≈ sqrt(h·m). If I divide watts by CdA I get something proportional to watts/sqrt(kg), or I can use watts²/kg as a benchmark for riders of a given height.

The m / (h · ρ) terms give the correct answer if the rider is fully recumbent. In that case, cross-sectional area matters, not height directly.

So for example, consider the following two riders:

A: 400 watts, 80 kg
B: 300 watts, 50 kg
which is faster on the flat? If I square the watts/100, I get 16/80 for the first rider = 0.2, and for the second, I get 9/50 = 0.18. So the first rider is faster.
If i look at W/kg, the first rider = 5, the second rider = 6, so the second rider has more W/kg.

BTW, I cannot understand the functional form of that equation.

What would make more sense to me is:

Frontal area = (sin λ) sqrt [ rel · h · m / ρ ] + (cos λ) m / (h · ρ)

To be correct, if in small ascents the weight is disadvantageous, in similar small descents, the weight should be an advantage, right?

In fact, if accelerating the heaviest weight need more energy to be accelerated, in a similar way, when you want to brake, the heaviest weight requires more energy to brake as well.
If two cyclists of different weights stop pedaling at the same time, which stops first, the light or the heavy?
So once in motion, which of the two needs to invest more energy in maintaining speed? the one that stops earlier when it stops providing energy, or the one that takes longer to stop?

In practice, they do not add weight to the bikes, which can complicate their maneuverability.
However, they do use solid disc-wheels precisely looking for inertial forces. They don’t use light carbon wheels like climbers.

And in fact, the winning cyclists on the track are never thin and light as climbers who are potential winners in Alpe D’Huez.
Neither do the time trialists.
Using 5w/kg a professional is a world/olympic medalist (or champion) in time trial and no slim climber can even try to get close even if he applies 7w/kg, not even when there are climbs on the routes. These things happen for a reason.
Greg Van Avermaet weighs 86kg and no lightweight rider can come close to competing with him on terrain with acceleration, ups and downs. However, he has nothing to do against them climbing a high mountain pass of an hour of continuous climb.

As a “rough prediction” based on experience observing ITTs, the 80kg and 400W rider will be around 3km/h faster in avg speed.
Someone with that UCI world tour professional numbers, sure can fly at 52km/h or more, depending on how flat is terrain …
Comparison as Nairo Quintana VS Victor Campenaerts or something like that … :thinking: :grin:

I don’t think that’s really true. Lots of good time trialists are small. Colby Pierce used to have the US hour record. Chris Froome was 3rd in the 2012 Olympic Time Trial.

In my example, 80 kg w/ 400W, vs 50 kg with 300 W, the heavier rider would be faster by 60 seconds per hour, neglecting rolling resistance, assuming the simple CdA model applies and neglecting the bike wind resistance.

An experimental approach to mass and CdA (or at least A) was done by Heil (2001):

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That’s why I prefer competition in real life… :rofl:

Not depending on how the “model” is done, which can favor some riders or others… just nature.

A very good example, as Cancellara was heavier, and puting not much more power, he won.

Another example.
Data from Olympics 2016.
54km AND +700m acumulated slope.

You all can see clearly what I’m trying to say.
Even between good time trialists, weight differences 68 to 80kg (not even 50-80…)

Cancellara (taller and heavier) with similar or even less w/kg can beat those guys in a terrain with multiple climbs and descents!!! Note this, specially Mr. @Twoshihtzu as we were before talking about “accelerating and decelerating” and other who said that weight is important as no-totally flat terrains…etc


To see full terrain profile:

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Oops - bunch of wrong things in there. Track riders use solid disc wheels for longer events, because of aero gain, nothing to do with inertia. Most high-end track bikes are carbon, so are the wheels. There’s no climbing on the track, so weight is less of an issue. Same considerations as when comparing a TT bike with a climbing one. The TT bike is optimized for aero, the climbing one for weight. The former will be heavier, not because the weight is a gain, but because it’s not the optimization point.

Inertia only has an impact when your speed changes. Track events are either at pretty constant speed, or are sprints - in neither of these cases will a higher inertia help.

There are different track riders for different events. Of course if you compare track sprinters to Tour climbing specialists, it’s exactly like comparing 100m sprinters to 10 000m runners. Yeah, they tend to look different. And for a good reason.

I was mainly referring to how even if you think you are riding a steady flat pace, you actually aren’t. We all constantly have small positive and negative accelerations. Basically the steady state that you claim heavier riders have an advantage at doesn’t exist in the real world.

The guy with the best w/kg overall may not have won overall, but was the guy with the best w/kg on the climb the fastest guy up the climb? The graphic you shared was kinda blurry on my device

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I already know this.
Just for avoid turbulence generated with rotation by spokes…same reason of using 3-ray wheels or bigger rims (shorter spokes):


Of course, and that’s why inertia has impact in order to maintain high speeds. A “problem” when you accelerate, but helps to avoid speed loses.
Not the main benefit of disc wheels (which is aerodynamic) but also and advantage.

I said that only to remark that even if there are some climbs…etc, overall absolute watts and inertia of heavier riders make them faster even if w/kg is similar or even less.

A situation as the Olympics will not be possible into Zwift in any case.
Even without the 700+ accumulated slope, on a flat, the result would be just the opposite to the real life… :grin:

It’s certainly an interesting discussion,
I’m certainly not smart enough to fully understand it all but it seems like zwift has to make certain generalizations in order to make it all work simply. I don’t doubt that zwift could be incorrectly modeling in some cases. To me it’s good enough, but others maybe want more realism.

