Thoughts on Zwift’s Experimental Pack Dynamics v4

UPDATE (April 5, 2023): After lots of tweaking and testing, Zwift has taken Pack Dynamics 4 live game-wide. Read more about it here >

On Monday, October 24, Zwift held its first-ever public ride using their experimental “Pack Dynamics v4”. The ride was James Bailey’s weekly “Ride with James” and its description explained the overall goals of PD4:

The Goal:

• This new version of PD has the goal of creating a more realistic racing experience (compared to IRL racing). The main changes introduced focus on reducing the forward and backwards movement of cyclists in the pack while also reducing the overall speed of larger groups.
• We encourage racers to try a more “aggressive” approach and try breakaway opportunities to create more dynamic racing scenarios.

What you can expect:

• Less speed in groups of cyclists where there isn’t a concerted effort to “pull” the group by taking pulls at the front.
• Double draft is activated by default therefore riding in a group should feel a bit easier than with the standard draft model.

It’s safe to say that the vast majority of Zwift racers, and probably the vast majority of Zwifters, are supportive of Zwift investing more time into improved pack dynamics. While the current pack dynamics (v3, rolled out in October 2021) are a great improvement over the previous version, with riders moving more like a “school of fishes” as Wes Salmon used to say, they aren’t perfect. Overall group speeds are still too high thanks to the endless slingshot-n-churn on the front, making breakaways nearly impossible. And sitting in the group draft is harder than you would find IRL.

I participated in that first ride on Monday, and recorded it:

But that first ride wasn’t the best test of PD4. It had been misconfigured for the wrong route, which meant much of the ride was spent climbing or descending a dirt KOM. And the overall ride was only 16km long, which simply didn’t leave much time to get the “feel” of PD4.

So I volunteered to use PD4 in the Zwift Insider Tiny Races happening the following Saturday. Testing PD4 at “race pace” with a sizeable pack of riders would give us a much better picture of how it behaved in comparison to PD3.

I recorded the full set of four Tiny Races, including me on the suffercam:

Experimental Feature

I want to be sure to note here at PD4 is very much an experimental feature at this point. It’s not ready for prime time, and Zwift knows this. That’s why it’s only being used on a very limited set of events, and participants are being asked to share their impressions in Zwift’s forum once they’ve completed their ride.

See the Pack Dynamics v4 thread >

This is exactly how it should work, of course. Zwift can develop new Pack Dynamics and test them in-house to some degree, but the best feedback and testing will come from a much larger body of experienced riders found in the Zwift community.

My Notes on Pack Dynamics v4

Objectively testing something like Zwift Pack Dynamics can be challenging, because you’re really comparing what you’re experiencing to what you remember from past experiences. Does PD4 feel like PD3? How does it compare to outdoor riding?

Everyone’s impressions will vary slightly. But there are also some definite themes I’m seeing in feedback from other riders which matches the impressions I had. Here are my notes on my PD4 experiences thus far, written as objectively as possible.

Draft Feel

Simply put, if you’re not near the front of the front group, the draft with PD4 feels unrealistically weak. While Zwift says double draft is built into PD4 and “riding in a group should feel a bit easier than with the standard draft model”, that wasn’t my experience.

While Zwift Racers know they can tailgun at the back and still enjoy a strong draft with PD3, PD4 gives you no such luxury. In the Tiny Race 1 I stayed in the front pack, and to be honest things didn’t look or feel much different from PD3 up there.

But in races 2, 3, and 4 I found myself toward the back of the front pack. And that’s where things got ugly. I was dropped in the Jungle in race 2 (more on that below), dropped in Central Park in race 3, and dropped along the Thames in race 4. It was painful!

It could be argued that the front pack just contained very strong riders this week, and I didn’t have what it took to hang in there. But after racing Tiny Races almost weekly for 14 weeks, and looking at pack speeds and power numbers, I’d say this wasn’t a case of strong riders stretching out the group. The pack speeds are still high, and the draft doesn’t feel strong enough, so you end up (as a B racer) feeling like you’re riding a TT if you’re trying to get back into the middle of the large front group.

Reviewing my race video, it’s possible that this isn’t a draft problem as much as a speed problem with PD4. There are times in my race where I’m just off the back of the front group, pushing high watts, but my speed is lower than it would typically be on Zwift with PD3. So perhaps PD4 is slowing me, as part of its attempt to keep pack speeds slower, and that is what causes me to get dropped from the front group.

