How does watts/CdA work for TT in Zwift?

Racing
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In the real world, it’s about CdA, CRR, and watts. How does Zwift handle this?

To be honest, in the real world the top TT folks ain’t that big. Even the 6’ plus boys. Point being, I feel the algorithm may be skewed a bit towards watts.

I take it in stride, it’s fun, and I choose on my own to ride in my more restrictive aero tuck for racing while using my actual TT bike. We work really hard to get that CdA down in the 0.2XX’s and make power.

Up to about 40 feet per mile, maybe 50 in elevation, I can average 24 to 25mph on my race setup outdoors with only about 240w (3.4w/kg). Zwift it’s more like 23.5 on a pan flat route.

I’m just curious as to how it works it out between raw watts, w/kg, and a CdA estimate based on rider dimensions and equipment choice.

I tend to notice the heavier riders on flat TT’s having the easier time of it, which makes sense, to a point. But 80 to 85kg boys ain’t going to be sporting a 0.220 CdA either. And at those speeds that matters a lot.

Just curious.

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Height, weight, and frame and wheels matter in Zwift.

Check out ZwiftInsider for various speed tests (e.g.,frames, wheels, frame/wheel combos, climbing, rolling, TT, etc…) IIRC, he has also written about height and weight.

If you want to see TT times relative to power, height, and weight, you can check out results on ZwiftPower. Filter on “results” and “time trials”. Then check out some race results.

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Note: frame and wheel are not documented/recorded, so you don’t see those time differences which to be honest are going to be small given the typical length of a TT race

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I don’t agree with that statement, Look at this TTT on the Greatest London Flat. Team green weigh about 60kg and produced about 180w and won team Orange weighing about 90kg producing 280w.

Looking at team orange you can also see the heaviest rider had to produce significantly more power to stay with the lighter riders.

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Edit: Updated the picture to reflect race position better.

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Am I seeing orange losing at 47 minutes vs green winning in 50?

Seems weird

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That is a TTT and they start in different time blocks so the time is a bit confusing.

But here are the winners, same conclusion can be made.

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I will fix the Pic above.

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Again, it is what it is on the internet. So, grain of salt and all that. It isn’t that important. It is fun to theorize about.

That’s a TTT anyway, they use road bikes, and draft. So no idea there. Could be right, maybe not. You are ignoring though that two of the skinny dudes/dudettes were the same power as the bigger riders. Given the power output of the other team members, there were some wheel suckers on that TTT in both groups.

I’m talking about TT bikes and ITT with no draft. The TTT can be a totally different animal with lots of variables.

I’m just saying that IMO Zwift could make the ITT for TT bikes a bit more realistic (opinions are like butts, everyone has one and it usually stinks).

You’d have to dig , but if you went over to Slowtwitch and looked at the bike fits of larger riders versus smaller ones and have an idea of what CdA they probably ballpark out to…you’d get it.

The napkin math to go 27mph on the flats using some Corsa Speed 23mm tires (a known CRR) would be the following:
Weight, est. CdA possible, power
85kg, 0.270 (realistic IMO), 314w
85kg, 0.250 (a stretch), 293w
70kg, 0.220 (I’ve seen it), 256w

Let’s split the difference and call it 0.260. That’s still 304w. That’s a difference of nearly 50w. Yeah…

The guy up there making 320w, he won’t be at any 0.260 or 0.270 CdA. That 111kg dude is going to probably be around nearly a 0.290. That puts em at 344w to go 27mph.

That’s the math. The amount of belief you need here is that you cannot bike fit a TT fit the same for a 111kg rider as a 70kg and less rider. The positions you can use aren’t there. You can’t achieve the same hip angle on the same stacks.

2 Likes
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Good math I like it :+1: . Yes the TTT was probably not the best example. there was a TT race on Zwift a while ago and if I remember correctly it was on TT bikes. That should be a better example.

I would think Zwift has a CdA for different weight riders. I don’t know in what detail they have it per kg of just 3 groups small med and large.

Edit: Found it.

1 Like
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It can only be ballpark guesses when riding on Zwift. It may be close to IRL for one person but off by a bunch with another. Depends on the bike frame, the wheels for that frame, the position of the rider on that frame with those wheels, considering his/her physical characteristics. No way Zwift can figure in all that for each person.

