Wasting Watts with Zwift’s Pack Dynamics 4.1

September 5, 2023

LAST UPDATED September 19, 2023

UPDATE: Zwift has shared that this difficulty in moving up in larger packs is actually a bug, and will be fixed soon. Read the forum post >

Zwift’s Pack Dynamics 4.1 went live at the end of August, and the Zwift community has been debating its effects ever since.

We published TTT drafting tests on road bikes and TT bikes soon after PD4.1 was live, and in those posts I touched on the potential for wasting watts with PD4.1. Today we dig into this topic further, as it’s an important one for racers to grasp.

Defining “Wasted” Watts

In some sense, none of the watts we put into our pedals are wasted. It’s all hard, honest work that makes us stronger (if we skip the topic of overtraining, at least).

When I talk about “wasted” watts in the context of Zwift racing, I’m talking about power you’re putting into the pedals that isn’t making your avatar move faster. So much of racing is about conserving your energy so you have what it takes to go hard when needed. If you can hold your desired position in the peloton at 200W, but you’re putting out 225W to hold that same position, I would say you are wasting 25 watts.

As we’ll see, Zwift’s new Pack Dynamics 4.1 makes it very easy to waste watts in exactly this way.

Reducing Churn Through Stickiness

For years, Zwifters talked about the “sticky draft,” a feature of Zwift’s early pack dynamics which was put into place to help riders stay in the draft. The sticky draft was most apparent at slower speeds on climbs when you would come up behind another rider and “stick” to their back wheel instead of going around them. Sure, if you put out enough power you would break that stickiness and come around, but it sure hurt to feel like you were stuck on a wheel when you wanted to pass the rider!

Pack Dynamics 3 essentially got rid of the sticky draft. But I would say PD4.1 has brought it back in some sense. My guess is that the algorithm is different, but the effect is similar in packs: you can stay in the same fore/aft position, even if you put out more power.

Wasting Watts – an Example

I’ve seen complaints from riders saying PD4.1 is “harder” in the pack and “more like a TT feel.” One Zwifter commented that, “it took me 30W more than usual to hang with the RoboPacer.”

Having ridden with RoboPacers and race packs quite a bit using PD4.1, I have to say I haven’t noticed it feeling harder to hang in the pack. I certainly notice the packs are slower, and it’s harder to move forward, and especially to move into the wind. But sitting in the pack doesn’t seem to require any more work than it did in the past.

I believe riders are wasting watts in the peloton because they expect their avatar to move forward and backward in the group like it did in the previous version of Pack Dynamics. With PD4, in a steady group you could add a bit of power and easily move forward, even slingshotting around the front rider(s) and into the wind. But in PD4.1, adding a bit of power probably won’t change your position in the pack at all! That means you might be able to hold your position in the pack at 200W… and bumping your power up to 230W might not change your position one bit.

Whatever Zwift implemented to reduce pack “churn” has made our fore/aft pack positions more “sticky,” creating a “power window” with 20-40W of wiggle room. Hold any wattage in that window and you’ll stay in your current position in the pack. Hold the top end of that wattage (say, 240W) and you’ll feel like it’s extra hard to hang with the group. Hold the bottom end of that wattage (say, 200W) and it should feel as easy as ever.

Here’s a quick video with my bot in RoboPacer Yumi’s group to illustrate this behavior:

In the video above, my bot begins by holding pack position on flat ground at 199W. I then bump it up to 239W, and my rider doesn’t change position in any noticeable, immediate way. Eventually my rider works his way to the front of the group. But if I had only bumped up the wattage to, say, 219W, he probably wouldn’t have moved forward much at all.

So in this video example:

199W is riding efficiently
239W is riding inefficiently

If this was a race, that 40W margin could very well be the difference between winning and losing.

How to Ride Efficiently

So how do you ride efficiently with PD4.1? First, stay off the front. If you find yourself rotating into the wind, you’re putting out way more power than necessary.

Second, test your power. When you’re sitting in the pack, at the position you want, ease up on your power slightly, until your rider begins to drop back. This tells you the minimum power you can hold to stay in your position. Do this often enough (RoboPacers are a great place to practice) and efficient riding will become second nature.

Is this a bug?

You may ask: given this potential for wasted watts, did PD4.1 break proper pack behavior? Does Zwift need to fix PD4.1?

I say no. This “sticky draft” issue is one riders can easily learn to use to their benefit, plus PD4.1 offers some key improvements:

Slower pack speeds (more realistic, and more chances for the breakaway)
Reduced churn (more realistic pack appearance)

Zwift continues to iteratively improve pack dynamics, which are a difficult thing to get right in a virtual cycling environment. If you have any doubts about whether Zwift’s changes are actually making things better, I would submit this video of me racing with Pack Dynamics 2.0 just over 2 years ago: