# Large differences in power: light/heavy riders

Would anybody explain to me why there are such dramatic differences in power between heavy and light riders? Some short flat races indicate 1 W/kg difference for riders finishing in the same bunch which is unusual. Yes, in the real world we have power-to-drag that matters and big watts win but from my race experience with men it is never that dramatic. One example from Zwift Insider Tiny Race on 19 November: Woman 55kg 4.0 W/kg. Man 110kg 3.0 W/kg. Both of them top ten finishers with the same time, flat course. How does the Category Enforcement take this into account? And will the new point system be different @gloscherrybomb?

Welcome to the forum.

Weight does not have that big effect on flat roads so looking at w/kg is a meaningless metric. For flat races you need to look at raw power so the light rider did 4w/kg (220w) and the man did 3w/kg (330w).

w/kg is not a good metric to use on flat roads.

1 Like

Generally the physics in Zwift are very well modelled. To answer your question, in ZwiftRacing.app there are two things that help with the current weight issues that are problematic with the existing category systems.

1. The seeding uses Compound Score which normalises for weight

2. The ranks changes are then based on actual results

3 Likes

Hi Gerrie, thanks for your answer. Mathematically you are right, hence I mentioned power-to-drag ratio. In a standard environment, a 55kg woman needs roughly 310W to reach 40 km/h bunch speed. A 110kg man needs 340W. However, if a tiny woman drafts behind such a large man, or even hides in the bunch full of large men, her savings are significant. It could be somewhere up to 40% in the slipstream of a solo rider and up to 65% in the bunch. The large man will also save but never as much as she does. When we look at the race result, the woman saved 93W (30%) but the man made basically no save. And that is the root of my question: why the differences are like this? Is the pack dynamics to blame?

You assume the woman was only on the draft and it was perfectly flat.

30% savings in a bunch sound about right, I have not seen 65% savings.

Maybe i donât understand the question.

Iâd say some of the key variables are drafting skill, not producing excess power in the draft, and a bit of luck with positioning in the pack (no random gaps opening right in front of you all the time). Of course, better drafting feedback in the UI would help a lotâŚ

1 Like

There are various studies showing drag savings up to 50%. But they are performed with men. From my personal experience as a track cyclist a woman can save a lot in the bunch with men. As for my issue: My concern is that model used by Zwift for light and heavy riders movement is perhaps mathematically correct but when it comes to drafting and its impact on power the results do not make sense. In the race example above, the woman saved 30% in the bunch from start to finish while in reality her saving benefit should be much higher. The guy needed 340W to ride solo, he also spent all his race time in the bunch but his average power eventually was 331W.

I wouldnât worry about it too much at this point. Zwift is currently working on a new pack dynamics model which will âsoonâ become the new default.

âCompetitive Experiences - Pack Dynamics V4: We have implemented new tweaks based on community feedback following successful tests of Pack Dynamics V4. We will continue testing following the release of game version 1.32 in a similar fashion. Keep your eyes on the forum for the announcement posts!â.

I assume they are only talking about the reduction of drag witch is only one factor in the equation.

so if both were in the draft they would have the same draft reduction Cd, so the difference in power was due to the bigger aria of the man and more weight thus higher rolling resistance.

If the Man was only riding in the front no draft you would see a bigger difference in power.

I donât know the exact formulas zwift use.

In Zwift I think the physics of modelling cycling is a lot more accurate than the physics of modelling drafting. From the discussions in the forum regarding the new PD4 one gets the impression that their drafting physics is a mix of much simplified fluid dynamics mixed with lots of kludges (such as autobraking) to try to get it to seem realistic in various situations.

3 Likes

Aside from the pure mathematics and clean metricsâŚ which would likely play out consistently in a perfect environmentâŚ in a race itâs much different, even on zwift. Doing well in a zwift race has a lot to do with technique. When and how you apply power, whom and how you draft, and how you use the course to your advantage. The differences are subtle and take time to master.
I often try to be the lowest average power in a race just to practice this. Robopacers are another good place to practice feathering the power output to maximize efficiency and get a feel for the dynamics

2 Likes

My impression is they do not want to touch the math model so they keep introducing various hacks such as auto-breaking that may work well in some scenarios but not in all of them. Another problem I perceive is that Zwift does not ease resistance on pedals once you enter a draft and uses just speed manipulations. That goes against habits and patterns you acquire in your training outside. Letâs hope that new pack dynamics will improve at least something.

2 Likes

In my experience, riding IRL & on Zwift with riders both much lighter and heavier than I am, Zwift seems to give more of a penalty on the flat to the heavier rider than reality.

