Zwift took their latest pack physics version (Pack Dynamics v4 or PD4) live on April 4. PD4 was developed to meet the following goals:

Reduce pack speed somewhat for large packs
Make it less punishing if you’re dropped/easier to bridge back up
Make breakaways have a slightly better chance of sticking if the attackers work well together
Reduce the number of bunch sprint finishes in races
Reduce churn and rider movement in large packs

Now that it’s been live for 2 weeks, I thought it would be helpful to dig in and see if each of those goals was attained.

But first, a disclaimer: figuring out precisely how PD4 has actually affected Zwift pack dynamics is no easy task! Yes, precise tests can be done to show certain effects, but they don’t show the whole picture. There are many, many variables at play. So what you see below is a mix of conclusions from Zwift Insider tests, my own experience riding on Zwift, and feedback I’ve seen from many other Zwifters. I believe my conclusions are mostly right, but given the lack of solid analytics data, I won’t claim they’re perfect. And I would love to hear if you disagree with any of what I wrote below!

This is sort of the linchpin goal in the list, isn’t it? If Zwift can reduce the speed of large packs it would make it easier to bridge back if you’re dropped, it would give breakaways a chance, and it would reduce the number of bunch finishes.

There’s just one problem: it doesn’t seem to have happened.

It’s hard to find empirical evidence showing that packs are now faster with PD4 than they were with PD3. There are many variables in play, and I don’t have access to the full universe of Zwift data. But if you look up category-separated races that consistently run on the same route, you notice a trend.

Example: Team DRAFT Races

Look at the times for B and C categories (the largest categories, so most comparable across events) of Team DRAFT’s Thursday Races just prior to PD4’s release (Mar 23 and Mar 30) compared to the race using PD4 (Apr 6). PD4 times are faster for both categories, despite fields being larger in most of the pre-PD4 events.

Similar results can be seen for Team DRAFT’s Monday Races. Mar 27 and Apr 3 show significantly slower times than Apr 10 and Apr 17.

Why are pack speeds higher with PD4? It’s not because Zwift’s fundamental individual rider physics have changed – we still have the same wind resistance, Crr, CDA, etc. Tests with a solo rider at constant power produced the same results before and after PD4’s release. A single rider sitting on the front of a pack still turns in the same times as well.

The big change is the stronger draft. Double-draft is now used game-wide, so riders sitting in can now hold lower power yet maintain the same speed as before. (See our recent TTT speed tests for details.)

The stronger draft does two things that affect overall pack speeds:

Riders get more rest in the draft. Putting it another way: the combined power of the group is now lower for the same speed. This means a race group holding the same power as it did pre-PD4 will now move faster than it did pre-PD4.
The slingshot is stronger. When you slingshot around the front rider and into the wind, double-draft shoots you out into the wind stronger than the old draft did. This increases your speed just a bit more, which lets the pack move a bit faster. Multiply that over hundreds of “slingshot events” in a race and you’ll have a slightly faster pack.

Many Zwifters are saying PD4 has slightly changed their RoboPacer experience, making it harder to hang with the pack and increasing the speed of the group. This makes sense if you consider that many Zwifters have learned to try to stay ahead of the RoboPacer in the pack. This means the RoboPacer isn’t on the front setting the pace – rather, the bot is in the draft.

Increase that draft while keeping the bot’s power the same, and now the bot is moving faster than before. This means riders will need to put out 5-10% more power than before to stay on the front of the RoboPacer group, while sitting in the pack draft shouldn’t feel much different even though you’re going a bit faster.

Zwift wanted to make it a bit easier to bridge back to a group if you’re dropped. Has this happened with PD4? I would say yes… and no!

Yes: if you drop just a bit off the back of the group, the stronger draft gives you a bit more “cover” than before, so you can chase back on.

But if you drop too far back, the increased pack speed (see above) means you’ll be chasing an even faster pack, without any help from the draft. This means bridging from one group to the next (say, a group 20 seconds up the road) is now harder than before.

Jordan Cheyne attacking the pack in the 2020 UCI Esports Worlds

Before PD4, seeing a small breakaway win in a flattish Zwift race against a pack of 30+ was a rare thing indeed. This is because the pack would churn on the front, driving its speed up, while the small breakaway wasn’t churning nearly as much.

PD4 doesn’t help solo breakaways – in fact, it hurts their chances, since pack speeds are higher, but solo speeds are not.

But PD4 does give small bunch breakaways a better chance, since double-draft lets a well-organized breakaway work together TTT-style to efficiently keep speeds high. If a small bunch can rotate in TTT formation, using the double-draft slingshot to boost speeds each time a new rider goes to the front, they stand a better chance now than they did pre-PD4.

Are we seeing fewer bunch finishes with PD4? I’m not sure. It’s another hard thing to test given the data and analytics tools I have on-hand.

But my gut (and recent Tiny Race stories) say we’re seeing a slight increase in bunch finishes, and we’ll see more as breakaways learn to work together with Zwift’s updated physics.

The goal of reducing forward/backward movement of riders in the pack is, I think, the one PD4 goal in Zwift’s list which has most clearly been met. You only have to hop into a RoboPacer group to see there’s less of that churn, and a more realistic feel of needing to modulate your power to move forward/backward in the group.

But there’s a problem: we’ve still got too much churn, and it’s driving up pack speeds (see above).

The good news is, Zwift knows this, and they’re working on it…

There’s a massive forum thread on PD4, and David P. from Zwift, who headed up the development of PD4, has been chiming in. Here are some excerpts from what he said in a post just yesterday:

I guess it was already said and explained, and I can confirm, the so called “churn” is responsible for the higher speeds of the packs, and with PD4, like the ZI tests demonstrated because of increased draft if riders keep a constant pace the speed will increase.

I’m currently exploring what Dejan previously suggested, of implementing a more aggressive type of churn prevention, by only allowing to overtake to the front (someone not drafting), if you overcome the power to maintain the same speed if not drafting. So… if you want to go to the front of the group you need to put out more power (net power, not absolute w or w/kg) than the person in front.

So I can confirm I’m working on a possible set of PD 4.1 improvements, only testable on racing events, at least initially. ETA unknown.

So what does it all mean?

I’d say Pack Dynamics v4 is an improvement to Zwift, but there’s more that could be done. The work David P. is doing seems to be exactly what’s needed in order to tighten pack physics even more, so Zwift’s stated PD4 goals can be emphatically met.

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