# How should Zwift calculate rider speed?

Im currently around 78 KG. That means I have to push extreme power during the race to follow or attack. I understand it’s the same for everybody because it’s based on w/kg. However, it seems not the best way to implement the speed on flats, downhill or uphill with w/kg.

On the road, the heavy weight riders (I call myself one) have a big advantage because somebody of 50 kg can never perform a steady pace with 50 k/h on the road, in Zwift it’s no problem for them…

Please make Zwift more honest and fair for each rider, I think completely changing the way of using the w/kg rule on the flats or uphill would be already a solution. I think if Zwift could use the absolute power in some cases ( flat and downhill/uphill) It will totally change the way of riding/racing on Zwift. In that case you don’t have the problem with weight in and people losing weight for races (or cheating with their weight).

I know it may sound difficult to realize that, but there are plenty of very smart engineers inside Zwift, so the should be able to pull this off and create a more realistic and fair riding/racing experience in Zwift.

And to be very honest, I think it would be also a lot healthier for all those riders who are are pushing themselves to be as light as possible…in a non healthy way.

Won’t using absolute power just hugely favour the heavier riders though?

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The topic of rider speed on the flat is also discussed here Fix physics simulation on the flat The formula to model speed of the rider seems pretty clear, but there are some variables, like the frontal area of the rider based on height and weight, that are not absolute and open for discussion. From the discussion in this other thread, it seems like heavier riders are penalized too much in the calculation of aerodynamic drag.

To comment on the absolute power suggestion. I would like physics in Zwift to be as close to IRL as possible. I think there are better ways to eliminate the need to show weight and height, like a result-based category system.

Maybe, I think this should be tested properly, but at least it’s more realistic. Do you see another (better) solution?

A better solution would be to tweak the current algorithm so it’s a bit more realistic. Right now young, very small and light riders can seem to go too fast on the flat for their low watts.

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Anyone got any evidence of this?

You can’t omit the impact of weight completely though. Even on the flats it’s important for accelerations.

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Find a flat road on a day with little to no wind. load zwift on your phone, connect to a power meter on your bike, turn off mobile data so you won’t draft other riders, cycle a mile in real life, check your phone to see if the distance is similar.

get people of differing weights to do the same and see if any weight advantage is gained.

Maybe it is time for someone to write a white paper and publish it to calculate speed from power.

This is the best I could find over the years, Bicycle Speed (Velocity) And Power Calculator

It’s been done over and over. Wilson’s Bicycling Science is on its 4th edition, originally published in 1975. It covers that topic, among many others. There’s no mystery there.

There is a lot of mystery in mathematically modeling bicycle handling dynamics, however.

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Without having access to the math model used by Zwift, the most probable source of divergence from real life is the estimation of CdA from rider height and weight. There are a number of papers published on that topic, but since human beings have the bad habit of coming in all sorts of shapes, estimating CdA from two parameters is bound to be quite limitative; in addition, it is easy to settle on a model that sort of fits a reference population, and then extrapolate it outside the bounds of the data used to generate it, and create abnormal results.

This may explain why short riders have a very marked advantage in Zwift: probably the CdA vs height relationship is pushed far outside its source data boundaries.

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Of course, if the frontal area calculation from FAQ of “Bicycle Speed & Power Calculator” (the same as @Gerrie_Delport_ODZ linked to)

is used, one will see that they have a fixed width of the rider from head to toe. It is obvious that this is a very approximate formula. An elliptical form would at least be a bit better.

Anyways, it is likely that the discrepancies we see compared to IRL are related to CdA as @Robert_C said.

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I see that RGT has put a lot of work into getting this accurate:

INFORMED AND ACCURATE PHYSICS MODELLING

Firstly, we worked hard from the beginning to create a physics algorithm that was based on real-world research. It took us a long time and 1000’s of iterations and we’re still not done with this, but we’re getting closer to a hyper accurate representation of the real world speeds and movement.

Physics on RGT

Now it is time for Zwift to tune their algorithm. This seems to be a good walkthrough of the physics in Zwift The Physics of Zwift Cycling if you want to be a nerd about it.

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Completely agree with this post!

In day life, I’m not facing any problems with my weight, however on Zwift it is an obsession to be as light as possible.

I’m also between 75 and 80 kg, in the European road races I can perform on the top level. In Zwift I can barely hang on… even after 2 month of serious training including a training stage in Spain, I could not finish in the top 40 of the premier league. I think it would be time for some more realistic gameplay.

And also, in real life you have the race tactics who determine the result of the race. In Zwift it’s only based on the right position and the highest W/kg, and the less better rider who could go for an early attack doesn’t stand a chance…

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Remember on Zwift you compete against the whole world. Getting in the top 40 in the Premier league is going to be hard.

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Im not saying there isn’t problems with the speed calculations, but do remember that most of the field in the premier div races are very strong with a lot of top end conti riders like yourself or otherwise talented people. Looking at the results it seems that you are both not good enough at saving energy having a higher w/kg average that riders weighing the same as you and your power profile is not ideal for Zwift.

For zwift racing it is usually those who has the ability for high variablity between NP and avg. watts more than those putting out the highest avg. watts and with your power profile you fall firmly into the second group not being able to put out the wattage or w/kg required to match the best on zwift in any short period of time. The short time period also means that there are some riders like myself who train for these races and is able to perform well in anything under 1-1.5 hours of time, but would get dropped in a longer outdoor race

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Your point is taken but Zwift is unlike any real road race.
Excelling in one may not mean you will excell in the other.

At 78 kg, you are not a heavy Zwift rider.

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sounds good 78Kg isn’t heavy with a weight of 107Kg-ish I can put out lots of power (but go slow up hills because of the lower w/kg… ;). ( I never seem to make it up going down hill though )

I’d assume the goal at zwift wasn’t a scientific accurate simulator but something that has been designed to entertain

It has been suggested that they (Zwift) have taken some “creative freedom” to make the W/kg category system work. Meaning that lighter riders can compete with heavier/stronger riders on the flat. Most races on Zwift are flattish and will normally favor absolute watts. By adding some bias they can neutralize this advantage and make more “entertaining” races. I hope that is not the case.