As I said I’m 169 and 67kg so I assume on zwift a little lower than the 0.279.
The 0.269 I got IRL is in full skinsuit, aero overshoes, super aggressive position with 36cm bars and turned in hoods. This is on a solo roadbike TT - so zero draft.
So you can see it requires a lot of kit and positioning to get a low cda on a road bike.
You can see other has a worse cda even on a TT bike.
I’m not having a go at anyone, just saying whoever decided on cda value for zwift was feeling very generous and explains the speeds
Feels like 0.3 to 0.33 would be more appropriate.
But I guess everyone will lose their **** if everyone slows that much
So far so good (the 50 kg, 160 cm case seems reasonable)?
But, how realistic are CdA values at the upper-end of the (physique) scale?
The pack is only as fast as those pulling at the front and taking the brunt of the wind. I know from experience that many cat C races are packed with big, power-house riders easily capable of maintaining (well) above 250 W for prolonged periods. Are their CdA values a simple linear extrapolation from that of the “average” rider?
Too generous a value at the top end of the scale would also contribute to warp-speed descending.
No one decided CdA values. We use a formula from some scientific study, that may or may not be accurate is some situations.
I decided to stir up this discussion on CdA values really to see the opinions and personal experiences, because where do we draw the line between what’s accurate and what isn’t? Like you said, there are many variables like clothing, position, equipment, etc.
I’m however experimenting with dynamic CdA changes like someone already suggested, so depending on the situation CdA may change. All experimental at the moment, so nothing defined.
@DavidP A little bit more context. This is the same setup, skinsuit, standard road bike (giant TCR so not even aero bike), 20 mins before the TT then the TT itself
20min warm up not trying to be aero - CdA 0.319 - 169cm 67kg
1:05 TT trying to be as aero as possible - CdA 0.269 - 169cm 67kg
There would be tears every year when Rapha 500 comes along.
As regards existing records, IRL roads and trails change, there’s rerouting, junctions are added, all sorts. The KOM community just gets on with it. There’s no reason that improving Zwift should be held back on account of this.
All sounds good - only trying to give some real world, real rider data.
I have loads of mywindsock cda data from different scenarios if you need any, a friend who does the same events is more like 188cm and 85kg if you want some alternative data.
Personally, I think that would be a great real-world comparison
This would remove (or, at least, reduce) some of the variables that can affect unrelated data comparisons (road conditions, terrain, wind speed/direction). Your estimated CdA versus your (much bigger) mate’s value for the same event would give more of an apples-to-apples comparison.
I don’t really look at cda data but here are one from a TT effort at the end of a race. (Just me chasing the pack…spoiler I never caught them)
95kg 1.83cm in the drops on the hoods
(I don’t think I trust that number… )
But as I noted above Cda will only slow everything down it won’t improve pac dynamics (except if I am missing something)
I don’t really care if we go slower or faster, so if Zwift decide to change cda then it is all good.
Won’t increasing the CdA for everyone significantly increase the Watts saving benefits of riding in a pack, behind the front row?
In real life, there is a small benefit of having someone in your draft.
Solo riders will need to signifacantly increase power to achieve the same speed as a lower CdA, something I’m very familar with IRL, as my 58cm Cube Attain GTC Disc is an aero brick with 610mm stack… Fitting an adjustable Deda stem last spring made a world of difference, lowering the bars by ~4.5cm and giving an effective stack of ~565mm, similar to a non-endurance frame of 388mm reach.
If it’s going to take longer races to see any kind of results in a break away why even implement it for the select few. The thing zwift needs to figure out, is what side of the spectrum to real life riding/racing it is headed towards. If it’s headed towards being close to IRL, it needs to change the dynamics of algorithm to match what is capable to outside riding….
Nothing in my post said I wanted to make breakaways more difficult.
I suggested a consequence of increasing riders’ CdA in Zwift and how in real life, my endurance bike’s very upright geometry means I have to put out more Watts for the same speed compared to many other local keen Strava users.
i think i actually did a zwift group ride that had drafting enabled with people on TT bikes (so everyone had an equally lower cda). the power and the drafting dynamics didn’t change, the ride just went slightly faster. probably, anyway. i was more interested in watching tv
If we’re talking the bell curve of ‘normal’ cyclists.
Under 0.300 is far too optimistic. What are we trying to model? A normal rider riding on the hoods, average jersey and shorts, normal bike, normal vented helmet?
Or are we talking about a skinsuit, aero bike, aggressive bent arms TT position, aero helmet, aero socks etc?
These are very different parameters.
Obviously, Zwift have opted to model everyone as near optimal super athletes in sustained aero positions for all in game riding.
That, is a substantial mistake in my view. It’s given the physics an unrealistic correlation to real world conditions. It’s further increased the pack speeds and the speeds required to escape them. It’s also left Zwift with almost no scope for tactical powerups as everyone is already an elite athlete with an ideal aero body type.
The solution seems simple.
Also, some of the tiny number of people contributing here are light years from average riders. So, I’d be very careful of using their n=1 data.
Just getting around to running this through the Silca calculator. The result is remarkably almost identical. To achieve a 40kmh speed, using .241 CdA, results in requiring 229.52 watts. To do the same with instead using an entered height of 160cm results in 229.25 watts. So… good news I guess for lighter riders anyway.
Should point out that the entries are all for fairly ideal conditions and everything optimum – ie. no wind, no gradient, 21 degree Celsius, normal barometric pressure, riding in the drops, with a hot waxed chain, on new pavement and latex tubes installed.
I would suggest talking to John Hamann at Velocomp His company has produced power meters for well over a decade that calculate and and use cda for power calculation . My self have used several of his meters since 2008 because the ability to adjust my position on the bike. I would say he has the most knowledge when it comes to cda and real world data.
However, it has (perhaps) also added an incentive to use Zwift to meet any distance/speed related challenges on the likes of Strava, such as the Rapha 500, if they allow indoor riding. A casual glance at the group sizes on Tempus Fugit and the favourite RoboPacers suggests that many Zwifters want to ramp up as much distance as they can for the least effort.
no, higher CdA, slower the pack, the more chance for a break away. Physics.
The main factor in a breakaway is overcoming air resistance which get exponentially harder the faster your are going. As you can see the amount of extra Watts required to go 2.5kph faster gets higher and higher the faster you are going. Hence why at Flanders and Roubaix the break took hours to establish, with everyone already going 50kph it’s near impossible to have the spare extra power to ride away and only once the pack slows is it possible to get away.
So if pack speed on Zwift was 20kph and someone did 25kph then it’s 45w extra to get away.
BUT if pack speed on Zwift was 35kph and someone did 40kph then it’s 104w extra to get away.
More than double the watts for the same 5kph speed increase because of the faster initial speed. That’s why CdA is important.
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