While doing some 2-rider drafting tests on Zwift recently, I noticed a pack dynamics “quirk” that I haven’t seen discussed elsewhere. You know how we love Zwift minutiae here at Zwift Insider… I couldn’t help myself! I went down the rabbit hole. For science. For Zwifting! And now you get to read the results. Let’s dive in…
The Watt Wasting Window
I’m calling this quirk the “watt wasting window” because it works like this: if you’re sitting in second wheel, there is a “window” or range of power you can hold to maintain your position.
There’s the minimum amount. This is what you want to hold in order to race efficiently.
But there’s also a maximum amount. This is the maximum power you can hold without coming around the rider in front of you and getting into the wind yourself.
The range of watts between minimum and maximum is large, which means you can waste a lot of watts in this situation. For example, in my video below, I show how you can sit on the wheel of a rider holding 300W at either ~230W (the minimum) or ~295W (the maximum). That’s a 65-watt swing. That’s big!
Here’s a quick video demonstrating how it works:
Before anyone asks: the overlays in the video are from Sauce for Zwift. It’s a very useful third-party Zwift add-on, but I used it for this video particularly so I could see more accurate rider speed and show the stats of both riders.
Applicable Scenarios
The most obvious scenario where this quirk matters is in a team time trial. Ever TTT racer knows that when you’re not at the front of the train putting in a pull, you have two jobs:
- Position yourself correctly
- Ride as efficiently as possible
It’s especially important to do these jobs well when you’re in the second position because messing them up can slow down your whole team or unnecessarily increase your fatigue right before you’re about to take a hard pull. To put it simply: you don’t want to waste any watts just before taking a pull in a TTT.
The watt wasting window grows larger the faster your front rider moves, too. That’s just how the math works! So if the front rider is holding 400W, the second rider can sit in at ~290W, but will also be sitting in at 395W – a range of 105 watts! Screenshots from recent tests:
It’s useful to know about this quirk in other scenarios too. In fact, just one day after running these tests, I found myself in a race sitting in a chase group with two other riders. As I sat second wheel putting out around 300W, I remembered what I’d seen the day before, and I eased up to around 250W. Guess what? I stayed on that rider’s wheel.
Now I wonder how many watts I’ve wasted since Zwift changed Pack Dynamics to behave this way.
Non-Applicable Scenarios
If you’re wondering why you’ve never noticed the watt wasting window before, it’s because this quirk is only noticeable in specific scenarios.
In a typical Zwift race you’ll spend a lot of time moving around in the peloton, just trying to sit in efficiently. You may also be attacking off the front, or maybe even putting in a hard pull on the front of the group to keep the pace high.
In these scenarios, the watt wasting window doesn’t apply. You can move freely around the peloton, you can fly off the front with your leg-snapping attack, or you can sit in the wind and put in a big pull to keep the pace high so your teammate can sprint to glory in the final 300 meters. All without worrying about wasted watts. (Well, at least not the wasted watts I’m talking about today.)
In fact, you may even find yourself accidentally rolling through the peloton and onto the front, into the wind, due to the virtual momentum your avatar had from the power you put down several seconds before. While this may be wasted watts as well, they’re wasted watts of a different type, and not the watt wasting window I’m discussing here.
It’s Not a Bug, It’s a Feature
The watt wasting window applies to very specific scenario: when you’re stably sitting second wheel.
Zwifters may see this pack dynamics quirk as a bug that Zwift needs to fix, but I don’t think that’s the case. Perhaps avatar behavior could be tweaked a bit, but there’s good reason why it works the way it does.
To put it another way: just because Zwift isn’t behaving like outdoor riding doesn’t mean something is wrong. There are limits to the simulation.
Consider this: what’s the alternative? Instead of staying on the front rider’s wheel, what should happen to the second rider when they bump their watts up to just below the front rider’s power?
Outdoors, if you rode at 295W behind a rider holding 300W (all other things being equal) you would ride into their back wheel and crash. Clearly, that’s not going to happen in Zwift. So how should Zwift position your avatar? There are three possibilities:
- You stay on the wheel of the rider ahead – this is what’s currently happening.
- You move forward, so your avatar melds with the one ahead. But this looks bad visually.
- You move forward and off to the side of the rider, like you might do outside to avoid crashing.
#2 is a nonstarter for me. I’d be interested in experiencing #3 to see if it creates a more intuitive simulation, where I begin to automatically modulate my power for maximum efficiency.
But #1, the way it currently works, also seems like a sensible approach. And it works in such a way as to help you find that magical minimum power number, if you pay close attention…
Finding The Magical Minimum
Zwift could modify the UI to help riders find that minimum power level in this scenario, perhaps by showing some sort of “watts wasted” graph. But that may prove confusing to riders, and Zwift (rightly) wants to keep their UI very simple.
The good news is, if you find yourself in this very specific scenario where the watt wasting window applies, you can find that magical minimum today as you ride! The key is to keep a gap to the rider just ahead. Outdoors, a 6″ gap feels super efficient. But on Zwift, that probably means you’re putting out too much power! Let the gap go out to 2-3 meters (you’ll get a “close the gap” message), then try to hold it there. You won’t lose any draft benefit. If the gap starts to close, you’re putting out too much power. And if it starts to stretch, you need to add a few watts.
Give it a try, and let me know how it goes for you. Ride on!
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