Pack Dynamics 4.1 - Test Events

Hey Zwift Racers!

Here we go again for another round of test events with possible improvements to the Pack Dynamics physics.
After the discussion coming out of the official release of Pack Dynamics 4, we decided to do an extra push to try and further improve the system. The changes are essentially all towards a more realistic pack behaviour in an effort to get us closer to more dynamic race scenarios.

I’m going to highlight the changes without going much into detail because one of the main goal of testing is to see how things feel without you knowing details that can bias the “test results”.

  • Overtaking someone that is not drafting should now be harder. With the goal of reducing “churning” at the front of groups, for you to overtake someone that is not drafting you will need to put out enough “net power”.
    Net power, is the power to overcome the opposing forces like if you are not drafting. So basically you will be tested like if you are not drafting to see if your power is enough to produce the speed to overtake. This doesn’t mean you need to do more raw power or watts/kg than the person in front.

  • Dynamic CdA changes. We now have the possibility to configure CdA penalties or bonuses, depending on 3 situations:

  1. While drafting
  2. While not drafting and “attacking”
  3. While not drafting

For the initial iteration, there are only changes to the first two.

  • So you will have an increased CdA while drafting someone, which is the equivalent of you raising your upper body and have a greater frontal area, just like you see your avatar doing.
  • If you are not drafting and your power is 10% above your last 10 sec. avg power you get a 5% CdA decrease, which is the equivalent of a more aero position.

So far we haven’t changed the baseline CdA (while not drafting), because that will involve a big discussion into slowing everything down on Zwift, so that is what I would call a last resort measure. But we can still potentially test it in some of these events.

  • The other final major change is related to drafting. So we can now configure some important parameters of the drafting mechanics:
  1. Draft cone angle
  2. Draft fall-off
  3. Draft scale

For the first test events the draft cone will be narrower, the fall-off will be “steeper” (meaning that draft will be stronger closer to someone but will diminish greatly when there is a gap), and also the draft scale will be higher in “high draft situations”, i.e: deep in a big group the draft will be stronger.

We are planning the first test events for next Tuesday. Here is the link to the scheduled test events: The at Home Cycling & Running Virtual Training App
(keep this link to check for further events using this new version)

Disclaimer: It’s possible that the behaviour in the first events is very different than usual and not even up to what we hope it is, that is why they are called test events :slight_smile: However if the changes are in a good state, other race organizers can soon request the specific event configuration so that more events start using it, just like last time with the PD 4 tests.

Ride On!


this is the way :pray:


So if you’re catching someone fast (eg due to inertia from earlier hard effort) but not pedalling hard when you reach them you’re going to basically slam into a wall and stick behind them?

Sounds fun. Especially when the rider in front learns to put out a few big pedal strokes just when they are being caught. That could stop a sprinter in their tracks very effectively.


No, it won’t work as wall and the power of the rider in front in not taken into the equation. You’re basically just “tested” like if there is no draft. It doesn’t matter if the rider in front is doing 2000w.
Besides that, there is also the speed difference threshold, where if you are going much faster than those in front you are not stopped.


thanks for the work. the weather is nice today but i’ll try it soon

edit: i should stop skimming posts longer than 2 sentences before i reply. i’ll try it on tuesday then


@DavidP everything seems awesome but just a question will increasing Cda when in a group make group riding even harder and make races more of a tt ?

In theory no, because drafting will be stronger inside the group. But we will see how it goes. This is the part where real test events are needed, because bots don’t know how to complain if it feels harder or easier :joy:


Forum bots specially programmed to provide destructive criticism.

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So…not what you wrote in your post then.

It would avoid misunderstandings if you could describe the algorithms more accurately.

Fair enough. I wasn’t clear enough about what net power is. I will correct it.

this would apply during sprint scenarios too, right? no idea if that’s good or bad yet since i haven’t tried it, but just want to clarify

Yes, it will. I guess positioning for the sprint will be more important than before.


Does that mean the strength of the draft benefit is increased? Because if CdA is increased but it’s easier to draft then that means draft benefit is going to have to increase significantly.

I like the sounds of where this is heading but I also have some reservations - because simulations aren’t often as fun. See Gran Turismo vs. Project Gotham Racing from back in the day. GT is full sim…and not as enjoyable as a lot more arcadey racing games.

Does that mean the strength of the draft benefit is increased? Because if CdA is increased but it’s easier to draft then that means draft benefit is going to have to increase significantly.

The draft strength increasing does not mean drafting is easier. Like I also said, at least for this first iteration, the draft strength is increased if you are close to someone, but diminishes greatly if the distance increases.

This is still far from a simulation.

So the first ‘official’ advantage to steering?

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Is the autobrake disabled for this? People are going to get kicked out the back mid-pack if there are sharper draft gradients than there are now.

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Why do you think so? If you try to steer when inside the pack (surrounded by lots of riders) you can’t steer over others. If you can steer and go out of the draft you might be in a disadvantage. Doesn’t seem like a clear advantage to me :thinking:

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If they are “mid-pack” then drafting should be as strong as it can be. If they have more than 1-2 bike lengths of distance to those in front, then things might get more complicated, but there is no auto-braking in that situation. Auto braking only happens with less than 1 bike length of distance.

Half the time I get braked I’m sitting mid-pack.

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People have recently said they’re able to steer right through people next to them.