Would we really trust our fellow zwifters to enter their correct drag coefficient? I mean, many zwifters can’t even figure out the right race category to enter :slight_smile:


Hi - I don’t fully understand the physics but I have been puzzled (albiet it is a game/training tool so not too bothered). I am heavy with a fair amount of power compared to my clubmates. I can hit the front and over 20-30 min push hard and eventually the lighter ones drop off and cant keep up. I am going hard but not at 100%. On TT of 17.6km (TF) where i bury myself I am 30 to 40 seconds behind. On Zwift. i have better equipment than they do. they weight and mine are accurately in Zwift.

So I see that Zwift does not represent real world on flat for heavy riders.

Just my observation…


Thanks for share your oppinion, not all are taking the time to look for this forum and come here to tell this.
Hope someday in this forum some people can understand that it’s a clear evidence, not an appreciation of some riders.

I would say that seems like if Zwift is made (I don’t know if intentional or not) to favor the performance of 50-60kg climbers (or weight cheaters) on the flat…
:shushing_face: :smile:


Having a bit of time I went ahead to do the test in Zwift.

The rider data from the Olympics 2016 was used to perform a TT test on Zwift Fuego Flats Reverse, it is a nice 7.1km flat route.

I had the rider do a average pace changing the rider parameters for each segment. The parameters was changed after crossing the line then the new rider rode a lap so that the riders were all starting the timed segment moving.

Input: from Olympics 2016 as posted above.
1st test:
Rider: Fabian Cancellara
Hight: 186cm
Weight: 80 kg
Avr Power: 440w
Time: 9:07.5

2nd test:
Rider: Simon Geschke
Hight: 170cm
Weight: 63 kg
Avr Power: 342w
Time: 9:21.6

3rd test:
Rider: Georg Preidler
Hight: 190cm
Weight: 68 kg
Avr Power: 365w
Time: 9:31.0

This seem to correspond to the results in the Olympics 2016.

Removed conclusion:


  • The route is not the same profile
  • Zwift riders kept constant power, no accelerations on the inclines like Rider IRL would do
  • All Zwift riders used the same bike, where IRL riders used different bikes.
  • This is a bit of a Apples and Oranges comparison, but this is the data we had.

Using Zwift to model the Olympics without a +700 slope give similar finishing positions.
This prove that the statement above is not valid.


Thanks for that info. I think some runs where you only change 1 variable at a time would provide even more insight. For example:
-different weight, same power, same height.
-same weight, different power, same height.
-same weight, same power, different height.

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I don’t think your conclusion is supported by the evidence, and using the evidence I reach the opposite conclusion.

Comparison of Zwift with Olympic TT data

In the Olympics the W/kg were (5.75, 5.74, and 5.70). Here the spread is (5.5, 5.42, and 5.36) - almost three times as wide as spread and smaller. So we’d expect the heavier rider to perform better on the flats compared to the Olympics (because the are producing relatively more power ber kilo).

Also the Olympic course is hillier - again we’d expect the heavier rider to perform relatively better in this Zwift trial agaisnt the Olympics.

The absense of wind speed effects in Zwift would also be expected to favour the heavier rider.

These suggest that in this comparison the heavier rider in Zwift will go significantly faster than his lighter competitor relative to the performance in the Olympics

The measured speeds in Zwift are (46.68, 45.51, and 44.76) kph - first goes 1.17 kph faster than second, and 1.92 kph faster than third. In the Olympics the differences in average speeds were 2.19 kph, and 2.31 kph.

Therefore on a scenario where you’d expect the performance of the heaviest rider vs the lightest rider compar, it is measured to be significantly worse based on the difference in avarage speeds.

Preliminary Conclusion: Zwift simulates heavier riders more harshly when compared to an example of an external event…

Comparison of Zwift and the Kreuzotter Model (KM).

KM used the triathalon setting and an 8kg bike for this comparison.

Speed difference between actual Olympics and KM are (-1.46, -1.45, -1.87) - pretty consistent, and in line with the hills and other real world effects for the Olympics leading to an expected slower speed for all riders.

For Zwift vs KM-Model the differences are (-0.82, 1.81, 0.86) - heavier riders go substantially slower than KM predicts for these rides, and lighter riders go substantially faster than KM predicts for these rides.


Using this limited sample of rides to compare Zwift rides against both a real world event and a model of the cycling power equation suggests that Zwift gives a substantial advantage to lighter riders with their algorithms.


The point was not to see if Zwift simulate speed the same as IRL the point of the test was to see if the comment that Zwift would simulate the opposite to the Olympic data is true or false.

Those Olympic numbers was taken from normalized power, I used average power.

The Olimpic riders did not ride at constant power, look at Cancellara he did a peak 5 min power of 547w.

There are to many differences to draw any conclusion apart from the fact that the comment made above is not true.

If I wanted to test IRL speed compare to Zwift I would use better sample data. :rofl: But as I said that was not the point of the test.

Not even KM can simulate a route with so many changes in elevation. You will need to use the raw KM formulas and apply it for every second using the power and grade as input.

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The sole conclusion in your post was

“Zwift does a good job modeling bike speed for different weight and length riders.”

I don’t think the evidence presented by you supports this conclusion