Zwift calculates CdA using height and weight, so based on my experimental model for Zwift (Itâs not perfect, I donât have the actual calcs Zwift uses, but seems to hold up pretty well):
(40kph on flat)
59kg rider - 242W / 4.1 W/kg @ 40kph
82kg rider - 298W / 3.6 W/kg @ 40kph
110kg rider - 366W / 3.3 W/kg @ 40kph

30% power seems to be roughly the max of what youâll save in a smallish group, you might get a little more in a huge bunch. Bigger rider/smaller rider doesnât seem to be a huge effect on your % savings, though sometimes there does seem to be a smaller draft behind tiny people in Zwift.

1 Like

I donât agree. I do ride sometimes in the pack irl, we ride around 40 km/h average, my average watts are 220w, my weight 56kg and I am 168cmâŚ thatâs in a big pack (10-20 riders) riding in a compact bunch.
310w is for going solo at 40km/h. (In fact a little less for me, more something like 280w, which I cannot sustain long).

1 Like

A few points/cans of worms here from previous discussionsâŚ

Zwiftâs standard draft in small groups is half of IRL. The Tiny Races use the standard draft. âDouble Draftâ is IRL full draft (in small groups).

Zwiftâs group draft is not at all accurate.

Pack Dynamics has to do only with how riders move in the virtual pack. As you say, Zwift does not seem to want to touch the mathematical model of drafting/group drafting.

Zwift does seem to penalize larger/heavier riders on the flat. The feeling is that the cause of this has to do with how Zwift calculates CdA.

If you want to âfeelâ the pressure on the pedals ease as you move into the draft, try RGT (now Wahoo RGT). Their drafting model, particularly the draft in a large group, is much closer to IRL.

2 Likes

RGT penalizes lighter riders in my experienceâŚ maybe not lighter per se but riders with a lower watt capacity. Granted there are techniques to lessen that to some degree but i feel itâs still not quite enough to equalize things to the degree zwift has. Iâm not starting a zwift vs rgt argument here, just observing through my experience with the two.

Maybe I do need to try RGT just to understand this. Because I kinda donât get what people mean here.

If Iâm doing X watts at Y RPM and 40kph on Zwift, and I move into someoneâs draft (say someone merges from another road in front of me, same speed), I now have to do <X watts to do 40kph. So the resistance that I have to fight in order to maintain 40kph lessens. It doesnât react like a gradient change, but once I get that draft, I am able to downshiftâin fact I might have to downshiftâto maintain speed. So continuing on at 40kph both feels easier and is easier.

What happens on RGT? Does it tell your trainer to treat a draft like a negative (or smaller) gradient? So youâre doing X watts and Y RPM at 40kph, and you get a draft. The resistance eases from your trainer? So your Y RPM in the same gear gets you less wattage, because you need less, and so you stay in the draft? Isnât that what my own downshifting does in Zwift?

Having not ridden RGT yet, if thatâs whatâs happening, is it really a huge benefit? My experience with drafting on Zwift, considering that I downshift to accommodate, is that the resistance is less, because if I keep up the same resistance and wattage, I wonât stay in the draft. Not skeptical, just trying to understand whatâs happening over there

Think in terms of IRL cycling.
(Sorry if this is all understood. Just setting the stage.)

Youâre riding along a flat road holding a steady cadence. The road turns up hill, so you have to increase the force on the pedals in order to maintain cadence/speed. You feel this in your feet. As you crest the hill, the slope decreases, and the force on the pedals decreases. All this is because the force due to the component of gravity youâre working against changes with the slope.

Now imagine youâre riding along a flat road. Thereâs a sudden wind gust from behind. Now youâre effectively moving more slowly through the air and the force due to drag decreases. If you maintain the same cadence, the force on the pedals decreases. This is the same effect that happens if you move into a draft.

RGT feeds changes in the force due to drag through the resistance setting of the trainer, so you feel it in the pedals. Zwift does not. Zwift just calculates the decrease in drag and you go a bit faster, so YOU have to decrease the force on the pedals to compensate.

Better or worse? Just different. In this area RGT is closer to IRL cycling.

1 Like

But in Zwift, when I moved into a draft, in order to maintain the same speed and not overshoot, I have to put in less force. So the effect of âless force through the pedalsâ is still there. It takes less force through my pedals for me to do 40kph in a draft than 40kph in the wind. Zwift isnât changing anything with my trainer, but if I continue to apply the same force, I leave the draft. And so in order to maintain the same cadence at the same speed, I need to downshift. To me, thatâs a difference I feel. Less effort to move at the same speed.

I keep thinking Iâm missing something, but when I draft on Zwift, it feels like drafting after I adjust for the speed difference.

1 Like