Fix physics simulation on the flat

You are right.
Everybody knows what you compiled in that data.
It looks like is a w/kg relationship also on the flat, but just by the FANTASY aerodinamic calculations in Zwift.
The translation of more kg of rider weight into a wortst-CdA makes the effect like if were the gravity on climbs, that’s why it looks a W/kg relation on the flats as well.

This thing does NOT happen in real life as everybody knows.
BUT Zwift will not do anything.
People like this fantasy of going 40km/h with 200W (can happen if they are light or they put a lower weight on the profile) that never will be possible outdoors.

Fantasy is happiness, then people are happier on Zwift, then will pay subscriptions.
Enjoy the videogame.
:wink: :joy:

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A 100 kg rider has double the kinetic energy of a 50 kg rider when both are traveling at the same speed. So if the increase in drag (aero + rolling resistance) doesn’t outweigh the difference in kinetic energy, the heavier rider should have to apply less W/kg to maintain the speed than the lighter rider. If they apply the same W/kg the heavier riders will go faster. This is my understanding of physics involved, but I might be wrong :slight_smile: We can ignore wind, air density and other factors that are the same for all riders.


Quite simply, kinetic energy is the energy of motion. In this case, the energy contained in you — and your bike — when you’re already in motion. The measurement units for energy (either potential or released) are joules. Power is expressed in watts, which is a joule per second.

So if I’m 80kg and I’m riding at 20km/h, my kinetic energy will be roughly 1,230 joules (Ek = ½ x 80kg x 5.55m/s2, where 5.5m/s is the same as 20km/h).

If I’m doing 50km/h, my kinetic energy will be nearly seven times higher — 7,720 joules — because as mentioned in the equation, velocity needs to be squared.

If you’re travelling 50km/hr on a flat road and you stop pedalling, you’re still moving quickly and won’t slow down very much. You’ve got all this energy which helps to overcome the drag and rolling resistance. So you’ve got a lot kinetic energy in the system, but the forces that are retarding you are relatively minor, so you don’t slow down very quickly.

Reference: The Physics of Moving a Bike -

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I agree, and I have to remark as well the kinetic energy.
None of the “models” (speed predictors) around there are considering that!!!
All of them are ONLY based in slope, Crr and aerodynamics, some of them including wind.

The kinetic energy is easily to “see” with the simple test of riding paired at same speed, then both riders stop pedalling and let’s see who stop first. :wink:

As easy as that.
Everyone observed this in real life.

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There certainly is something wrong when these light 13-year-old boys can hold B-level easily on the flat.
Talking about kinetic energy makes it clear that the simulation has room for improvement.


The kinetic energy has no impact if speed is constant. As Sir Newton said so eloquently - F = m a. At constant speed, a equals zero, thus all forces must sum to zero, and mass does not matter in the motion equation.

Now - mass does matter in the various forces that have to balance to zero, namely in the gravity forces if you’re not on a flat road, in the rolling resistance (proportional to weight), and in the wonky Zwift-invented CdA to weight and height relationship.

Kinetic Energy is 1/2 mv2. With the same speed (v), KE is proportional to m (mass).

Absolutely correct.

But changes nothing to F = ma.

But my statement that you quoted is correct, or?

Yes - that is the definition of kinetic energy for a translating body.

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So the 100 kg rider has double the force of the 50 kg rider that has to be deaccelerated to stop.

Yes. Again, F = ma. If you want the same (negative) acceleration, double the mass requires double the force.

Seems like we agree then :slight_smile:

Not on this:

The speed at which two riders end up at a given power on the flat but with different weight does not have anything to do with “kinetic energy”, since these are constant speed cases. The simulation does a simple summation of forces (driving power, gravity, aero and rolling), calculates an acceleration, integrates it and gets a speed (and integrates speed to get a position). There’s nothing missing in the model. The one place where it diverges from reality is in estimating the CdA from the rider weight and height - since no such closely-correlated relationship exists in real life, they had to invent one.

We agreed that the heavier rider have double the energy (kinetic) when they are riding at the same speed at the back/in in a group. On flat there is no gravity to care about. This extra force can be used to overcome the drag (CdA+RR). So if we see that the lighter rider can keep up with the heavier rider, one should think that heavier rider can outperform the the lighter rider if the delta KE > delta CdA+RR. The heavier rider will be able to take smaller gradient changes better than the lighter rider. That is how it works IRL.

Kinetic energy does not equal force. There is no “extra force” from kinetic energy on the flat at constant speed. However, you are correct that the heavier rider can take small gradient changes easier - these are accelerated cases, where mass has an impact (same force, higher mass, lower acceleration - Newton again). But keep in mind that the heavier rider has to put in a higher driving force (i.e. power at a given speed) to increase speed too, so he is at a disadvantage over the lighter one in those situations.

You are right that “force” was not the right term. My fault :slight_smile: Well-trained heavier riders can also produce more driving force than the lighter rider.
When you see these young boys keeping up with riders that can produce very high driving power, one might think that this is not right. This does not happen IRL.

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That effect is caused by the CdA relationship to weight and height.

And not rolling resistance?

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There is an impact of rolling resistance, but it is small, and it does not involve some weird correlation. It’s a direct use of the Crr concept, which is a good enough approximation of reality to be used in most vehicle simulations.

Keep in mind rolling resistance is proportional to speed, while aero drag is proportional to speed squared.

So weight has a too big factor in the CdA-formula so that Eric can keep up in his races :wink: Which of the formulas here Estimation of CdA from anthropometric data do you think is used?