I rode triathlons for over 20 years. I’m 6’3" and weighed in at race weight at about 165. I have very long legs and arms. My drop from seat to elbow pads was 24cm. I also rode with forearms angled down slightly, allowing air to flow through my long legs instead of around my narrow hips. This also had to dictate which frame and wheels I had to use. Given my bike setup and body type, there is no way Zwift will ever reproduce that cda by using my height and weight. Look at my Avatar… It’s all there.

It is what it is! A game! A great alternative to riding in traffic.

3 Likes
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I will be happy with 190, LOL

Nice photo.
Yes we are all different and weight and length don’t really describe cda, maybe very loosely. It also depends on how flexible and where that weight sit.

You can say that again.

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I’d love to know if the height penalty is the same between a road frame and a TT frame, because the height penalty is huge, should be less of a penalty with TT frames.

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Zwift clearly penalise weight, supposing more weight is always become the aerodynamics of a truck.
That is a fact.
So, for Zwift all is… WEIGHT, the lighter you are, the better you climb and… also ITT!!!
Nothing to be with things IRL.

You can see Zwiftinsider tests, and same conditions of height and bike.
If you put 300W with 80kg, speed is 39 km/h
If you put 300W with 60kg, speed is 42,5 km/h

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It seems that is not considered.
Also, keep in mind that there is only 1 minute difference between using a “fast” aero bike VS the TT bike after 50 minutes, based on the investigations of Zwiftinsider people.

If you are in a short TT (Bologna?) with some climb… maybe you would prefer to use an aero bike. :roll_eyes: :shushing_face:

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Exactly. Watts are easy to measure and CRR reasonable to calculate. Trying to figure out CdA when the only inputs are height and weight is never going to be accurate.

A while back I played around a little with the equations and speeds from Zwift and failed to get a fit. I have no idea how Zwift is calculating drag, but based on experience riding Zwift and on the road, my feeling is that however Zwift is doing it (tables perhaps?) the result generally overestimates Cd and/or A for heavier riders.

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Of course, drag is overstimated as you can find that here in Zwift if you are 10kg less and 10cm shorter… you are even 2km/h faster @300W… :smirk: :joy:

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Just seems like the differences between riders in CdA for TT bikes should be much smaller than on the regular bikes.

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W/KG on a flat course is irrelevant, the rider with the higher power output will always be faster (cda being equal)
Zwift clearly are only using w/kg which means lighter riders win everywhere, which is about as realistic as a Picasso :frowning:

w/kg only matters and becomes relevant as the gradient increases, and as the gradient increases, the cda is the thing that becomes irrelevant.

It would appear Zwift’s physics modelling is inadequate and this is particularly apparent on flat courses and flat TT’s (even Alex Dowsett has talked about it after a TT on his youtube channnel)

2 Likes
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Do you have any insight into their formulas?

Zwift display w/kg but that does not mean they use it in their formulas.

Calculating speed from power is a bit more complex than w/kg.

This is a random race I picked. on Crit City so very flat.

This is not a TT but the physics should at-least work the same.

1st 3.0w/kg
4th 3.3w/kg.

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Please choose a better example.
That is Zwiftpower, maybe more riders involved who could not be registered there… and it’s a RACE, so probably 1st was in another (lead) group different than 4th, and group maybe was faster.

If you choose a TT, without groups and draft, maybe we can consider something about.

On the other hand, I totally agree with @Leon_Evans_Cardiff_A , even if Zwift is not really using w/kg.
I understand what he wanted to mean.
In real life everyone who has ITT raced, knows that if put 15-20W more (not even 40, neither 100W) a 80kg rider will be CLEARLY faster than a 50-60kg rider on a “flat” terrain. Around 2 minutes faster maybe in a 16-17 or…20km long ITT.
We say “flat” even when we have 120-150m of acumulated positive slope in 15-18km. No needed a total Taiwan flat of 20m or Holland +60m… :relieved:

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Sure then you are welcome to find one.

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The ridiculous ITT of this morning…
In line with what we have talked before, VERY strage things we can see:

:joy: :relaxed: