zacster

When you approach a rider from behind you will go right past them if you don't back off to match the speed though. Resistance won't change, only the displayed speed does. You need fewer watts to go the same speed. In real drafting you feel the resistance change and that reduces the effort and watts. One of my favorite tricks when riding the park loops in NYC, either Central Park or Prospect Park, is when somebody comes up to suck my wheel without asking or other acknowledgement, is to back off a little bit at the front to get them comfy, and then speed up again and swerve to get them out of my draft. Their increase in resistance feels like hitting a brick wall and spits them off the back, just like in a race. With no cars allowed in the park on the three lane wide road you can do that safely. They always get the message. You couldn't do that in Zwift because the resistance never changes, plus you can't swerve anyway. You would see your speed reduce but it would feel the same. I should add that nobody bothers to draft me anymore. I'm never riding that fast now that I'm much older.

PeteHski

This is all very interesting. What happened to the “double draft” feature that was optional when setting up group rides for a more realistic draft effect? It seemed to disappear at some point.

Also what trainer are you using? My Kickr Bike is certainly not in sync with Zwift avatar speed. It actually follows Wahoo’s own power/speed model for the slope Zwift broadcasts, which is far more conservative than Zwift! They only match when climbing at relatively low speed.

Edit; Just to add that the Zwift speed / trainer cadence mismatch is most obvious on descents where, for some reason, Zwift only broadcasts 50% of the negative slope regardless of Trainer Difficulty set to 100%.

unterhausen

Okay, now I have to go sit on the floor in the basement and install netshark on my zwift computer.

FE-C has a whole list of parameters, like rider weight, height, rolling resistance, slope, and I'm too lazy to go look up the rest. The question I have is if zwift trusts the trainer manufacturers to come up with physics based on those parameters. Seems unwise to me.

ingo

I have a Kickr Bike V1. Double draft is still available but appears in very few events, last was for one of the Zwift race series last year?. My speed in Zwift matches the riders around me for the power, slope, and in game equipment. Even though Zwift sends 50% slope to the trainer on downhills, the speed calculation uses the in game slope, e.g. descending Bologna or the radio tower. Trainer Difficulty setting has no effect on the speed calculation uphill or downhill. Zwift Insider covered all of the physics model over time. I find my speed in zwift matches very closely to real life, once you account for equipment weight, drafting, stoplights, and wind. I have power meters on my outdoor bikes for comparison.

PeteHski

I've just finished a Zwift session on my Kickr Bike and the only time I can feel a clear change in trainer resistance is when the slope value changes, even by a small amount. It's really obvious. What I don't feel is any change in resistance when approaching and passing other riders. The draft seems to work more by allowing you to soft pedal behind other riders without losing speed. This does have the effect of reducing resistance as my trainer backs off the resistance temporarily after soft-pedalling. But if I ride at a steady cadence and power I don't feel any change of resistance.

So I have a strong suspicion that Zwift simply tosses out the slope value to your trainer and leaves your trainer to provide resistance based on its own slope sim mode. When I view my trainer parameters passively in Wahoo while Zwifting, my trainer speed and Zwift speed can be quite different, especially at higher speeds.

PeteHski

I was talking about trainer resistance curves, not Zwift virtual avatar speed. They are not the same thing.

Redbullet

I guess it can be done without that: if a complex algorithm controls very close your position around the pack, then it can give you a personalized variable "bonus" in speed depending on your position. You would feel that as a reduction in power, because you will ease up the force to stay in the pack. It is somehow similar with real life. But if the whole controls run on the server, it might not have resources to control those complex parameters for each of the tens thousands riders that ride in the same time on Zwift (not to mention network data transmission delays). Therefore, I guess that in Zwift the server simplifies and gives to everybody a rough speed bonus (not personalized) around a pack, regardless their position as leaders or "drafters". This is far from real drafting.
There was a single app. on the market that simulated drafting very well (maybe using that individual "speed bonus?) - it was RGT, later Wahoo RGT, which was recently discontinued due to obscure deals between Zwift and Wahoo. Maybe RGT ran some controls on every user's computer instead of server, thus, using users' PC resources to implement a very realistic personalized drafting. Briefly, I guess (I only guess) that such performance might not be technically possible due to lack of resources, if full controls are managed on the server only.

zacster

And to think it wasn't that long ago that we could only dream of such functionality. I'm 69 years old and still remember buying my first fluid trainer at REI when I was already 50+. I did Computrainer classes at an indoor studio over 20 years ago and that setup was so high tech at the time but so expensive that only a well endowed studio could afford it. We could actually ride against each other!! I bought my first power meter when I was 63, and how cool was that? I could actually track what I was doing in The Sufferfest and not using their guesstimate, and my real power was about 50 watts lower. And then I got my Kickr Core and got on Zwift, maybe 5 years ago, when prices on a smart trainer broke the $1k barrier. I've seen everything improve over the last 5 years, but it still isn't perfect.