Speed Variation

While PD4’s draft feel doesn’t feel right to me, the biggest problem I’ve seen with PD4 is something completely different. I’ll call it “Speed Variation” for lack of a better term, but basically what I’m seeing is rider speeds becoming unrealistically fast. It seems to happen on descents for me, and the best example I can show is my descent through the Jungle in Tiny Race #2. Here’s the video cued up to a good spot:

You’ll notice the video starts with me in 105th place. I’ve been dropped from the front, which is really a snaky line of riders. As the descent gets a bit steeper (going from -1-2% to -2-3%) my speed inexplicably picks up from ~39kph to ~53kph. I’m not pushing hard, either! It felt like I had a steamroller powerup, or perhaps an anvil. Without even pushing I magically moved from 105th to 45th by the time the Jungle dirt turned to Alpe pavement.

105th to 60th is a huge move, too, considering the pack was very strung out and I was catching riders one at a time. Also worth mentioning: I was on the Tron bike, while many riders were on faster gravel bikes!

I saw it with other riders as well. Here’s me getting dropped in New York. Watch the rider in the Jensie kit (“T Ward”) pass me, then watch the “A Vella” rider in the default grey kit pass as well. Both aren’t doing any more power than I am, but they fly past (and A Vuella just keeps going!)

I noticed this on Monday’s initial test ride, too. On my descent from the Temple KOM, I picked up momentum and began flying past other riders, despite putting out pretty low power in comparison to those around me:

Forward/Backward Movement

On a positive note, comparing videos of PD4 Tiny Races with PD3, I would say there is less forward/backward movement with PD4. That’s a good thing, since Zwift’s notes for PD4 say, “The main changes introduced focus on reducing the forward and backwards movement of cyclists in the pack…”

Here are two example videos, cued up to an ideal comparison spot:

Pack Speeds

Zwift’s notes for PD4 also include “reducing the overall speed of larger groups.”

It’s hard to compare group speeds with much precision outside of a controlled test, but just looking at Strava times, Tiny Races in weeks past, I would say the speed of the front pack isn’t appreciably slower. I compared the first 2 km of a few different events held on similar routes, and their times were nearly identical, with PD4 races actually slightly faster in some cases.

High pack speeds are further evidenced by the lack of any breakaways in our Tiny Races, although admittedly these aren’t the best environments for testing “breakawayability” since the pace will be quite high due to short race durations. I’d like to see PD4 used in an hourlong flat/rolling race, to see if breakaways are possible.

If Zwift is looking to reduce the speed of large groups (and I agree they should be looking at reduce pack speeds by 5-10% if they’re going for IRL realism), I don’t think PD4 is there yet.

Pack Shape

One final point (and I’ll admit this feels a bit nit-picky since PD4 isn’t trying to address this issue): I would love to see more realistic pack shapes at race speeds.

Then the pack is flying along (like you’d see in a 5-minute race), the front of the pack should be very narrow – just one or two riders – and it should widen as it goes back. A super-wide pack with, say, 15 riders all in the wind is what you see during neutral starts, or when everyone is taking it easy in a race.

I have a theory which I would love to test: that if riders could feel the draft in Zwift, it would encourage better pack positioning (more drafting, less in the wind). Right now if you’re in the wind, your trainer resistance is the same as when you’re in the draft. But if you felt yourself hitting the wind by getting more resistance, that might help you subconsciously ease up and get back in the draft.

More on this topic: “Opinion: Let Me Feel the Draft On Zwift” >

Conclusions

Zwift’s stated goals for PD4 are laudable. They show that Zwift knows the shortcomings of their current pack dynamics, and they want to make them more realistic.

That said, PD4 isn’t fully baked quite yet. Hopefully the feedback above, and the feedback Zwift is receiving from hundreds of Zwifters on their forum thread, will help Zwift’s developers dial PD4 in and thus improve the pack riding experience for everyone.

(Oh, and for you Tiny Racers: due to buggy speed inconsistencies mentioned above, we won’t be using PD4 in its current state in future Tiny Races. But once Zwift updates PD4, it’ll be time for another Tiny Race experiment!)

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