PeteHski

The question is whether or not drafting in Zwift actually has any real dynamic effect on your trainer resistance. Do you feel an increase in pedal resistance as you move out of the draft like you would IRL? Or does your on-screen avatar simply slow down, with no change in pedal resistance? I think it’s the latter. But if trainer resistance is changing dynamically then there has to be a control variable sent by Zwift to action it. The only dynamic resistance change I feel on Zwift is whenever the slope changes. I don’t feel any change in resistance when moving in or out of the draft. My avatar simply speeds up or slows down. Of course I can reduce my power output in the draft to maintain position, but that is not the same thing.

zacster

This is what I found on ZwiftInsider that I'd read before. You DO NOT feel the draft, you can see when you are drafting by your position, you can't steer yourself in and out. I think not feeling the draft is what confuses everybody about it. Unless you are watching your speed and not just watching your effort in watts, you just don't know it is happening.

https://zwiftinsider.com/feeling-the-draft/
https://zwiftinsider.com/zwift-drafting/

ingo

If you speed up entering the draft your cadence goes up, resistance stays the same so your power increases. If you reduce power by keeping the the same cadence and lowering torque then you should feel that. Exactly what happens in real life. Whether you perceive the draft effect it is on you, because the effect is there. When Schlange was asked the same question why he did not perceive the draft effect, he had no answer and acknowledged that the power decrease in the draft has to exist.

zacster

Let's look at it this way, or actually, let's close our eyes. If you close your eyes during a Zwift session you would never know you were drafting. If you maintain your power and cadence you will blow right by the rider. It is only seeing the rider, seeing a position change and seeing your speed, cadence and power that you know you are drafting. If you could close your eyes when riding outdoors, you would suddenly feel an easing of the wind resistance when you started drafting and feel like you were being pulled along.

PeteHski

I’m pretty sure trainer resistance doesn’t change when you enter the draft. Your avatar speed increases, but your power and cadence stay the same ie trainer resistance is unaffected. I’ve read a few of the Zwiftinsider articles on this and they all clearly state this.

There was one article discussing how Zwift could introduce a draft resistance factor as there is a “draft factor” included in the trainer comms profile, although it is only included in the ANT+ profile and not in the BT profile. They mentioned that some other apps are using this draft control parameter, but not Zwift.

I think it’s all just a mind game. When you enter the draft you know you can back off the power and of course then it feels easier. But if you deliberately enter the draft at a steady power and cadence, nothing changes other than your virtual avatar speed. I tried it multiple times to confirm. The draft itself actually allows a power window within which you stick to the wheel in front. So you can back off the power more than you might think before losing the wheel and adding more power does nothing until you exceed the upper end of the draft window and move through the rider in front. To be honest it all feels quite natural most of the time.

PeteHski

Having now read a few of the Zwiftinsider blogs about drafting it makes more sense now and fits in with my personal experience ie no resistance change when drafting. This article was most useful

https://zwiftinsider.com/feeling-the-draft/

A few snippets from the above link:-

“On Zwift, you feel no change, as resistance does not change based on your drafting status.”

“Ideally, Zwift would be able to send a simple “drafting factor” to our smart trainers, just as they send wind resistance, slope, track resistance, weight, and other parameters.”

“smart trainers use two different wireless protocols: ANT+ FE-C (ANT) and Bluetooth Smart FTMS (Bluetooth). The ANT specification does include a drafting factor. But the Bluetooth specification does not.”

The last comment is interesting and suggests that it would be very easy for Zwift to add a drafting effect to trainer resistance for users with an ANT connection. But it appears that they don’t currently use this parameter.

zacster

Most trainers support both ANT+ and BT these days, but not everybody has an ANT+ dongle. They are cheap enough though that if Zwift were to support it you could just add one to your setup if you don't already have it.

PeteHski

Just to complicate things, I read that Zwift sends rider weight over a BT connection but not with ANT connection. So resistance is not weight corrected when using ANT.

zacster

This was fixed back in 2018: https://zwiftinsider.com/update-1-0-29337/ (unless it was broken again later)

PeteHski

Ah I see now. I actually read that blog without realising it was talking about the fix!